Photophysics and Photochemistry of Pterins in Aqueous Solution

Abstract

Pterins belong to a family of heterocyclic compounds present in a wide range of living systems and participate in relevant biological functions. Interest in the photochemistry and photophysics of this group of compounds has increased since the participation of pterin derivatives in different photobiological processes has been suggested or demonstrated in recent decades. This account describes and connects basic studies on the fluorescence emission, the photooxidation, and the photosensitizing properties of oxidized six-substituted pterins in aqueous solution under UV-A irradiation. The biological implications of these studies are also discussed.

Theoretical Study on the Relative Energies of Neutral Pterin Tautomers

“…Pterins participate in relevant biological functions behaving as inhibitors (1), enzymes (2)(3)(4), coenzymes (5,6), sensitizers (7)(8)(9), and pigments (10). In the past decade, the photophysical and photochemical properties of pterins have been systematically studied (11). The photochemical properties of pterins have been found to be dependent on chemical structure, pH, the availability of oxygen, and the immediate availability of appropriate electron donors (11)(12)(13)(14)(15)(16).…”

“…In the past decade, the photophysical and photochemical properties of pterins have been systematically studied (11). The photochemical properties of pterins have been found to be dependent on chemical structure, pH, the availability of oxygen, and the immediate availability of appropriate electron donors (11)(12)(13)(14)(15)(16). More recently, Lorente and Thomas reported their experimental study on pterins in aqueous solution assigning the absorption and emission spectra of pterins and proposed plausible decay mechanism of excited pterins (17,18).…”

Photoinduced Generation of H2O2 and O2•- by 6-formylpterin in Aqueous Solutions

“…As reported by Albert (8) for several pterin derivatives, dominant equilibrium at pH higher than 5 involves an amide group (acid form) and a phenolate group (basic form) (Scheme 1). The pK a of this equilibrium is around 8 for different studied pterin derivatives (9). Other functional groups of the pterin moiety (e.g., 2-amino group or ring nitrogen atoms) have pK a values lower than 2 (8 ).…”

“…Thus at pH 7.2, both the acid and the basic forms of this compound arc present in significant proportions. Differences on the photochemistry and photophysies of the acid and the basic forms of many pterin derivatives have been reported (9). Therefore, in order to analyse separately the behaviour of both acidbase forms, we performed our experiments in the pH ranges 5.0-5.0. where pterins are present at more than 99" η in the acid form, and 10.0-10.5, where pterins are present at more than 99% in the basic form.…”

“…Other functional groups of the pterin moiety (e.g., 2-amino group or ring nitrogen atoms) have pK a values lower than 2 (8 ). The mechanism involved in the photooxidation of 6-substituted pterins strongly depends on the nature of the substituent at C(6) as well as on the pH (9), (e.g., the photochemistry of the acid and basic forms of the pterin derivatives arc different). Some pterin derivatives present a characteristic photochemistry.…”