PHOTO-CIDNP OF AMINO-ACIDS AND PROTEINS - EFFECTS OF COMPETITION FOR FLAVIN TRIPLETS

The photo-CIDNP intensities of amino acid residues in proteins depend on the competition of the various exposed aromatic sidechains for phot o-excited flavin triplets. The effects of this process on CIDNP enhanc ements are investigated using mixtures of the N-acetyl derivatives of histidine, tryptophan and tyrosine. Measurements for binary mixtures o f the three amino acids are used to extract values for the relative ra tes of formation of radical pairs from triplet flavin mononucleotide ( FMN). The concentration dependence of the CIDNP intensities of the thr ee amino acids is interpreted by including the competition between deg enerate hydrogen atom exchange and nuclear spin lattice relaxation in the free radicals derived from the amino acids. Finally, short peptide s containing both tryptophan and tyrosine are investigated, to monitor possible proximity effects.