SYNTHESIS AND STEREOCHEMISTRY OF OPTICALLY-ACTIVE BILIVERDIN CYCLIC ESTERS

(beta S,beta'S)-Dimethylmesobiliverdin-XIII alpha dimethyl ester exhib its a positive long wavelength circular dichroism Cotton effect (Delta epsilon(644)(max)=+6.1) in CHCl3, indicating an excess of the P-helic al 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 p ositive when n=1, 5 and 6, to negative when n=2, 3 and 4.