Photooxidation of Histidine by 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution: Time-Resolved and Field-Dependent CIDNP Study

Histidine oxidation photosensitized by pterin: pH dependent mechanism

“…This amino acid, free in aqueous solution, has 3 relevant acid-base forms, a protonated form (HisH + ), a neutral form (His) and anionic form (His(−H) − ), with pK a values of 6.0 and 9.2 (Fig. 1) [14]. In the neutral form of histidine (His), the imidazole ring is not protonated, the net charge is zero but, as for the most common α-amino acids, a zwitterionic form predominates where the carboxylic and the amino groups of the amino acid function are respectively deprotonated (−COO − ) and protonated (−N + H 3 ) (corresponding pK a values of 9.2 and 2.2) [14].…”

“…1) [14]. In the neutral form of histidine (His), the imidazole ring is not protonated, the net charge is zero but, as for the most common α-amino acids, a zwitterionic form predominates where the carboxylic and the amino groups of the amino acid function are respectively deprotonated (−COO − ) and protonated (−N + H 3 ) (corresponding pK a values of 9.2 and 2.2) [14]. Degradation of histidine under direct excitation by UV or visible radiation is irrelevant, but photosensitized processes were reported to oxidize His residues [1].…”

Performance and Mechanism of UV/Immobilized Cu-TiO2System to Degradation Histidine

“…The degradation rate of DON was up to 50% when pH value is 7. In this study, the point of zero charge of TiO 2 is 6.5, and the isoionic point of histidine is 7.59 [32]. We may discover that both the catalysts and histidine are negatively charged when pH value of solution is higher than 7.59.…”

Reduction of transient carnosine radicals depends on β-alanyl amino group charge

“…1 (top) and represents a superposition of CIDNP patterns typical for CIDNP spectra corresponding to those for the reactions of triplet TCBP with β-Ala-His (reaction (2)), and TCBP with Trp (reaction (3)): absorptive enhancements are observed for H2, H4 of β-Ala-His, H2, H4, H6 of Trp, and H5,5′ of TCBP, and emissive signals are observed for β-protons of the His residue, β-protons and H5 of Trp, H2,2′, and H6,6′ of TCBP. These enhancements are formed in reactions (2) and (3) since the g -factor of TCBP radicals 22 is larger than the g -factor of His 23 and Trp radicals. 24 The non-reactive β-Ala residue does not show up in the CIDNP spectra.…”

“…The CIDNP spectrum obtained immediately after the laser pulse during the photoreaction of TCBP with carnosine and tryptophan is shown in Fig. 1 3) since the g-factor of TCBP radicals 22 is larger than the g-factor of His 23 and Trp radicals. 24 The nonreactive b-Ala residue does not show up in the CIDNP spectra.…”