Formation of specific dipolar microenvironments complementary to dipolar betaine dye by nonionic peptide lipids in nonpolar medium

Abstract

This paper describes the host-guest interaction between nonionic peptide lipids and solvatochromic dipolar betaine dyes in nonpolar aprotic organic solvent. We have serendipitously found that the colour of Reichardt's Dye (referred to as ET(30) hereafter, although the term ET(30) has been used as a polarity parameter) in chlorobenzene unusually blue-shifted in the presence of L-glutamic acid-derived peptide lipid 1 with a benzyloxycarbonylated Gly headgroup. Since it is widely accepted that ET(30) shows negative solvatochromism, i.e., the visible absorption band of this dye blue-shifts as the solvent polarity increases, the blue-shift indicates that ET(30) was in contact with the more polar microenvironment produced by the peptide lipid 1 rather than chlorobenzene under aggregate-free conditions. The binding site was assumed to be N-H(delta+) and CO(delta-) attached to both sides of the Gly residue, respectively, i.e., the O- and N+ of ET(30) complementarily bound to N-H(delta+) and CO(delta-) through hydrogen bonding and ion-dipole interaction, respectively. Since ET(30) is practically non-fluorescent, it was not feasible to use fluorescence spectrometry, which is a powerful method for the study of host-guest interactions, in order to specify the binding mode of ET(30). Therefore, a synthetic approach, although very laborious but reliable, has been used in conjunction with solvatochromic probing using visible absorption spectroscopy to specify the binding site on peptide lipid 1. The binding site has been found to be located on two dipoles, i.e., N-H(delta+) and CO(delta-) attached to both sides of the Gly residue, respectively, because introducing steric hindrance into the Gly moiety using several L-alpha-amino acids with bulky -substituents interfered with the binding of ET(30). Similar specific binding behaviour of ET(30) was observed by replacing the Gly residue of the lipid 1 with sarcosine (Sar). It was found that self-assembly of the peptide lipid was necessary for effective capture of ET(30). The molecular structural requirements of the peptide lipids that form such specific polar microenvironments complementary to dipolar betaine dyes have also been investigated.