Aggregation, hydrogen bonding and thermodynamic studies on tetrapeptide micelles

Abstract

The temperature dependence of critical micelle concentration (c.m.c.), aggregation number, standard Gibbs energy change (Δ_mG°), enthalpy change (Δ_mH°) and entropy change (Δ_mS°) of micelle formation, and proton chemical shifts of the TFA · Tyr-Gly-Phe-Ala-OBz peptide (TFA is trifluoroacetic acid) have been presented in the light of NH⋯OC hydrogen-bond formation and forces implicated in micellization. It has been found that the c.m.c. of the tetrapeptide increases with increase in temperature from 20 to 40 °C, whereas it decreases at temperatures above 40 °C. Moreover, the temperature dependence of the aggregation number, proton chemical shifts and thermodynamic results suggest that intramolecular H bonds are formed at temperatures in the range 20–40 °C (secondary level of structure) owing to hindrance of micelle formation, whereas above 40 °C the intramolecular H bonds of the peptide are broken as micellization increases which demonstrates a tertiary level of structure. It has also been found that the temperature of 40 °C appears very significant in that it is close to the temperature at which both the enthalpy and entropy of micellization change sign, which presumably relates to the opposing temperature dependences of hydrophobic and electrostatic interactions, the inverse temperature dependence of hydrophobic interactions, reflecting changes in water structure.