CONFORMATIONAL TRANSITION OF INSULIN-INDUCED BY N-ALKYLTRIMETHYLAMMONIUM BROMIDES IN AQUEOUS-SOLUTION

A surfactant-induced conformational transition of bovine insulin has b een detected by difference spectroscopy for a homologous series of n-a lkyltrimethylammonium bromides, chain length C-10-C-16 at pH 10.0, 25 degrees C. The transition was followed as a function of surfactant con centration by absorbance measurements at 275 nm and the data were anal ysed to obtain the Gibbs energy of the transition in water (Delta G de grees(w)) and in a hydrophobic environment (Delta G degrees(hc)) for s aturated protein-surfactant complexes. A value of Delta G degrees(w) o f -11.8 +/- 1.8 kJ mol(-1) was found independent of n-alkyl chain leng th, which is similar to the value found for the n-alkylsulfate-induced transition in a previous study (-14.6 +/- 3.0 kJ mol(-1)). The values of Delta G degrees(hc) were in the range similar to -88 to -100 kJ mo l(-1) for chain lengths from C-10 to C-16. The values of Delta G degre es(hc) vs. chain length for both the n-alkyltrimethylammonium bromides and the n-alkylsulfates lie on the same curve, demonstrating that Del ta G degrees(hc) is independent of the nature of the surfactant head g roup.