Structure of aggregates and helix–coil transition of polypeptides by dielectric measurements. I. Poly‐γ‐benzyl‐L‐glutamate in dioxane and dioxane–dichloroacetic acid

Abstract

SynopsisDielectric dispersion measurements on poly-y-benzyl-L-glutamate (PBLG) in dioxane and dioxane-dichloroacetic acid (DCA) mixtures in the frequency range 203 Hz-2 MHz were made in order to study the structure of molecular aggregates. The structure of aggregates is explained on the basis of the variation of dipole moment and relaxation time with degree of aggregation. PBLG was found to form linear head-to-tail-type aggregates in dioxane. These aggregates gradually reduce in size without loosing their a-helical structure during the process of disaggregation obtained by either adding DCA to the solution in dioxane or by heating. J t was confirmed that the addition of 30 wt ' % DCA completely destroys the aggregation of PBLG in dioxane at 30°C. Thermal disaggregation, however, was not complete even at a temperature approaching the boiling point of the solvent. A reaction scheme for aggregation is proposed and equilibrium constants are calculated a t various stages of aggregation. The enthalpy of aggregate formation is found to be -3 kcal/mol. Results of optical rotatory dispersion measurements of the helixcoil transition in this system are also presented.

Dielectric Properties of Polypeptides in Solution. II. Aggregation of Poly(γ-benzyl L-glutamate) in Various Helicogenic Solvents

“…In all the cases, the solute concentration was 0.36±0.09 (mg/ml). D, the value calculated by eq 1, using the value of the dielectric increment shown in Figure 1 of the article by Gupta, et al 4 in chloroform-DMF mixtures were decidedly smaller than those in EDC-DMF mixtures indicating the degradation of the molecule by the presence of phosgene created from chloroform. Since the dipole moment per peptide residue obtained by Gupta, et al, in the chloroform-formamide mixtures is about the same as that obtained by the present author in chloroform-DMF, the mixtures of chloroform-formamide should be inadequate as aggegation-free solvents, and the dipole moment in dioxane must be compared with that in EDC-DMF or EDCm-cresol.…”

“…9 The conclusion of Gupta. et al,4 that the aggegate formed in the mixtures of dioxane and DCA would be of the HT type does not seem correct. However, the dipole moment per peptide residue in pure dioxane, which is calculated to be 2.5 D/residue by the BAM equation from the data shown in Figure 1 of their article, is in good agreement with the present result, as shown in Figure 2.…”

“…Watanabe 2 and Powers and Peticolas 3 have used electric birefringence to show that at least the aggregate of SS type appears. On the contrary, Gupta,et al,4 maintain that only the HT type is formed. The discrepancy among these investigators remains to be solved.…”

Physical Properties of Liquid Crystalline Solution of Once-Broken Rod Poly(γ-benzyl L-glutamate)

“…Differences in V 2 (v/v) at A points among the two solvents are obvious. S-PBLG molecules associate themselves in helicogenic solvents [17][18][19][20][21] and the type and extent of molecular association depend on the nature of solvent and concentration of polymer.…”

Dipole–dipole interaction and the relative stability of different types of aggregates of helical polypeptides

“…6 The fact that the enthalpy of aggregation thus obtained, A H -3 kcal/mol, corresponds typically to the attractive interaction between two H T T point dipoles of 3 Debye, roughly 5 A apart, has not been recognized. It suggests however that dipole-dipole interaction might be truly the leading term in the interaction energy.…”