Novel saccharide-induced conformational changes in a boronic acid-appended poly(L-lysine) as detected by circular dichroism and fluorescence

Abstract

Poly(L-lysine) has been modified with a 4-phenylboronic acid derivative which acts as a sugar-binding site and a dansyl group which acts as a fluorescence probing site. The helix content of this poly(L-lysine) derivative as estimated by CD spectroscopy became maximum at pH 10.3. The helix content was increased from 51 to 79% by complexation of D-fructose with the pendant boronic acid group and the maximum pH shifted from 10.3 to 7.9. Examination of the CD spectra revealed that the conformation changes in the order of β-sheet → α-helix → random coil with increasing medium pH. The D-glucose addition induced a similar increase in the helix content and a similar low pH shift of the helix content maximum but the mechanism was somewhat different: one D-glucose was bound to two boronic acid groups to form an intrapolymeric bridge and the resultant CD spectrum was similar to that of the β-turn structure. These saccharide-induced conformational changes were well reflected by plots of fluorescence intensity vs. pH. The decrease in the fluorescence intensity in the high pH region (random coil region) was confirmed by fluorescence polarization to be due to the increase in the molecular motion. This is a novel system to control the poly(amino acid) conformation by saccharides and to detect the conformational changes by convenient spectroscopic methods.