INVESTIGADORES
THOMAS Andres Hector
congresos y reuniones científicas
Título:
The role of triplet states in the photooxidation of folic acid
Autor/es:
M. PAULA DENOFRIO; M. LAURA DÁNTOLA; PETER R. OGILBY; CAROLINA LORENTE; ANDRÉS H. THOMAS
Lugar:
Mendoza
Reunión:
Congreso; XXI Inter-American Photochemical Society Conference; 2011
Resumen:
Folic acid (PteGlu) and its derivatives
(folates) are conjugated pterins widespread in biological systems. Their
chemical structure is composed of three moieties: 6-methylpterin (Mep), p-aminobenzoic acid (PABA), and glutamic
acid (Glu). Folate deficiency in pregnant women has clearly been shown to be
related to neural tube defects.1 It has been proposed that one of
the main functions of skin pigmentation is to avoid photolysis of folates.2
Due to the biological implications of the photodegradation of folates in
humans, the photochemical behavior of PteGlu as a model compound becomes very
interesting.
In the absence of oxygen, PteGlu is
photostable. However, excitation of PteGlu in air-equilibrated solutions leads
to cleavage and oxidation of the molecule, yielding 6-formylpterin (Fop) and p-aminobenzoylglutamic acid (PABA-Glu)
as photoproducts.3,4 When an
air-equilibrated aqueous solution of PteGlu is exposed to UV-A radiation, the rate of PteGlu degradation increases with
irradiation time.3,4 Analysis of this ?auto-photocatalytic?
effect on the basis of the general photochemical behavior of pterins5
suggests that the reaction consists in a photosensitized
process in which the photoproduct (Fop) acts as a sensitizer of its own
production. The first step would involve an electron transfer from the PABA
ring of the substituent of PteGlu to the triplet excited state of the
sensitizer.
The aim of this work was to prove the
interaction between the triplet state of unconjugated pterins and the
substituent of PteGlu. With this in mind, quenching studies by means of laser
flash photolysis were performed as previously described.6 To avoid the
excitation of the substrate and photolysis of the sensitizer, the experiments
were carried out using Mep, a photostable unconjugated pterin, as sensitizer
and PABA and PABA-Glu as quenchers. The results obtained in these flash
photolysis experiments provide direct evidence for the reaction between the
isolated substituent of PteGlu and the triplet state of Mep. Moreover, the
values of the quenching rate constants obtained indicate a very efficient
interaction. Most importantly, the data support the assumption that triplet
states of oxidized pterins participate in the mechanism of the photosensitized
oxidation of PteGlu.
References
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135, 88.
2 R. F. Branda, J. W. Eaton, Science, 1978, 201, 625.
3 A. H.
Thomas, G. Suárez, F. M. Cabrerizo, R. Martino, A. L. Capparelli, J.
Photochem. Photobiol.
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4 M. K.
Off, A. E. Steindal, A. C. Porojnicu, A. Juzeniene, A. Vorobey, A. Johnsson, J.
Moan, J. Photochem. Photobiol.
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80,
47.
5 M. L. Dántola, M. Vignoni, C. González, C.
Lorente, P. Vicendo, E. Oliveros, A. H. Thomas, Free Radic. Biol. Med., 2010, 49 (6), 1014.
6 T.
Keszthelyi, D. Weldon, T. N. Andersen, T. D. Poulsen, K. V. Mikkelsen, P. R.
Ogilby, Photochem. Photobiol., 1999, 70, 531.