Complete chiral discrimination in bilipeptides: the chiroptical properties of the bilatriene helix

Abstract

Biliverdin oligopeptides have been prepared with the aim of complete chiral discrimination of the conformationally labile bilatriene helix. Evidence is given that appropriately constituted biliverdin–tripeptides and -tetrapeptides exhibit diastereoisomeric homogenity in solution. To ascertain the homochirality of the bilatriene helices present in a bilipeptide two independent approaches were used: invariability of the chiroptical properties towards (i) successive peptide chain lengthening and towards (ii) the polarity of the solvent. The c.d. parameters of the inherently chiral bilatriene (P)-helix are thus evaluated to be Δε+ 100 to + 110 and Δε–130 to –150 for the red and blue band, respectively. These parameters belong to a bilatriene helix whose torsional angles closely resemble those of biliverdin dimethyl ester.