SYNTHESIS AND STEREOCHEMISTRY OF OPTICALLY-ACTIVE BILIVERDIN CYCLIC ESTERS

(beta S,beta'S)-Dimethylmesobiliverdin-XIII alpha dimethyl ester exhibits a positive long wavelength circular dichroism Cotton effect (Delta epsilon(644)(max)=+6.1) in CHCl3, indicating an excess of the P-helical conformation of the pigment. But when the pigment's propionic acids are linked together by -(CH2)(n)- chains in cyclic esters, the choice of cyclic ester conformation and pigment helicity is governed by the number (n) of CH2 units, with Delta epsilon near 640 nm varying from positive when n=1, 5 and 6, to negative when n=2, 3 and 4.