ON THE STRUCTURE OF BILIRUBIN IN SOLUTION - C-13(H-1) HETERONUCLEAR OVERHAUSER EFFECT NMR ANALYSES IN AQUEOUS BUFFER AND ORGANIC-SOLVENTS

Select hydrogen-carbon distances have been determined from C-13(H-1) h eteronuclear Overhauser effects observed in a 99% carbon-13 enriched ( CO2H)-C-13 bilirubin analog, [8(3),12(3)-C-13(2)]-mesobilirubin XIII a lpha. Analysis of the data confirms that the propionic acid carbonyl l ies within hydrogen bonding distance to the dipyrrinone lactam and pyr role N-H groups in chloroform and indicates, surprisingly, that those distances are only slightly longer in dimethyl sulfoxide solvent or wh en the carboxyl group is ionized in pH 7.4 aqueous buffered solutions of the pigment. The data supports the presence and persistence of fold ed, intramolecularly hydrogen-bonded bilirubin conformations in soluti on.