synthesis, spectroscopic studies and biological activity of stereochemically constrained enkephalin analogs containing alpha- aminoisbutyric acid

Abstract

The discovery of endogenous opioids has stimulated considerable interest in the development of structure-activity correlations for enkephalins (Tyr-Gly-Gly-Phe-Met/Leu). However, the flexibility introduced by the Gly-Gly segment in the molecule made such an attempt difficult. This thesis presents results of an investigation aimed at establishing a correlation between in vivo biological activity and conformation, by synthesizing stereochemically constrained analogs. Aib residues, which are known to favor conformation, have been introduced instead, and a good correlation could be drawn between conformation (studied by ¹H NMR) and in vivo biological activity (as measured by their induced behavioral effects in mice).

Enkephalin analogs of the type (Tyr-Aib-Phe-Met/Leu) or (Tyr-Phe-Aib-Met/Leu, with dansyl group) have been studied in mice using a behavioral assay. Aib², Aib²-Aib³ analogs are shown to possess high biological activity, whereas some other analogs are found to be less active.

In the course of this study, two novel enkephalin analogs (Aib-Aib-Phe-Met-NH and Phe-Aib-Phe-Met-NH ) have been developed, which act differently in the presence of the narcotic antagonist naloxone.

Extensive conformational studies carried out with various enkephalin analogs and related peptides revealed that analogs with a single -turn centered around residues 3 and 4 are substantially active. Highly folded helical analogs having consecutive -turns centered around residues 2-3 and 5-4 are also very active.

Fluorescence studies with dansyl enkephalin analogs indicated their utility in the study of conformation and interaction of these peptides with - and -opioid receptors and membrane receptors.