On the structure of bilirubin in solution. 13C[1H] heteronuclear Overhauser effect NMR analyses in aqueous buffer and organic solvents.

Select hydrogen-carbon distances have been determined from 13C[1H] heteronuclear Overhauser effects observed in a 99% carbon-13 enriched 13CO2H bilirubin analog, [8(3), 12(3)-13C2]-mesobilirubin XIII alpha. Analysis of the data confirms that the propionic acid carbonyl lies within hydrogen bonding distance to the dipyrrinone lactam and pyrrole N-H groups in chloroform and indicates, surprisingly, that those distances are only slightly longer in dimethyl sulfoxide solvent or when the carboxyl group is ionized in pH 7.4 aqueous buffered solutions of the pigment. The data supports the presence and persistence of folded, intramolecularly hydrogen-bonded bilirubin conformations in solution.