Diffusion of Labeled Polyelectrolyte Probes in Unlabeled Polyelectrolyte Matrix Solutions

Fluorescence photobleaching recovery, FPR, was used to investigate the diffusion of fluoresceinamine-labeled probe poly(styrene sulfonate sodium salt), LNaPSS, in solutions containing unlabeled poly(styrene sulfonate sodium salt), NaPSS, as a matrix polymer.  Various probe molecular weights were studied, while the weight average of the matrix polymer was limited to just one value, Mw = 990000.  For the solutions without added salt, the matrix polymer slowed the self diffusion of LNaPSS, but the effect became less dramatic at higher matrix concentrations.  Sealed-cell and in situ dialysis FPR studies showed that diffusion increased with addition of salt, but the rise depended on matrix concentration in a complicated way.  Long-duration residence of probe polyelectrolyte in temporal aggregates, which would be expected to result in a weak dependence of diffusion on probe molecular weight, was not detected.  Instead, the dependence of diffusion on probe molecular weight became stronger with matrix concentration, though the expectations of scaling theories were not reached in the molecular weight range that was covered.

Elucidating the Kinetics of β-Amyloid Fibril Formation

Nadia J. Edwin, Grigor B. Bantchev, Paul S. Russo, Robert P. Hammer and Robin L. McCarley in “New Polymeric Materials”, ACS Symposium Series #916, Ljiljana S. Korugic-Karasz, William J. MacKnight and Ezio Martuscelli, eds. American Chemical Society: Washington, DC,  2005; Ch. 9, pp. 106-118.

The formation of β-Amyloid peptide (Aβ1-40) aggregates was monitored by dynamic light scattering.  Various sizes of materials may be present throughout the aggregation process, but small scatterers are difficult to detect in the presence of large ones.  Fluorescence photobleaching recovery studies on 5-carboxyfluorescein-labeled Aβ1-40 peptide solutions readily confirmed the presence of large and small species simultaneously.  The effects of dye substitution on the aggregation behavior of Aβ1-40 peptide are subtle, but should not prevent further investigations by fluorescence photobleaching recovery or other fluorescence methods. 

A detergent-like mechanism of action of the cytolytic toxin Cyt1A from Bacillus thuringiensis var. israelensis

S. D. Manceva, M. Pusztai-Carey, P. S. Russo and P. Butko, Biochemistry 2005, 44 (2): 589-597.  

The cytolytic delta-endotoxin Cyt1A from Bacillus thuringiensis var. israelensis is used in commercial preparations of environmentally safe insecticides. The current hypothesis on its mode of action is that the toxin self-assembles into well-defined cation-selective channels or pores, which results in colloid-osmotic lysis of the cell. Recently, a new hypothesis has been put forward suggesting that Cyt1A rather nonspecifically aggregates on the membrane surface and acts in a detergent-like manner. To distinguish between these two hypotheses, we investigated whether in the presence of lipid Cyt1A self-assembles into stoichiometric oligomers, which are characteristic of pores or channels, or aggregates into nonstoichiometric complexes, which would support the detergent-like model. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that in the presence of lipid Cyt1A forms protein aggregates with a broad range of molecular weights, some being too large to enter the gel. Cyt1A tryptophan (Trp) fluorescence in the presence of lipid exhibited a decrease in anisotropy and quantum yield, but an unchanged lifetime, which is consistent with the presence of toxin aggregates in the membrane. Electrostatic interactions between the charged amino acid residues and the lipid headgroups are responsible for bringing the protein to the membrane surface, while hydrophobic and/or van der Waals interactions make the membrane binding irreversible. Fluorescence photobleaching recovery, a technique that measures the diffusion coefficient of fluorescently labeled particles, and epifluorescence microscopy revealed that upon addition of Cyt1A lipid vesicles were broken into smaller, faster diffusing objects. Since no change in size or morphology of the vesicles is expected when pores are formed in the osmotically equilibrated membranes, our results support the detergent-like mode of action of Cyt1A.

Rotational and Translational Diffusion of Tobacco Mosaic Virus in Extended and Globular Polymer Solutions

Randy Cush, Derek Dorman, Paul Russo, Macromolecules 2004, 37(25); 9577-9584. 

Depolarized dynamic light scattering is used to measure the translational and rotational diffusion of a rodlike probe, tobacco mosaic virus, in matrix solutions of dextran, an extended polymer with some branching, and globular Ficoll.  Translation and rotation both decline almost exponentially as the concentration of either matrix rises.  The ratio of rotational to translational diffusion is similar in dextran or Ficoll solutions, and close to the values expected from continuum theories of the friction of rodlike particles.  Reinforcing a continuum picture in which hydrodynamic effects surpass any due to topological constraints, the declines in rotational and translational motion are almost inversely proportional to the solution viscosity.  Only modest and gradual deviations from Stokes-Einstein behavior are observed, even at high matrix concentrations.  This stands in stark contrast to an earlier study by this group (Macromolecules 1997, 30(17), 4920-4926).  The difference may be traced to the subtle effects of optical rotation (dextran and Ficoll are chiral) on instrument alignment, coupled to the weak depolarization of the strongly scattering tobacco mosaic virus and the very slow rotations encountered at high matrix content.  In optically inactive solutions, and even in optically active ones studied with the correct and tedious alignment, a particle the shape and size of tobacco mosaic virus can serve as an effective microrheological probe.  Confirming this conclusion, the apparent microviscosity obtained by inverting rotational or translational diffusion coefficients reflected the molecular weight trend, at a particular concentration, of shear viscosity measured in a cone and plate device. 

Structural Changes and Aggregation of Human Influenza Virus

Jason N. Campbell, Richard Epand and Paul S. Russo, Biomacromolecules, 2004, 5, 1728-1735. 

The pH-induced change in the structure and aggregation state of the PR-8 and    X-31 strains of intact human influenza virus has been studied in vitro.  Reducing the pH from 7.4 to 5.0 produces a large increase in the intensity of light scattered to low angles.  A modest increase in the polydispersity parameter from cumulants fits to the dynamic light scattering correlograms accompanies the increase, as does a change in how that parameter varies with scattering angle.  These trends imply that the virus particles are not uniform, even at pH 7.4, and tend to aggregate as pH is reduced.  The scattering profiles (angular dependence of intensity) never match those of isolated, spherical particles of uniform size, but the deviations from that simple model remain modest at pH 7.4.  At pH 5.0, scattering profiles calculated for aggregates of uniformly sized spheres come much closer to matching the experimental data than those computed for isolated particles.  While these observations, indicating acid-induced aggregation, develop over a period of minutes to hours after acidification, a nearly instantaneous increase in hydrodynamic size is the first response of intact virus particles to lower pH. 

 A New Synthesis of Fluorescein Isothiocyanate Labeled Poly(styrenesulfonate sodium salt)

Rongjuan Cong, Sibel Turksen and Paul Russo, Macromolecules, in press (2004)

 An efficient synthesis of fluorescein-labeled poly(allylamine-co-styrene sulfonate sodium salt) (NaPSS) is reported.  This new approach takes advantage of the low reactivity of allylamine in free radical polymerizations and begins with sulfonated monomer, thus giving a product with a high molar fraction of sulfonated repeat units, 0.98.  After reaction with FITC, the polymers have about the right dye content for fluorescence photobleaching recovery (FPR) experiments and other applications.  Polymerization in a solvent/nonsolvent mixture results in polydispersities only slightly worse than anionically polymerized samples at low molecular weights.  Even tighter control of polydispersion can be achieved for useful quantities of fluorescently tagged materials after analytical scale gel permeation chromatography. 

Colloidal Crystals of Silica-Homopolypeptide Composite Particles.

Brian Fong, Sibel Turksen, Paul S. Russo, and Wieslaw Stryjewski, Langmuir, 20(1), 266 - 269 (2004)

The synthesis of colloidal particles containing a silica core and poly(carbobenzoxy-L-lysine) shell is described.  Dynamic light scattering and electron microscopy show the particles to be very uniform in size.  Infrared spectroscopy suggests the homopolypeptide shell exists primarily in the a-helical conformation.  Between crossed polars in an optical microscope, aged suspensions of the particles develop a lower phase that is characterized by the bright colors and banding of cubic colloidal crystals.  The domain sizes are not large, which is one of several factors that seem to obfuscate optical analysis of the crystals, but light transmitted through each is very pure, consistent with dynamic diffraction. 

Factors Affecting the Size of Aqueous Poly(vinylphenol-co-potassium styrenesulfonate)/Poly(ethylene oxide) Complexes.

Cong, Rongjuan; Pelton, Robert; Russo, Paul; Doucet, Garrett. Department of Chemical Engineering,  Macromolecules (2003), 36(1), 204-209.

The diffusion properties of complexes formed between poly(ethylene oxide) (PEO) and poly(vinylphenol-co-styrenesulfonate) (PKS) in aq. medium were investigated by diffusion ordered NMR, fluorescence photobleaching recovery, and viscosity measurements. All 3 techniques showed that PEO/PKS complexes range from single PEO coils with bound PKS mols. to large complex species contg. many PEO chains. For a given PKS structure, there are 2 important transitional PEO mol. wts. The lowest one, .apprx.8000 Da, corresponds to the onset of PEO/PKS complex formation. The second transitional PEO mol. wt. is between 105 and 106 Da and corresponds to the onset of multi-PEO chain complex species which are important for flocculation. PKS functions as a phys. crosslinking agent for PEO. If there is too little PKS, multiple PEO chains are not bound together. Similarly, high concns. of PKS give small complexes because there are few opportunities for connecting multiple PEO chains together, since all the PEO chains are satd. with bound PKS.

NMR investigations of the structure of water-soluble poly(ethylene oxide) complexes with polystyrene sulfonate copolymers.

R. Cong,  R. Pelton, P.S. Russo, A. D. Bain, I. Negulescu and Z. Zhou, Colloid and Polymer Science , 281(2); 150-156 (2003).

Macromolecular complexes of styrene sulfonate copolymers, poly(vinyl phenol-co-styrene sulfonate) (PKS) and poly(styrene-co-styrene sulfonate) (PS-co-SSS) with poly(ethylene oxide) (PEO) in aqueous solution were studied by NMR. The shift to lower frequency of the 1H chemical shift for the PEO ethylene group and the existence of strong nuclear Overhauser effect cross-peaks between the aromatic rings of PKS and the ethylene proton of PEO indicated that arom. rings interacted with the PEO ethylene groups. Since the spectra of PS-co-SSS/PEO mixtures  revealed complexes with structures similar to those formed with the phenolic copolymers, it was concluded that hydrogen bonding was not necessary for complex formation.

Self-diffusion of a Rodlike Virus in the Isotropic Phase

R. C.; Cush and P. S. Russo, Macromolecules, 35(23); 8659-8662 (2002).  (doi:  10.1021/ma0205459)

The optical tracer self diffusion of tobacco mosaic virus (TMV) was measured as a function of concentration in the isotropic phase.  Diffusion decreases almost exponentially with concentration, leveling off at about 40% of the dilute solution value.  In comparison to the semidilute behavior of the semiflexible polymer, poly(benzylglutamate), diffusion-reducing interactions begin at lower reduced concentrations for TMV but are ultimately not as severe.  The diffusion decreases about three times faster than expected from the Brownian dynamics simulations of thin rods by Doi, Yamamoto and Kano (J. Phys. Soc. Japan, 1984, 53, 3000-3003), suggesting the importance of finite thickness, hydrodynamic interactions, or both. 

Synthesis of Paucidisperse Poly(g-benzyl-a,L-glutamate) Oligomers and Star Polymers with Rigid Arms

X. Wang, W. H. Daly, P. S. Russo, M. Ngu-Schwemlein, Biomacromolecules, 2(4), 1214-1219 (2001). 

The synthesis of highly uniform g-benzyl-a,L-glutamate (BLG) oligomers via a convergent solution phase approach is reported.  BLG oligomers were produced with designed lengths of 4, 8, 12, and 16 as a first step to production of BLG-4-mer and BLG-8-mer rod stars.  The star oligomers were purified by size-exclusion chromatography and reversed phase HPLC, and characterized by MALDI-TOF mass spectrometry and reversed phase HPLC.  These star-shaped BLG oligomers could be used as initiators for growing larger stars.

On the Use of Pattern Fluorescence Photobleaching Recovery with Modulation Detection to Obtain Colloidal Size Distributions. 

Fong, Brian; Stryjewski, Wieslaw; Russo, Paul S.,  J. Colloid Interface Sci. , 239(2), 374-379 (2001). 

Fluorescence photobleaching recovery, long a staple tool of complex fluid experimentation, is discussed as a method for size characterization of colloidal particles. Specific comparisons are made to dynamic light scattering, which is often used for sizing particles despite some important limitations. The capabilities of fluorescence photobleaching recovery for size determination are demonstrated for aggregating core-shell silica-polypeptide composite particles suspended in polar organic solvents. 

 

Study of Rodlike Homopolypeptides by Gel Permeation Chromatography with Light Scattering Detection:  Validity of Universal Calibration and Stiffness Assessment, 

E. Temyanko, P. S. Russo and H. Ricks, Macromolecules, 34, 582-586 (2001).  

Gel permeation chromatography with simultaneous light scattering measurements have been performed on poly(g-benzyl-a,L-glutamate) and poly(g-stearyl-a,L-glutamate) in N,N’-dimethylformamide and tetrahydrofuran, respectively.  Intrinsic viscosities were computed by combining the measured molecular weights with literature values for the Mark-Houwink parameters.  The product of intrinsic viscosity and molecular weight for the rodlike polymers, when plotted against elution volume, overlaid the results for polystyrene, a random coil.  In the excluded volume limit, similar convergence was observed for the second virial coefficient of osmotic pressure as a function of polymer number density.  In the case of poly(g-benzyl-a,L-glutamate) the radius of gyration increased linearly with mass up to at least M = 100000, corresponding to a contour length of 70 nm.  Slight deviations from linear behavior at higher mass suggest a persistence length in the upper range of literature values based on measurements that did not have the advantage of chromatographic separation. 

 

Thermoreversible Gelation of Isotropic and Liquid Crystalline Solutions of a "Sticky" Rodlike Polymer

Sarah Schmidtke, Paul Russo, Javier Nakamatsu, Ebru Buyuktanir, Bilge Turfan and Elena Temyanko and Ioan Negulescu

Thermoreversible gelation of rodlike poly(g-stearyl-a,L-glutamate) in n-dodecane has been studied by differential scanning calorimetry, polarized light microscopy, visual observations and temperature ramped fluorescence photobleaching recovery. Solutions spanning a wide range of concentrations can produce gels. In solutions that are initially liquid crystalline, long-range features such as cholesteric twist appear to be frozen by gelation. Although gelation is rapid, some of the polymer molecules retain significant mobility, allowing the gels to "cure" slowly.


Teaching  Light Scattering to Exemplify and Reinforce Basic Principles

D. S. Poche', P. S. Russo, B. Fong, E. Temyanko and H. Ricks, J. Chem. Ed., 76 (November), 1534-1538 (1999).

There is no abstract for this article, but it uses simple concepts like osmotic pressure and wave interference to explain the scattering by polymers.  Modern, visual programs for the analysis of scattering data are also shown.  These programs are designed with both teaching and research in mind. 


Organophilic Colloidal Particles with a Synthetic Polypeptide Coating

B. Fong and P.S. Russo, Langmuir,  15(13); 4421-4426 (1999).

Composite colloidal particles with a silica core and a synthetic, homopolypeptide shell have been produced by initiation of benzyl-L-glutamate N-carboxyanhydride monomer from primary amine functionalized silica particles. The resulting poly(-benzyl-,L-glutamate)-coated spheres were characterized by electron microscopy, dynamic light scattering, infrared spectroscopy, and thermogravimetry. The polypeptide shell accounts for about 20% of the total mass of the particles, which are reasonably uniform in size. Infrared spectra show an -helical secondary structure, but other conformations are not excluded. A geometrical analysis is applied to calculate the maximum number of amino groups that might realistically participate in initiation. The actual shell thickness is smaller than expected on the basis of these geometrical considerations, which reflects undesired termination steps or the conversion of some monomer to unattached polymer by trace initiator impurities.

 


Hydrodynamic Studies on the Manganese-Stabilizing Protein of Photosystem II

I. Z. Aubrzycki, L.K. Frankel, P.S. Russo and T. M. Bricker, Biochemistry,  37(39); 13553-13558 (1998).

The solution conformation of the manganese-stabilizing protein of photosystem II was examined by analytical ultracentrifugation.  Sedimentation velocity and sedimentation equilibrium studies were performed.  These experiments yielded values for so20,w of 2.26 S with a diffusion constant, D, of 7.7 x 10-7 cm2s-1.   This s value is significantly lower than the apparent s value of 2.6 S previously reported [Miyao, M., and Murata, N. (1989) Biochim.Biophys. Acta 997, 315-321].  The molecular mass of the protein, 26.531 kDa, was verified by MALDI mass spectrometry.  The diffusion coefficient was als determined by dynamic light scattering.  The z-weighted average of D was 6.8 x 10-7 cm2s-1.  This result was somewhat lower than that observed by analytical ultracentrifugation due to the presence of slowly diffusing components in the sample.  A two-component exponential fit of the dynamic light scattering data, however, gave D = 7.52 x 10-7 cm2s-1 for the major component of the sample, which is in excellent agreement with the value determined by analytical ultracentrifugation.  The value of s*, the apparent sedimentation coefficient, was found to depend on the concentration of the protein and varied about 4% per milligram of protein.  This is a feature of proteins which are asymmetric in solution.  This asymmetry was examined using both the v-bar and Teller methods.  Both methods indicated a significant degree of asymmetry for the manganese-stabilizing protein.  Our findings indicate that the prolate ellipsoid model for the manganese-stabilizing protein is elongated in solution, with approximate dimensions of about 12.6 nm x 3.0 nm, yielding an axial ratio of 4.2. 


Self Diffusion of a Semiflexible Polymer Measured Across the Lyotropic Liquid Crystalline Phase Boundary. 

Paul S. Russo, Michael Baylis, Zimei Bu**, Wieslaw Stryjewski, Garrett Doucet, Elena Temyanko, Debbie Tipton, J. Chem. Phys., 111(4), 1746-1752 (1999).

The self diffusion of fluorescently tagged poly(g-benzyl-a,L- glutamate), a helical, semiflexible synthetic homopolypeptide, has been measured in isotropic and cholesteric liquid crystalline solutions by pattern fluorescence photobleaching recovery. On the isotropic side of the sharp isotropic-liquid crystalline (ISO-LC) phase boundary, the rodlike polymers assume all possible orientations in a three-dimensional space, becoming enmeshed. In liquid crystalline solutions, as first shown by Robinson (Trans.Faraday Soc., 52, 571-592, 1956), spontaneous alignment of the cholesteric screw axis parallel to the optical (z) axis of the instrument produces small monodomains in which parallel rodlike polymers are organized into planes. Each horizontal plane is twisted slightly compared to its neighbors. Over the thickness of the sample, the rodlike polymers assume all possible orientations in this two-dimensional space. Despite the small size of the monodomains, it was possible to determine the self diffusion coefficient of the semiflexible rods, orientationally averaged in two dimensions. Crossing the sharp ISO-LC phase boundary corresponds to the sudden release of any putative topological constraints active in the isotropic phase, and produced a modest but significant increase in diffusion. A relationship developed by Hess, Frenkel and Allen (Mol.Phys., 74(4), 765-774) is used to show that diffusion perpendicular to the rod axis is about 10 times slower than diffusion parallel to the rod axis in the liquid crystalline phase. In dilute solution, the comparable number would be 2. The perpendicular diffusion had decreased to about 8% of its initial value in dilute, isotropic solution. The parallel diffusion decreased to about 40% of its initial value. These results were obtained by neglecting the uncertain effects of semiflexibility. Likewise, the effects of modest polydispersity have not been treated explicitly.


Light Scattering Study of Magnetic Latex Particles and Their Interaction with Polyelectrolytes

D. Sohn, P. S. Russo, A. Davila, D. S. Poche’ and M. L. McLaughlin, Journal of Colloid and Interface Science, 177, 31-44 (1996).

Commercially available latex particles with superparamagnetic inclusions have been characterized by polarized and depolarized dynamic light scattering. The particles are spherical, but the larger ones (Rg 0.1 mm) with many magnetic inclusions behave in dynamic light scattering experiments like symmetric tops while the smaller particles do not. The as-received materials had a broad size distribution, but more narrowly dispersed particles were prepared by selective filtration for use in probe diffusion and stability studies. The interaction of these negatively charged particles with a linear polyelectrolyte, negatively charged sodium polystyrene sulfonate, was studied by static and dynamic light scattering. Depolarized dynamic light scattering is particularly sensitive to changes in the stability of the colloids or interaction with the polyelectrolyte. The most unusual result is that polyelectrolyte can prevent or reverse salt-induced aggregation of the superparamagnetic latex particles, apparently by a method other than steric stabilization.


Interactions between Colloidal Poly(tetrafluoroethylene) Latex and Sodium Poly(styrene sulfonate)

T. Jamil and P.S. Russo, Langmuir, 14, 264-270 (1998).

The translational and rotational diffusion of poly(tetrafluoroethylene) latex in aqueous solutions of unbound sodium poly(styrenesulfonate) were measured by dynamic light scattering. Both quantities were affected by the viscosity of the poly(styrenesulfonate), which was itself a function of added salt. Adjusted for viscosity, the rotational diffusion decreased with added salt but did not depend on polymer concentration in the range studied. This suggests that rotational diffusion of a probe particle can be used to sense the local ionic environment. Visual observations and static light scattering showed that salt-induced aggregation of the latex can be prevented or reversed by small amounts (several milligrams per milliliter) of sodium poly(styrenesulfonate).


Light Scattering and Fluorescence Photobleaching Recovery Study of Poly(amidoamine) Cascade Polymers in Aqueous Solution

K. Yu and P. S. Russo, J. Polym. Sci.--Polym. Phys. Ed., 34, 1467-1475 (1996).

Diffusion coefficients of polyamidoamine cascade polymers (PAMAM's) were measured in aqueous solutions by dynamic light scattering and, after labeling with fluorescein isothiocyanate, by fluorescence photobleaching recovery. The dynamic light scattering results depended weakly on pH at a high salt concentration, but varied strongly with the concentration of added salt in the low-salt limit. The fluorescence photobleaching recovery values were almost independent of salt concentration. The difference between the two techniques is that thermodynamic nonideality strongly affects light scattering at the concentrations that are experimentally accessible. The hydrodynamic sizes from fluorescence photobleaching recovery were somewhat smaller than those from dynamic light scattering in the high-salt limit, despite attachment of the dye. Nevertheless, fluorescently tagged PAMAM’s should make suitable markers and diffusion probes.


Thermoreversible Gelation of a Rodlike Polymer

D. L. Tipton and P. S. Russo, Macromolecules, 29(23), 7402-7411 (1996)

Thermoreversible gelation is one of the most poorly understood aspects of polymer science. Random flight polymers, by virtue of their great flexibility, can easily form multifunctional crystalline or semicrystalline crosslinks to stabilize a gel network, so it is at least easy to visualize gelation. It is less clear how rodlike polymers can participate extensively in intermolecular crosslinks, and yet rodlike polymers often do gel.{96,326,412,279,376,303} This is not only a curiosity, but a matter of practical significance. During fiber and film fabrication, rodlike polymer solutions are often quenched into non-solvents, causing gelation and inhomogeneities that can adversely affect performance. Production of three-dimensional objects from rodlike polymers may be facilitated by polymer gelation, followed by solvent removal. A wide variety of gel structures are available, and the potential exists for new materials processed from them.

Good model systems for thermoreversible gelation are always hard to find. Lacking configurational entropy, rods are even more sensitive to poor solvent conditions than are other polymers. Since its behavior in good solvents is largely understood,{8,17,28,860,122,65,370,867,20,1,117,118,8,72,73,65,122, 860,281,261,858} the helical polypeptide, poly(g-benzyl-a,L-glutamate), PBLG, is a common choice for studying rodlike polymer gels.{941,326, 412, 364, 304, 303,859,317,861,433,869,287,750} In this paper, we use differential scanning calorimetry (DSC) and static and dynamic light scattering (SLS and DLS) to explore the wide range of behavior that can occur in PBLG/toluene


Self Diffusion of Rodlike Polymers in Isotropic Solutions

Z. Bu, P. S. Russo, D. L. Tipton and I. I. Negulescu, Macromolecules, 27, 6871-6882 (1994)

The self diffusion coefficient, Ds, of fluorescently tagged poly(g-benzyl-a,L-glutamate) has been measured throughout the isotropic regime in pyridine. Intrinsic viscosity, phase boundary studies, epifluorescence microscopy, computer modeling and the diffusion rates themselves show that the labeling did not appreciably perturb the semiflexible rod structure of the polymer. It is also demonstrated that pyridine is a good solvent for poly(g-benzyl-a,L-glutamate), comparable to N,N'-dimethylformamide. As concentration increased, two or three regimes were found depending on the rod length. Dilute behavior holds to number densities, n, well exceeding L-3, where L is the rod length. A somewhat better estimate of the point at which Ds begins to decrease is the classical criterion, c [h]-1 where c is the weight/volume concentration and [h] is the intrinsic viscosity. Over the whole range of molecular weights, the condition ndL2 0.5-1, where d is the rod diameter, well describes the downturn number density. This corresponds to n/n* 0.1-0.2, where n* =16/(pdL2) is the Onsager critical number density associated with the lyotropic liquid crystal transition. Once diffusion begins to decrease, it does so strongly, obeying Ds ~ c-1.13 0.04 L-1.8 0.2. The third regime of diffusion, evident only in the two longest samples, is again relatively level and lasts to the lyotropic transition. Diffusion does not cease at any concentration, and the ratio of Ds is reasonably well scaled when number density is normalized by n*, which underscores the importance of finite thickness effects for semidilute solutions of poly(g-benzyl-a,L-glutamate). The self and mutual friction coefficients differ, with the former being lower in dilute solutions.


Synthesis and Some Solution Properties of Poly(g-stearyl-a,L-glutamate)

D. S. Poche’, W. H.Daly and P. S. Russo, Macromolecules, 28, 6745-6753 (1995).

The synthesis of poly(g-stearyl-L-glutamate) is reported, starting from glutamic acid. The g-stearyl-L-glutamate amino acid (SLGAA) was prepared from stearyl alcohol and L-glutamic acid. Ring closure of SLGAA to the cyclic N-carboxyanhydride (NCA) monomer was accomplished using bis(trichloromethyl)carbonate (triphosgene). Polymerization of the SLGNCA monomer in the presence of either primary amines or sodium methoxide produced a series of polypeptides with molecular weights ranging from 20,000 to 300,000 g/mol. The polydisperse polymers were characterized by static and dynamic light scattering, solution viscometry, 1H and 13C NMR, and infrared spectroscopy. The polymer was also modeled using the SYBYL molecular graphics package. The combined data indicate a semiflexible structure, with PSLG sidechains being well extended from the polymer backbone in the good solvent, tetrahydrofuran. GPC and DLS data indicated polydispersity ratios (Mw/Mn) in the range of 1.1 - 2.0. The Mark-Houwink-Sakurada coefficients obtained were: K = 1.26 0.3x10-5 cm3/g and a = 1.29 0.09 for Mw in the range of 38,000-250,000. The hydrodynamic diameter of the PSLG rod was calculated to be 3.7 0.2 nm; the diameter of mutual exclusion calculated from the Zimm-Schulz-Onsager equation was 3.6 0.7 nm. These values were in good agreement with the value obtained from the SYBYL molecular model.


Rotational and Translational Diffusion of a Rodlike Virus in Random Coil Polymer Solutions.

R. Cush, P.S. Russo, Z. Kucukyavuz, Z. Bu, D. Neau, D. Shih, S. Kucukyavuz, H. Ricks, Macromolecules, 30, 4920-4926 (1997).

Depolarized dynamic light scattering was used to measure the translational and rotational diffusion of tobacco mosaic virus, TMV, in aqueous solutions of dextran (M ~ 505,000). TMV is an electrically charged, nucleoprotein assembly with the shape of a stiff, rigid rod. Dextran is an uncharged, flexible carbohydrate polymer. The TMV was held at a fixed, dilute concentration (0.5 mg/mL) while the concentration of dextran spanned both dilute and semidilute regimes (0 - 14.5% by weight). There was no evidence of phase separation or strong aggregation of the TMV particles in the presence of the dextran. The TMV particles dominated the depolarized scattering at all dextran concentrations. The angular variation of the decay rates of the autocorrelation functions always followed the form expected for symmetric top molecules in the absence of translational-rotational coupling. Nevertheless, translational and rotational motions are almost surely coupled in most dextran-containing solutions. The apparent translational and rotational diffusion rates decreased with added dextran, but not exactly according to the rise in macroscopic solution viscosity. A transition occurred at about 6.5 % dextran. Beyond this concentration, pronounced failures of the continuum (Stokes-Einstein) relation between diffusion and viscosity were found. Translational diffusion continued more rapidly than expected based on the macroscopic viscosity, while rotational diffusion fell sharply below expectation. The quotient Dr/Dt of rotational and translational diffusion, which presumably cancels effects due to viscosity, also dropped suddenly above the transition point. These findings are consistent with a sudden onset of topological constraints to rotational motion of the TMV, without onset of severe constraints to translational motion. Temperature dependent studies showed that either the solution or the solvent viscosity can describe translation and rotation fairly well, at least at concentrations below the transition. Energies of activation for translational and rotational diffusion of TMV were similar and not strongly dependent on dextran concentration in this regime.


Polarized and Depolarized Dynamic Light Scattering of a Rodlike Polyelectrolyte in a Strong Acid.

D. Sohn, P. S. Russo and D. B. Roitman, Ber. Bunsenges. Phys. Chem., 100(6), 821-888 (1996).

Solutions of poly(p-phenylene-cis-benzobisoxazole) in methane sulfonic acid, maintained in an anhydrous state using methane sulfonic acid anhydride, were studied by polarized and depolarized dynamic light scattering. The semiflexible chains are protonated by the strong acid, and the ionic strength is governed by the addition of sodium methane sulfonate. Despite a high specific refractive index increment, the scattering was very weak at low salt, since interactions among the unscreened polyelectrolyte chains lead to high osmotic stiffness. At high salt, the polarized scattering was characterized by a diffusive mode corresponding to translational motion; the decay rate of this mode met the expectation for a rodlike polymer in dilute solution. The diffusive mode vanished at low salt, without appearance of any new very slow mode as has often been reported for polyelectrolytes. At any salt concen- tration, there was a very rapid, angle-independent decay mode. In the depolarized scattering, only a broad, rapid decay mode was observed, and its average decay rate was reasonably insensitive to salt. This mode is too fast to represent rotational diffusion, and its rate appears to exceed slightly that of the fastest polarized decay mode. Rapid, angle-independent "plasmon" modes have been proposed for the polarized scattering from polyelectrolytes. We are not aware of other work reporting depolarized dynamic light scattering from a rodlike polyelectrolyte, nor does there seem to be theoretical guidance for this case.


Following Polymer Gelation by Depolarized Dynamic Light Scattering form Optically and Geometrically Anisotropic Latex Particles

B. C. Camins and P. S. Russo, Langmuir, 10, 4053-4059 (1994).

The formation of polyacrylamide gels has been followed by zero angle dynamic depolarized light scattering from colloidal poly(tetrafluoroethylene) probe particles. The scattering is dominated by the probes, since polyacrylamide does not strongly depolarized light. Before gelation, the zero angle depolarized intensity time correlation function g(2)* lies in the fully homodyne limit with a high optical coherence parameter *******. As gelation proceeds, * decreases and the correlation functions take on a wide spectrum of decay rates centered about a decreasing average value. The average decay rate does not decrease to zero, however. Assuming a mixed homodyne-hetrodyne model, the decrease in * was used to measure the population of "mobile" vs "frozen" colloidal latex particles. Despite the large measured volume of the zero angle experiment, only one "speckle" of scattered light is often observed after the gel is formed, so the measurements do not represent an average over all possible configurations. Obtaining a proper average at zero angles would be, at best, tedious. At finite scattering angles, ensemble averaged correlation functions for depolarized scattering were generated over many speckles by rotating the sample. This was preformed for "soft" and "hard" gels, which differed only in cross-linker content. Substantial differences were found both as regards the portion of particles retaining some mobility and their dynamics.


Dynamic Light Scattering from Ternary Solutions of Semiflexible

Polymers

Paul S. Russo

Dynamic light scattering has been used to study ternary polymer solutions of the type long semiflexible rod/short semiflexible rod/solvent. The semiflexible rods are helical poly(y-benzyl a-L-glutamate). The correlation functions are well fit by two exponentials. The slow mode, corresponding to diffusion of the dilute longer rod, decreases to as little as 20% of its dilute binary solution value as concentration of the shorter rods is raised. Although the details of the decrease are not thoroughly clear, its magnitude is comparable to that seen earlier in concentrated binary solutions of poly(y-benzyl a-L-glutamate), where a thermodynamic correlation is required (Russo, P. S.; Langley, K. H.; Karasz, F. E. J. Chem. Phys. 1984,80,5312), but much smaller than that reported recently in concentrated binary solutions poly(n-butyl isocyanate) in CCl, (Statman,D.; Chu, B. Macromolecules 1984, 17, 1537) after an entirely different analysis. These results are discussed in terms of rod flexibility.