Tyrosine photo reaction with

Biochemical applications have been made more recently O, ). Here, cyclic photochemical reactions are employed to generate nuclear spin-polarization in biological macromolecules. The information obtained is of a structural nature rather than mechanistic. In the case of proteins aromatic amino acid residues (tyrosine, histidine and tryptophan) can be polarized by reversible hydrogen atom or electron transfer reactions with a photo-excited dye. These reactions require direct contact of the dye with the amino acid side-chains so that they only occur for residues lying at the surface of the protein. 

Nuclear spin polarization can be induced in a variety of biologically important molecules by laser irradiation of solutions in the presence of a dye. Initially we had found that using flavin dyes it is possible to polarize the amino acids tyrosine, histidine and tryptophan, both as free amino acids and as surface residues in proteins (, O. This occurs in cyclic reactions with a high degree of reversibility of the type discussed in the previous section. More recently, we found that the purine nucleotide bases can be polarized similarly (16). It appears that flavins are remarkable in that upon photo-excitation they react reversibly with several classes of compounds such as aromatic phenols, aromatic amines, and secondary and tertiary amines. In all these cases CIDNP can be observed. We shall now discuss the photo-CIDNP spectra of the amino acids in more detail. 

Deeble DJ, Schuchmann MN, Steenken S, von Sonntag C (1990) Direct evidence for the formation of thymine radical cations from the reaction of SO/" with thymine derivatives a pulse radiolysis study with optical and conductance detection. J Phys Chem 94 8186-8192 DeFelippis MR, Murthy CP, Faraggi M, Klapper MH (1989) Pulse radiolytic measurement of redox potentials the tyrosine and tryptophan radicals. Biochemistry 28 4847-4853 Delatour T, Douki T, D Ham C, Cadet J (1998) Photosensitization of thymine nucleobase by benzo-phenone through energy transfer, hydrogen abstraction and one-electron oxidation. J Photo-chem Photobiol 44 191-198... 

Photo induced interconversion of the configuration at an asymmetric C atom, has not been reported. For tyrosine it has been excluded even under high-intensity laser irradiation [66]. This chiral unit seems to be very stable. A bicyclic ketone system with (formal) double epimerization is described by Zandomeneghi et al. [67] (Eq. 33). This reaction proceeds, however, by a complex rearrangement mechanism and will be discussed in Sec. II.B. 

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