Reactivity of Conjugated and Unconjugated Pterins with Singlet Oxygen (O2(1Δg)): Physical Quenching and Chemical Reaction†

Abstract

Pterins (PTs) belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. We have investigated the reactivity of conjugated PTs (folic acid [FA], 10-methylfolic acid [MFA], pteroic acid [PA]) and unconjugated PTs (PT, 6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT]) with singlet oxygen (1O2) in aqueous solutions, and compared the efficiencies of chemical reaction and physical quenching. The chemical reactions between 1O2, produced by photosensitization, and PT derivatives were followed by UV-visible spectrophotometry and high-performance liquid chromatography, and corresponding rate constants (k(r)) were evaluated. Whenever possible, products were identified and quantified. Rate constants of 1O2 total quenching by the PT derivatives investigated were obtained from steady-state 1O2 luminescence measurements. Results show that the behavior of conjugated PTs differs considerably from that of unconjugated derivatives, and the mechanisms of 1O2 physical quenching by these compounds and of their chemical reaction with 1O2 are discussed in relation to their structural features.

Substituent Effects of Pterin Derivatives on Singlet Oxygen Scavenging Activity

“…The total rate constant of 1 O 2 quenching (k t ) by pterin derivatives (P) consisted of rate constant of the chemical reac-tion of 1 O 2 with pterin derivatives (k r ) and that of the 1 O 2 physical quenching by pterin derivatives (k q ) (k t ϭk r ϩk q ). 15,16) In our result, there was no difference in 1 O 2 scavenging activity among 6FP, Pn, P6COOH, NP and BP, despite different substitutions at the 6-position (Fig. 1, the column of R 6 ).…”

“…It was reported, however, that the k r value for Pn (2.5Ϯ0.3ϫ10 5 M Ϫ1 s Ϫ1 ) is very low in comparison with the corresponding k t value, indicating that the deactivation of 1 O 2 by Pn is mainly a physical quenching process (k q ϭ2.6Ϯ0.3ϫ10 6 M Ϫ1 s Ϫ1 is an order of magnitude larger than k r ). 15) This indicated that the chemical reaction did not play a major role in 6FP, P6COOH, NP and BP as well as Pn, which is possibly the reason why there was no difference in 1 O 2 scavenging activity among 6FP, Pn, P6COOH, NP and BP.…”

Production and quenching of reactive oxygen species by pterin derivatives, an intriguing class of biomolecules

“…OLIVEROS et al Table 2 Quantum yields of 1 O 2 production (Φ Δ ) by pterin derivatives, rate constants of 1 O 2 total quenching (k t = k r + k q ), physical quenching (k q ) and rate constants of the chemical reaction with 1 O 2 (k r ) in air-equilibrated aqueous solutions (data from refs. [23][24][25][26]28] …”

“…The efficiencies of these processes may be evaluated by determining the rate constants of 1 O 2 physical quenching and of the chemical reaction with 1 O 2 (k q and k r , reactions 6 and 7, respectively). Values of these rate constants have been published for aromatic conjugated and unconjugated pterins, as well as for 7,8-dihydropterins, in aqueous solutions [23][24][25]28,32] (Table 2). Oxidation products have also been investigated to evaluate potential consequences in vivo.…”

Theoretical Study on the Relative Energies of Neutral Pterin Tautomers

“…Under UV-A irradiation, the excited pterins may undergo photooxidation besides fluorescence emission. Pterins, chiefly 6-carboxy pterin, have been shown to photosensitize DNA damage by both type I (electron transfer) and type II (singlet oxygen) mechanisms (19)(20)(21)(22). Despite their important contribution, the electronic and geometrical features of related low-lying states as well as the explicit mechanism of photochemistry and photophysics of pterins are still unclear.…”