PHILICITY OF AMINO-ACID SIDE-CHAINS FOR PHOTOGENERATED CARBENES

The selectivity of a diazirine-photogenerated carbene towards amino acid side-chains was investigated by analysing amino acid retention following photocoupling with an immobilized carbene precursor. The heterobifunctional photocross-linker 3-(trifluoromethyl)-3-(m-isothiocyanophenyl)diazirine was synthesized and coupled to fibre glass. Photoinduced amino acid binding to the solid support was analysed. The immobilized diazirine-photogenerated carbene preferentially binds to cysteine and aromatic amino acids. Amino acids carrying sulphur or oxygen as side-chain heteroatoms are, in general, more carbene-philic than amino acids with aliphatic side-chains. Marginal carbene insertion is obtained with glycine. On the basis of the empirically determined photocoupling capacities of the applied amino acids, a carbene philicity scale has been established. For homologous amino acids, carbene selectivity partly correlates with their hydrophobicity and the number of chemical bonds. Consequences of this distinct binding capacity are discussed with respect to photoselective protein modification. © 1990.