ELECTRONIC EFFECTS ON THE FLUORESCENCE OF TYROSINE IN SMALL PEPTIDES

It is shown for a series of tyrosine-derivatives and tyrosine-containi ng peptides that the amide group in combination with electron-withdraw ing substituents quenches the fluorescence of the phenol moiety. The a mmonium group has the strongest electron-withdrawing effect and thus t he largest influence on the quenching rate. The peptide group itself d oes not quench the fluorescence. In a series of peptides with an incre asing number of alanines the decreasing quenching efficiency of the pe ptide group due to the greater distance of the ammonium group is demon strated. In tyrosine-containing di- and tripeptides a linear correlati on between the C-13-NMR chemical shift delta of the C(alpha) atom of v arious aliphatic amino acids and the fluorescence-quenching constant c onfirms the hypothesis that electron-withdrawing and -donating groups are modulating the fluorescence-quenching efficiency of the peptide gr oup. In small peptides the fluorescence lifetime of tyrosine is charac teristic for the neighboring amino acids. Using model substances the r edox properties of a peptide group and the phenol ring were studied el ectrochemically. The highest occupied molecular orbital of the tyrosin e (1.4 V vs saturated calomel electrode [SCE]) and the lowest unoccupi ed molecular orbital of the peptide group (3.12 V vs SCE) have appropr iate energies for a photoinduced electron transfer reaction. For solut e-quenching experiments quencher molecules can be systematically selec ted.