Disorder–order transitions induced in anionic homopolypeptides by cationic detergents

CD spectra have been obtained for poly(L‐glutamic acid) and poly(L‐aspartic acid) as functions of temperature and concentration of cationic detergents. Dodecylammonium chloride induces a coil–helix transition in fully ionized poly(L‐glutamic acid). The interaction of the monomeric detergent with the polypeptide is responsible for the conformational transition. The detergent concentration required to produce the transition is independent of temperature. The CD of fully ionized poly(L‐aspartic acid) is nearly unaffected by dodecylammonium chloride, in marked contrast to the situation found with poly(L‐glutamic acid). However, these results do not imply that dodecylammonium chloride interacts differently with aspartyl and glutamyl residues. The observed results can be accounted for by the well‐known fact that the glutamyl residue has a higher helix‐forming tendency that the aspartyl residue. Cetyltrimethylammonium chloride destabilizes the helical form of poly(L‐glutamic acid). This detergent presents an exception to the usual ability of ionic detergents to promote formation of ordered structures in oppositely charged homopolypeptides.