INVESTIGADORES
CABRERIZO Franco Martin
congresos y reuniones científicas
Título:
Photolysis of folic acid and 6-formylpterin in acid aqueous solutions
Autor/es:
ANDRÉS H. THOMAS; GUSTAVO SUÁREZ; FRANCO M. CABRERIZO; RAÚL MARTINO; ALBERTO L. CAPPARELLI
Lugar:
Dresden, Alemania
Reunión:
Congreso; XXVIII IUPAC Symposium of Photochemistry; 2000
Resumen:
Folates and pterins are heterocyclic compounds widespread in
biological systems. Folic acid or pteroylglutamic acid, a conjugated pterin, is
a vitamin of the B group and acts as a coenzyme in reactions related to the
synthesis of puric and pirimidinic bases. Pterins participate in relevant
biological functions too. Light sensitivity of pterins derivatives is well
known and they have been found in photosensitive organs such as the eyes.
Moreover, it has been suggested that pterins may play some role in
photosynthesis and may act as blue antennas in superior plants and other
organisms. Reports on photochemistry and photophysics of pterins can be found
in the literature. However, much is still left to be known on the
photochemistry of folic acid and pterins.
Amide group in pterins moiety suffers deprotonation to the
corresponding fenolate in alkaline media. The aim of this work is to study the
photolysis of the acid forms of folic acid and 6-formylpterin by UV-A light
(320 - 400 nm.). Thus, the pH conditions (4.5 - 6.0) were chosen, so that only
the acid forms are present.
In the present work, products of
photolysis of folic acid and 6-formylpterin were identified and quantified and
the role of O2 was analysed. The photochemical reactions were
followed by UV-VIS spectrophotometry, thin layer chromatography (TLC), high
performance liquid chromatography (HPLC) and the oxygen consumption was
measured with an O2 selective electrode. The quantum yields of
substrate disappearance and of photoproducts formation were determined in
experiments performed under different conditions.
Scheme below summarises the results obtained. In the absence
of oxygen, folic acid is photostable, whereas excitation in the presence of
oxygen leads to cleavage and oxidation of the molecule, yielding 6-formylpterin
and p-aminobenzoylglutamic acid as
photoproducts. No other photoproducts were detected under our experimental
conditions. This degradation reaction of folic acid is accelerated in the
presence of 6-formylpterin. This process requires photoexcitation of
6-formylpterin but not requires the absorption of light by folic acid.
Therefore, excitation of 6-formylpterin leads to the generation of some
reactive specie that induces the degradation of folic acid. As the photolysis
proceeds, 6-carboxypterin arises from 6-formylpterin, as observed when
solutions only containing the latter compound are irradiated in the presence of
oxygen.
Photolysis of
6-formylpterin, under anaerobic conditions, leads to the formation of a red
intermediate (RI) with a longwave absorbing band of low extinction at 480 nm.
RI is thermally oxidised producing 6-carboxypterin on admission of oxygen. Due
to its spectral features and reactivities, the "red intermediate"
detected in our experiments could be 6-carboxy-5,8-dihidropterin