Electronic Effects on the Fluorescence of Tyrosine in Small Peptides

Abstract

It is shown for a series of tyrosine-derivatives and tyrosine-containing peptides that the amide group in combination with electron-withdrawing substituents quenches the fluorescence of the phenol moiety. The ammonium group has the strongest electron-withdrawing effect and thus the largest influence on the quenching rate. The peptide group itself does not quench the fluorescence. In a series of peptides with an increasing number of alanines the decreasing quenching efficiency of the peptide group due to the greater distance of the ammonium group is demonstrated. In tyrosine-containing di- and tripeptides a linear correlation between the 13C-NMR chemical shift delta of the C alpha atom of various aliphatic amino acids and the fluorescence-quenching constant confirms the hypothesis that electron-withdrawing and -donating groups are modulating the fluorescence-quenching efficiency of the peptide group. In small peptides the fluorescence lifetime of tyrosine is characteristic for the neighboring amino acids. Using model substances the redox properties of a peptide group and the phenol ring were studied electrochemically. The highest occupied molecular orbital of the tyrosine (1.4 V vs saturated calomel electrode [SCE]) and the lowest unoccupied molecular orbital of the peptide group (-3.12 V vs SCE) have appropriate energies for a photoinduced electron transfer reaction. For solute-quenching experiments quencher molecules can be systematically selected.

Inductive Effects in Ruthenium-Modified Amino Acids

“…Figure 4 contains a plot of lnk versus emission energies which includes literature data reported for [Ru(bipy) 3 ] 2ϩ . [18] Further support for an explanation which only involves an energy gap law relationship comes from the behavior of our complexes at low pH values. It should be noted that [Ru(bipy) 3 ] 2ϩ cannot be included without caution since linear energy-gap relationships are only expected within a series of closely related complexes.…”

Kinetics of precursor cleavage at the dibasic sites

“…However, as observed for other peptides [16–18], their fluorescence quantum yields are smaller than that of the free residues. These lower values result mainly from peptide bonds and inductive effects as proton transfer and electron‐withdrawing [19–21]. These effects – more or less facilitated by peptide fluctuations – are induced by specific intramolecular interactions which define the particular rotamer distribution of each fluorophore inserted in the original A 13 peptide.…”

Photophysical Properties of Tyrosine and Its Simple Derivatives in Organic Solvents Studied by Time‐resolved Fluorescence Spectroscopy and Global Analysis¶

“…The different lifetimes of the rotamers arise from the interaction between the phenol fluorophore and the quenching groups. A charge transfer interaction between the excited aromatic chromophore (phenol ring) as a donor and the electrophilic units in the amino acid backbone (the carbonyl of the amide group) as an acceptor was proposed by Cowgill (31,32), Toumon et al (33) and Feitelson (10) and confirmed by others (7,9,24,26). The slow-exchange ground state rotamer model in the case of tyrosine derivatives predicts that the fluorescence intensity decay should be described by the sum of three exponents; this in which the phenol ring comes into the closest contact with the carbonyl group has the shortest fluorescence lifetime.…”