The transition of alpha-helix to beta-structure of poly(L-lysine) induced by phosphatidic acid vesicles and its kinetics at alkaline pH.

Static and dynamic circular dichroism (CD) measurements were carried out for poly(L-lysine) in suspensions of dilauroylphosphatidic acid (DLPA) vesicles at alkaline pH (8-11.5). The static experiments demonstrated that the alpha-helix of poly(L-lysine) induced by deprotonation in alkaline solutions is transformed to beta-structure by the addition of DLPA vesicles. Stopped-flow CD measurements for such order-to-order transition revealed that the rate determining step is the unfolding process of alpha-helix to random coil. Previously, we have reported the conformational change of poly(L-lysine) induced by DLPA vesicles at neutral pH, where the beta-structure transition from random coil was observed. Thus two types of transition of poly(L-lysine) are observed depending on bulk pH, i.e., from random coil to beta-structure and from alpha-helix to beta-structure. So far the phospholipid-induced conformations of poly(L-lysine) were interpreted in terms of counterbalance between the positively charged terminals of the lysyl chains and the negative headgroups of the phospholipid in vesicle. However, present work indicates the direct interaction other than electrostatic interaction between the lysyl chain and phosphate groups of the lipid.