Physicochemical characterization of a novel surfactant peptide containing an arginine cation and laurate anion

Abstract

A novel surfactant peptide consisting of an arginine cation with laurate anion has been synthesized, purified and characterized. The critical micellar concentration (cmc) of peptide in aqueous solutions has been determined using spectroscopic techniques and is found to increase from 0.06 to 0.11 mM with increasing temperature (15–45°C). Cmc is also determined in the presence of salts like NaCl, KCl and sodium acetate and it is found that these electrolytes hinder aggregation with a significant increase in the case of sodium acetate. The aggregation number of the surfactant peptide has been determined using fluorescence quenching measurements and is observed to decrease from 14 to 6 with increasing temperature (15–45°C). The standard free energy change (ΔG ⁰_m ) and standard enthalpy change (ΔH ⁰_m ) of the peptide aggregate are found to be negative with a small positive value for standard entropy change (ΔS ⁰_m ). The peptide aggregate seems to undergo phase transition above 50°C as observed from UV-vis and fluorescence spectroscopy. From pyrene binding studies, it is shown that the interior dielectric constant increases from 5.08 at 34°C to 8.77 at 50°C and further decreases with increase in temperature indicating a phase change at 50°C. Also, the ratio of excimer intensity to monomer intensity, which is a measure of microviscosity of the aggregate, decreases with increase in temperature with a change at 50°C indicating a phase change.