Investigation on intermolecular forces between bile pigments and polar model compounds mimicking the chromophore - protein interactions in biliproteins

Abstract

A systematic investigation of intermolecular interactions of biliverdin-IXα-dimethyl ester and 2,18-bridged helically fixed verdinoid and rubinoid analogues with a variety of chiral compounds possessing a limited number of donor and/or acceptor sites was performed. To evaluate interaction strengths the concentration dependence of the induced chiral discrimination between M and P helical species as detected by CD was used. Biliverdin esters show pronounced association only with compounds exhibiting strong hydrogen bonding donor properties. In particular, if the donor of the ligand is provided by a carboxylic acid group defined 1 : 1 complexes are formed but no protonation of the tetrapyrrole backbone takes place. 2,18-bridged helical bilirubins - being monomeric under the conditions employed - behave similarly but interact with acceptors, too. Association constants were determined by Scatchard plot analysis. The quantitative data gained allow to map the non-covalent, polar binding properties of helical biliverdins and bilirubins. The implications of results for the conformation determining interactions in biliverdin peptides and biliproteins are discussed.