INVESTIGATION ON INTERMOLECULAR FORCES BETWEEN BILE-PIGMENTS AND POLAR MODEL COMPOUNDS MIMICKING THE CHROMOPHORE - PROTEIN INTERACTIONS IN BILIPROTEINS

A systematic investigation of intermolecular interactions of biliverdi n-IXalpha-dimethyl ester and 2,18-bridged helically fixed verdinoid an d rubinoid analogues with a variety of chiral compounds possessing a l imited number of donor and/or acceptor sites was performed. To evaluat e interaction strengths the concentration dependence of the induced ch iral discrimination between M and P helical species as detected by CD was used. Biliverdin esters show pronounced association only with comp ounds exhibiting strong hydrogen bonding donor properties. In particul ar, if the donor of the ligand is provided by a carboxylic acid group defined 1 : 1 complexes are formed but no protonation of the tetrapyrr ole backbone takes place. 2,18-bridged helical bilirubins - being mono meric under the conditions employed - behave similarly but interact wi th acceptors, too. Association constants were determined by Scatchard plot analysis. The quantitative data gained allow to map the non-coval ent, polar binding properties of helical biliverdins and bilirubins. T he implications of results for the conformation determining interactio ns in biliverdin peptides and biliproteins are discussed.