Conformation of poly(L-glutamate) in cationic surfactant solutions with reference to binding behaviors

The conformational changes of sodium poly(L-glutamate) (P(Glu)) in surfactant solutions were measured by circular dichroism (CD) as a function of the degree of surfactant binding. The influences of both surfactant headgroup and chain length on the formation of the secondary structure of P(Glu) are investigated. The effect of headgroup is studied by comparing three kinds of cationic surfactants carrying a common dodecyl chain. The observed overall binding affinity is in the order of dodecylammonium chloride (DoA) > dodecylpyridinium chloride (DoP) > dodecyltrimethylammonium chloride(DoTA). Typical alpha-helix spectra of P(Glu) were found in DoA solutions, but no ordered structure of P(Glu) could be induced in DoP or DoTA solutions. Surfactant hydrophobicity is another parameter that controls the conformation of polypeptide, and it is studied by using n-alkylammonium chlorides with different chain length (C(n)A, n = 10, 12, 14). The conformational change of P(Glu) is proportional to the degree of binding in the solution of C(n)A with long alkyl chains but sigmoidal in the solution of short alkyl chains. The hydrophobic interaction couples with but is not necessarily compatible with the transition to the ordered conformation of the polypeptide.