Change in mobility of side chains due to neutralization of charged poly(L‐lysine) with dansyl group

Abstract

AbstractPoly(L‐lysine) having dansyl (5‐dimethylamino‐1‐naphthalene‐sulfonyl) groups to its side chains was prepared. The fluorescence spectra and fluorescence anisotropy ratios of the dansyl (DNS) group were measured in various conditions. In aqueous solution the increase in emission intensity was observed reflecting the alkali‐induced coil‐to‐helix transition. In aqueous‐methanolic solutions with methanol content above 60 wt %, the poly(L‐lysine) with DNS group (DNS‐PLL) was probed to show α‐helical conformation from CD spectra. With addition of alkali, the increase in fluorescence intensity of α‐helical DNS‐PLL and the drastic change in fluorescence anisotropy ratio were observed. In this case the rotational mobility of DNS probe decreases, gives a minimum at a certain concentration of added alkali, and then increases again up to approximately the initial level. At the concentration where the rotational mobility gives the minimum, intensity of scattered light gives a maximum. This shows that suppression of the mobility of DNS side chains is caused by the intermolecular aggregation of α‐helical DNS‐PLL. This concentration of added alkali corresponds to the midpoint of neutralization to charged side chains of the DNS‐PLL. The interaction that causes aggregate of α‐helical DNS‐PLL is suggested to be the intermolecular hydrogen bonding between neutralized and unneutralized side chains. © 1994 John Wiley & Sons, Inc.