Angelo Perico

Positional time correlation function for one‐dimensional systems with barrier crossing: Memory function corrections to the optimized Rouse–Zimm approximation

The one‐variable Smoluchowski equation is used to study the influence of barrier crossing processes on the positional time correlation function. The memory function of this correlation function is evaluated for a 2–4 potential as a function of the barrier height using the Mori continued fraction expansion and an equivalent but more efficient matrix formulation. Higher orders in the expansions are required to obtain numerical convergence as the barrier height increases. An exact integral solution for the correlation time is derived and is compared with the approximations. A biexponential approximation, which describes the independent motion in a potential well and the transition between wells, is found to be very accurate for high barriers. Numerical simulations provide checks on the approximations to the correlation function for a barrier height of 2 kBT. The possibility of including the influence of more rapid barrier crossing processes into the many variable Smoluchowski description of long time polymer...

Intramolecular excimers and microbrownian motion of flexible polymer molecules in solution

Abstract The fluorescence emission of pyrene end-labelled polyethyleneoxide samples ( M n = 0.16 · 10 4 –1.95 · 10 4 ) has been studied in dilute solution at 20°. Intramolecular “excimer” fluorescence of increasing intensity is observed with decreasing molecular weight. The influence of the molecular weight on the relative quantum yield of the “monomeric” pyrene groups has been analyzed according to the general kinetic treatment of diffusion controlled quenching processes.

Viscoelastic relaxation of segment orientation in dilute polymer solutions

Time autocorrelation and memory functions for segment orientation are derived for a general diffusion model of a linear polymer chain in solution. Exact analytical results for the orientation and alignment memories P1(t), P2(t) (averages of the first and second order Legendre polynomial of the cosine of the angle of rotation of a segment) are obtained as a function of the time autocorrelation function M1(t) of the segment vector. These expressions significantly depart from the results for the diffusional rotation of a sphere: P1=M1, P2=M31.

N‐terminal truncated pyroglutamyl β amyloid peptide Aβpy3‐42 shows a faster aggregation kinetics than the full‐length Aβ1‐42

We tested directly the differences in the aggregation kinetics of three important β amyloid peptides, the full‐length Aβ1‐42, and the two N‐terminal truncated and pyroglutamil modified Aβpy3‐42 and Aβpy11‐42 found in different relative concentrations in the brains in normal aging and in Alzheimer disease. By following the circular dichroism signal and the ThT fluorescence of the solution in phosphate buffer, we found substantially faster aggregation kinetics for Aβpy3‐42. This behavior is due to the particular sequence of this peptide, which is also responsible for the specific oligomeric aggregation states, found by TEM, during the fibrillization process, which are very different from those of Aβ1‐42, more prone to fibril formation. In addition, Aβpy3‐42 is found here to have an inhibitory effect on Aβ1‐42 fibrillogenesis, coherently with its known greater infective power. This is an indication of the important role of this peptide in the aggregation process of β‐peptides in Alzheimer disease.

Dynamics of chain molecules. I. Solutions to the hydrodynamic equation and intrinsic viscosity

An exact numerical solution to the equation of motion for a flexible macromolecule in solution is given for the bead and spring model incorporating the preaveraged Oseen hydrodynamic interaction. The limits to be imposed on the strength of the interaction are confirmed and discussed. The data obtained for the eigenvalues λK, the eigenvectors QjK, and the quantities μK show that the hydrodynamic interaction affects the relaxation times and the shapes, the mean square length of the modes remaining practically unchanged with respect to the free‐draining results. By comparison with the intrinsic viscosity experimental data, the values of r/b (hydrodynamic interaction strength) and b (mean dimension of the subchain) may be obtained. The method has been applied to PS and PDMS. Defined and reasonable values of r/b and b have been obtained. The range of r/b of major interest (0.2–0.3) gives small draining effects over a large molecular weight range; this makes the intrinsic viscosity depend approximately on M0.5,...

Intramolecular excimer formation in polymers: Pyrene Labelled Polyvinylacetate

Abstract The rate constant for intramolecular excimer formation between pyrenyl side-groups, in a polyvinylacetate chain at a mean separation of 200 bonds, has been measured as a function of molecular weight, solvent viscosity and solvent thermodynamic power. Above M = 1 × 10 5 , the rate constant is 1.4 × 10 7 sec −1 in low viscosity bad solvents. This value is about twenty times that for the rate constant of the analogous reaction between the two terminal groups in a chain with a mean end-to-end separation of 200 bonds. Increases of the viscosity and of the thermodynamic power of the solvent depress the rate constant, in agreement with the behaviour expected for a diffusion controlled reaction.

The Supramolecular Association of Polyelectrolytes to Complementary Charged Surfactants and Protein Assemblies

Cohesion matters! The correlation between the conformational rigidity of the polyelectrolyte and the size and stability of the globular assembly is discussed in this review article. Some examples of models for the association of polyelectrolytes to globular assemblies are shown here.Supramolecular complexes of strong polyelectrolytes and oppositely charged ionic micelles or protein assemblies derive their main stabilization from electrostatic interactions that include the counterion condensation/release mechanism and from hydrophobic interactions distributed along apolar sections of the components. The predicted and the experimental behavior of selected complexes differing in the flexibility of the polyelectrolyte and in the cohesion of the complementary assembly is reviewed and analyzed. Depending upon the rigidity of the polyelectrolyte, globular surfactant clusters persist in the final structure or are transformed into elongated assemblies. On the other hand, even the rather rigid DNA molecule is forced to bend by strongly associated cationic protein complexes such as the histone octamers. A general framework should allow the prediction of these structures in terms of the interplay between the persistence length of the polyelectrolyte and the association constant of the protein or the surfactant assembly.

Polypeptide dynamics: Experimental tests of an optimized Rouse–Zimm type model

A theory for long time random coil peptide dynamics is developed based on a generalization of the optimized Rouse–Zimm model of Perico et al. [J. Chem. Phys. 87, 3677 (1987)] and Perico [J. Chem. Phys. 88, 3996 (1988) and Biopolymers 28, 1527 (1989)]. The generalized model employs the rotational potential energy for specific amino acid residues and amino acid friction coefficients to compute all input parameters in the model. Calculations of the fluorescence depolarization correlation function P2(t ) and of the local persistence length are found to be sensitive to the amino acid sequence, the length of the polypeptide chain, and the location of the probe. Model computations of P2(t ) are compared with new experimentally determined rotational correlation times (of the order of nanoseconds) from fluorescence depolarization measurements of three different synthetic 17‐residue peptides, each containing a single tryptophan (TRP) residue as a probe. In addition, the previous anisotropy measurements on ACTH, glu...

Picosecond fluorescence studies of polypeptide dynamics: fluorescence anisotropies and lifetimes

Abstract Fluorescence lifetimes and anisotropies for single tryptophan-containing polypeptide hormones ACTH and glucagon, and a series of their fragments are reported. The anisotropy data are discussed in the context of the theory of Perico and Guenza (J. Chem. Phys. 84 (1986) 510). A persistence length of about 7 to 10 residues is obtained for the mobilities in the two hormones. The theory is able to account for chain length and probe location effects, but the calculated time dependence of the anisotropy does not fit the experimental curve well at short times.

Dynamics of chain molecules. II. Spectral distribution of the light scattered from flexible macromolecules

The separate contributions to the spectral density of the light scattered from a flexible macromolecule in solution have been calculated on the basis of the bead and spring model with hydrodynamic interaction. The interaction enhances the effect of the intramolecular motions with respect to the translational diffusion. A hypothetical homodyne spectrum has been calculated for a chain length of 1500 A, and the resulting linewidth and height are larger than in the free‐draining case.