Rose Bengal–Sensitized Photooxidation of the Dipeptides l-Tryptophyl-l-Phenylalanine, l-Tryptophyl-l-Tyrosine and l-Tryptophyl-l-Tryptophan: Kinetics, Mechanism and Photoproducts¶

Abstract

The Rose Bengal-sensitized photooxidations of the dipeptides l-tryptophyl-l-phenylalanine (Trp-Phe), l-tryptophyl-l-tyrosine (Trp-Tyr) and l-tryptophyl-l-tryptophan (Trp-Trp) have been studied in pH 7 water solution using static photolysis and time-resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O(2)((1)Delta(g))) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di- and polypeptides of photooxidizable amino acids. The ratio between chemical (k(r)) and overall (k(t)) rate constants for the interaction O(2)((1)Delta(g))-dipeptide indicates that Trp-Phe and Trp-Trp are good candidates to suffer photodynamic action, with k(r)/k(t) values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp-Tyr, a lower k(r)/k(t) value (0.18) has been found, likely as a result of the high component of physical deactivation of O(2)((1)Delta(g)) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O(2)((1)Delta(g)) attack is the Trp group and suggests a much lower accumulation of kynurenine-type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a-oxidized hydrogenated pyrrolo[2,3-b]indoles.

Lysozyme oxidation by singlet molecular oxygen: Peptide characterization using [18O]‐labeling oxygen and nLC‐MS/MS

“…This is not a common behavior using a dipeptide or other polypeptide, whose peptide bond influences considerably the reactivity. 42 However, it has been shown that the reaction with peptides containing tryptophan mainly forms compounds 1 and 3, and in smaller amounts kuinurenine derivative 5. 43 Reaction in aqueous solutions between tryptophan and 1 O 2 was carried out using different photosensitizers, 44,45 and the reaction products were identified by chromatographic techniques and spectrometric methods.…”

Oxidative modification of cytochrome c by singlet oxygen

“…However, LC-MS 2 analysis found that all of the mass altered peptides were derived solely from oxidative modification of Trp. When the reactions of Trp containing dipeptides with 1 O 2 were studied, structures corresponding to mass increments of +4, +16 and +32 were identified as suggested in Figure 1B [11] [49]. Consistent with our observation of +48 and -59 photochemical products, additional peaks attributed to addition of two O 2 molecules and subsequent fragmentations were observed when Trp-Phe was reacted with 1 O 2 [49] however mass spectrometry does not provide sufficient information to unequivocally assign structures to these modifications.…”

Characterization of Asparagine Deamidation in Immunodominant Myelin Oligodendrocyte Glycoprotein Peptide Potential Immunotherapy for the Treatment of Multiple Sclerosis

“…Both transformations are reversible. Tyr and Trp in peptide sequences should be protected from sunlight, as both amino acids are at risk of photo-oxidation [ 7 ]. The rate of racemization in solid-phase peptide synthesis is common and depends on many factors, including the electron-withdrawing effect of the amino acid side chain, the temperature, the reaction solvent, and the coupling reagents [ 8 , 9 , 10 , 11 ].…”