The transition of α-helix to β-structure of poly( d-lysine) induced by phosphatidic acid vesicles and its kinetics at alkaline pH

Abstract

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 α-helix of poly( l-lysine) induced by deprotonation in alkaline solutions is transformed to β-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 α-helix to random coil. Previously, we have reported the conformational change of poly( l-lysine) induced by DLPA vesicles at neutral pH, where the β-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 β-structure and from α-helix to β-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.