FLAVINYL PEPTIDES .2. INTRAMOLECULAR INTERACTIONS IN FLAVINYL AROMATIC AMINO ACID PEPTIDES

Close association of flavin coenzymes with aromatic amino acid residues as may occur in flavoproteins was simulated with a model system. L-Tryptophan, L-tyrosine, and L-phenylalanine were attached by peptide linkage to N-3 and N-10 ω-carboxyalkylflavins. The dependence of spectral and fluorescence properties of the flavin upon variation of the chain length separating the two chromophores was investigated. Fluorescence quenching due to collisions in the excited state, but especially that due to ground-state complex formation, increases as the chain length of the 10-substituted flavinyltryptophan and -tyrosine peptides decreases from 5 to 1 methylene groups. Hydrophobic forces are undoubtedly involved in complex formation in aqueous solution, but dispersion forces are also effective, since significant fluorescence quenching occurs in organic solvents. The flavinyltyrosine peptides exhibit intramolecular hydrogen bonding between the tyrosine hydroxyl and the 4-carbonyl group of the flavin in chloroform, but this is not the most important force in maintaining interaction in aqueous solution. Flavinyltryptophan and ω-tyrosine peptides (n = 1) in position 10 of the flavin have fluorescence properties which differ from those of the corresponding peptides substituted in position 3. This indicates that orientation of the complexing moieties influences the extent and type of quenching of the flavin fluorescence. Short-chain flavinyl peptides exhibit a degree of fluorescence quenching of sufficient magnitude to account for similar changes observed upon binding of flavinyl coenzymes to apoenzymes. © 1969, American Chemical Society. All rights reserved.