Photochemical behaviour of 6-methylpterin in Aqueous Solutions: Generation of Reactive Oxygen Species

Abstract

Pterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6-Methylpterin (MPT) was investigated for its efficiency of singlet-oxygen (1O2) production and quenching in aqueous solution. The quantum yields of 1O2 production (phi(delta)) was determined by measurements of the 1O2 luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of phi(delta) were found to be 0.10 +/- 0.02 and 0.14 +/- 0.02 in acidic and alkaline media, respectively. Studies of the photooxidation of MPT in acidic (pH = 5.0-6.0) and alkaline (pH = 10.2-10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV-visible spectrophotometry, high-performance liquid chromatography and an enzymatic method for H2O2 determination. MPT is not light sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several nonpteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4 (+/-0.5) x 10(-4) and 8.1 (+/-0.8) x 10(-4) were obtained in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between 1O2 and MPT (k(r)) was determined to be 4.9 x 10(6) M(-1) s(-1) in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed.

Substituent Effects on the Photophysical Properties of Pterin Derivatives in Acidic and Alkaline Aqueous Solutions

“…We determined these p K a , and values of 8.6 ± 0.1, 8.0 ± 0.1, 8.47 ± 0.04 and 8.4 ± 0.1 were found for DPT, RPT, PA and MFA, respectively. These values are similar to those previously determined for other related pterin derivatives (3–6,23).…”

“…The reported p K a values of the amide nitrogen (Fig. 1) of 6‐hydroxymethylpterin (HPT) and 6‐methylpterin (MPT) are 8.1 ± 0.1 (5) and 8.33 ± 0.06 (6), respectively. To the best of our knowledge, the corresponding p K a values of other compounds included in the present study have not been reported in the literature.…”

“…1). The p K a of this equilibrium is about 8 for several pterin derivatives (3–6). Other functional groups of the pterin moiety ( e.g.…”

“…In addition, they have been found in photosensitive organs such as the eyes of different animals (11). Finally, pterins participate in biologically important photosensitisation processes; for example, pterins act as sensitisers in photochemical reactions that induce DNA damage (12,13) and are able to generate singlet molecular oxygen ( 1 O 2 ) (5,6,14,15).…”

Encuentros Latinoamericanos de Fotoquímica y Fotobiología (ELAFOT): The Latin‐American Photochemical and Photobiological Community¶

“…Andrés H. Thomas, Alberto L. Cappareli and coworkers examined the ability of 6‐methylpterin (MPT) to photogenerate and quench in aqueous solution (6). Quantum yields for generation of singlet oxygen were 0.10 ± 0.02 and 0.14 ± 0.02 in acidic and alkaline media, respectively.…”