Inverse Raman Spectroscopy of Bilirubin and its Ditauride

The inverse Raman spectrum of bilirubin and bilirubin ditauride were measured under pre-resonant conditions. Spectra in chloroform and dimethyl sulfoxide were assigned by comparison with model compounds and on the basis of the known effects of the solvents on the internal hydrogen bonding in bilirubin. The band assignments were used to interpret the spectra of bilirubin ditauride in aqueous solution. Evidence is presented for strong hydrogen bonding to the pyrrole N-H bonds and to the lactam rings. It is proposed that hydrogen bonding to water is facilitated by photoactivation and, in turn, stabilizes photohiliruhin.