PHOTOPHYSICS AND PHOTOSENSITIZING PROPERTIES OF PTERINS IN AQUEOUS SOLUTION

FRANCO M. CABRERIZO; ANDR¨¦S H. THOMAS; ALBERTO L. CAPPARELLI

Salvador de Bah¨ªa, Brasil

Conferencia; 17th I-APS Conference on Photochemistry; 2006

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. These compounds derive from 2-amino-4-hydroxypteridin and most of them exist in two acid-base forms in the pH range between 3 and 13 (Scheme 1). As it is known, the photochemistry, as well as the photophysical behaviour of the different acid-base forms of each compound present significant differences.  Studies of the photooxidation of three pterins derivatives (pterin (PT), 6-methylpterin (MPT) and 6-hydroxymethylpterin (HPT)) in acidic and alkaline aqueous solutions at 350 nm and room temperature have been performed. The quantum yields of pterins disappearance were determined. The photochemical reactions were followed by UV-visible spectrophotometry, high-performance liquid chromatography and an enzymatic method for H2O2 determination. The photophysical properties of acid and basic forms of five compounds of the pterin family (HPT, MPT, 6,7-dimethylpterin (DPT), N-methylfolic acid (MFA) and pteroic acid (PA)) have been studied. The effects of the chemical nature of the substituents at position 6 of the pterin moiety as well as the effects of the pH on the absorption and emission properties are analyzed. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds have been investigated using the single photon counting technique. Results obtained for pterin derivatives containing small substituents with 1 carbon atom (HPT, MPT, DPT) are different from those found for pterin derivatives containing a p-aminobenzoic acid (PABA) moiety in the substituent (MFA and PA). Fluorescence quantum yields (¦µF) of the first group of compounds are relatively high (¡Ý 0.4), whereas MFA and PA exhibit very small ¦µF values (¡Ü 0.01). PT, MPT, HPT, DPT and MAF were investigated for theirs efficiency of singlet oxygen (1O2) production and quenching in D2O solution. The quantum yields of 1O2 production (¦µ¦¤) were determined by measurements of the 1O2 luminescence in the near-infrared upon continuous excitation of the sensitizer. These results show that all studied compounds, except MAF, are good 1O2 sensitizers under UV-A irradiation in aqueous solutions. For most of the studied compounds, values of FD for the basic forms are higher than the corresponding values for acid forms, and they are considerably affected by the nature of the substituent at position 6 on the pterin moiety. Above pH 11, the generation of 1O2 by pterins is efficiently quenched by hydroxide ions (HO-). This effect may be due to the deactivation of singlet excited states of pterins through quenching by HO-.