Formation of specific dipolar microenvironments complementary to dipolar betaine dye by nonionic peptide lipids in nonpolar medium
作者:Hiroshi Hachisako、Naoya Ryu、Hiromi Hashimoto、Ryoichi Murakami
DOI:10.1039/b818218c
日期:——
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-Hδ+ and COδ− attached to both sides of the Gly residue, respectively, i.e., the O− and N+ of ET(30) complementarily bound to N-Hδ+ and COδ− 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-Hδ+ and COδ− attached to both sides of the Gly residue, respectively, because introducing steric hindrance into the Gly moiety using several L-α-amino acids with bulky α-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.
本文描述了非离子肽脂与非极性非质子有机溶剂中的溶剂化变色二甲基染料之间的主客体相互作用。我们偶然发现,在氯苯中,当存在具有苄氧羰基化Gly头部基团的L-谷氨酸衍生肽脂1时,Reichardt染料(以下简称ET(30),尽管ET(30)曾被用作极性参数)的颜色异常蓝移。由于广泛认为ET(30)表现出负溶剂化变色性,即随着溶剂极性增加,这种染料的可见吸收带蓝移,因此蓝移表明,在无聚集条件下,ET(30)接触到由肽脂1产生的更极性的微环境,而不是氯苯。结合位点被假设为分别附着在Gly残基两侧的N-Hδ+和COδ-,即ET(30)的O-和N+分别通过氢键和离子-偶极相互作用与N-Hδ+和COδ-互补结合。由于ET(30)实际上是非荧光的,无法使用荧光光谱法(这是研究主客体相互作用的有力方法)来指定ET(30)的结合模式。因此,采用了一种合成方法(尽管非常费力但可靠),并与可见吸收光谱的溶剂化变色探针相结合,以指定肽脂1上的结合位点。结合位点已被发现位于两个偶极上,即分别附着在Gly残基两侧的N-Hδ+和COδ-,因为使用具有庞大α取代基的几种L-α-氨基酸引入Gly部分的立体阻碍干扰了ET(30)的结合。通过用肌氨酸(Sar)替换脂1的Gly残基,观察到ET(30)的类似特定结合行为。发现肽脂的自组装对于有效捕获ET(30)是必要的。还研究了形成与二甲基染料互补的特定极性微环境的肽脂的分子结构要求。