Glutamic Acid (Glu) p-amino benzoic acid (PABA) 6-methyl pterin ...

Investigation into the photochemical relationship between the
pterin and PABA portions of Folic Acid
I. Research Interests in Folic Acid
Folic acid (FA) is composed of three components: a pterin ring system, a p-amino
benzoic acid (PABA) portion, and the amino acid glutamic acid (Glu).
HO
Glutamic
Acid (Glu)
O
O OH
N
H
p-amino benzoic acid
(PABA)
O
Folic Acid
(FA)
N
6-methyl pterin
The Martin research group investigates the photochemical relationship between the two
aromatic portions of folic acid. Although it has been reported that folic acid decomposes when
exposed to light, 1-5 the specific mechanism of photodegradation as well as the intramolecular
photochemical relationship of the two aromatic portions of folic acid remains to be studied in
any great detail.
Although both the pterin and PABA rings both absorb UV light, a narrow region exists
where the pterin absorbs longer-wavelength UV light that allows for selective irradiation.
Excitation of the PABA ring may result in the same resulting photochemical degradation as
irradiation of the pterin ring, but through a very different mechanism. Therefore, it is possible
that energy can be transferred between these two aromatic rings based upon the relative energies
of their excited states, both singlet (S1) and triplet (T1). Additionally, an electron transfer
reaction may proceed between the aromatic rings that then result in the photodecomposition of
folic acid.
H
N
N
OH
N
N
NH 2

HO
HO
O
O
O OH
N
H
O
hν hν
PABA S 1
PABA T 1
O OH
N
H
O
2
N
H
energy transfer
NH 2
H 3C
N
N
pterin S 1
ISC ISC
? ?
energy transfer
pterin T 1
? ?
Electron
Transfer
PABA pterin
?
PABA 6-methyl pterin
N
N
OH
N
OH
N
N
N
NH 2
NH 2

II. Research Tools and Approaches
In our research group, we use a wide variety of tools for research. Theoretical
calculations, using the Gaussian software package, allow us to investigate the relative energies of
transient species as well as the distribution of unpaired electrons and bond-dissociation energies.
We also synthesize small libraries of organic compounds for use as models for the larger
biologically relevant targets. These libraries are then subjected at an array of irradiations,
spectrophotometric analyses, and product study investigations. Finally, we investigate the effect
of our target molecules on model phages, such as lambda phage, to investigate the possible antiviral
action that these substances may possess.
III. Background Information
Folic Acid, also known as vitamin M, is an essential vitamin that is yellow-orange in
color. It is found in liver, kidney, mushrooms, spinach, yeast, green leaves, and grasses. 6 Folic
acid is reported to be present in photosensitive organs, various mammalian metabolic pathways,
and possibly involved in photosynthesis. 7
The electrochemical behavior of folic acid has been well studied. 8 However, reports on
the photostability of folic acid are rare, 5 and those that exist often provide conflicting
information: some reports state that the vitamin is photosensitive only under aerobic conditions, 5
while most others report that folic acid is photosensitive under all conditions studied. Attempts
to elucidate the intramolecular photochemical behavior of folic acid from the literature is
difficult since studies of the aqueous stacking of the two aromatic portions of folic acid report
both intramolecular stacked 9 and unstacked forms. 10 Therefore, the apparently contradictory
nature of these reports makes it difficult to draw any clear conclusions, and more work is needed
in this area of study.
Literature indicates that photolysis of folic acid leads to cleavage between the pterin
benzylic position and the anilinic position of the PABA ring. 1,4,5,11 This may suggest that the
reactive species responsible for the degradation of folic acid lies with the PABA ring and not the
pterin core, although the pterin portion of folic acid is known to absorb UVA light and the PABA
portion does not. 12 It is known that pterins will undergo electron transfer reactions with electron
rich amino acids to generate a charged radical pair. 13 The possibility of intramolecular electron
transfer, or intramolecular energy transfer, has not been reported for folic acid. It is known that
folic acid enhances the generation of reactive oxygen species generated by riboflavin in
photolyzed solutions of cell culture media, although folic acid itself is not the source of the
reactive oxygen species. 14 It has also been reported that folic acid is photochemically inactivated
in the presence of riboflavin, and the reaction is inhibited by ascorbic acid, a known triplet state
quencher. 15 Therefore it is very likely that the photo-decomposition of folic acid proceeds
through an excited triplet state and folic acid may participate in photo-sensitization reactions.
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References
(1) Lowry, O. H.; Bessey, O. A.; Crawford, E. J. "Photolytic and enzymatic transformations
of pteroylglutamic acid," J. Biol. Chem. 1949, 182, 389-398.
(2) Roe, D. A. "Photodegredation of carotenoids in human subjects," Fed. Proc. 1987, 46,
1886-1889.
(3) Suarez, G.; Cabrerizo, F. M.; Lorente, C.; Thomas, A. H.; Capparelli, A. L. "Study of the
photolysis of 6-carboxypterin in acid and alkaline aqueous solutions," J. Photochem.
Photobiol. A 2000, 132, 53-57.
(4) Thomas, A.; Einschlag, F. G.; Feliz, M. R.; Capparelli, A. L. "First steps in the
photochemistry of folate in alkaline medium," J. Photochem. Photobio. A 1998, 116,
187-190.
(5) Thomas, A. H.; Suarez, G.; Cabrerizo, F. M.; Martino, R.; Capparelli, A. L. "Study of the
photolysis of folic acid and 6-formylpterin in acid aqueous solutions," J. Photochem.
Photobiol. A 2000, 135, 147-154.
(6) In The Merck Index; 12 ed.; Budavari, S., O'Neil, M. J., Smith, A., Heckelman, P. E.,
Kinneary, J. F., Eds.; Merck Research laboratories: Whitehouse Station, NJ, 1996, p
4253.
(7) Chahidi, C.; Aubailly, M.; Momzikoff, A.; Bazin, M. "Photophysical and
photosensitizing properies of 2-amino-4 pteridinone: a natural pigment," Photochem.
Photobiol. 1981, 33, 641-649.
(8) Spina Bifida and Hydrocephalus Association of Nova Scotia, "Folic Acid information,"
http://reseau.chebucto.ns.ca/Health/SBANS/Folic_Acid.html
(9) Thiery, C. "Etude spectroscopique de l'acide folique," Eur. J. Biochem. 1973, 37, 100-
108.
(10) Lam, Y.-F.; Kotowycz, G. "Self Association of Folic Acid in Aqueous Solution by
Proton Magnetic Resonance," Can. J. Chem. 1972, 50, 2357-2363.
(11) Lowry, O. H.; Bessey, O. A.; Crawford, E. J. "Pterine Oxidase," J. Biol. Chem. 1949,
180, 399-410.
(12) Chahidi, C.; Morliere, P.; Aubailly, M.; Dubertret, L.; Santus, R. "Photosensitization by
methotrexate photoproducts," Photochem. Photobiol. 1983, 38, 317-322.
(13) Aubailly, M.; Santus, R. In Chemistry and Biology of Pteridines; Walter de Gruyter &
Co.: Berlin, 1986, pp 99-102.
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(14) Grzelak, A.; Rychlik, B.; Bartosz, G. "Light-dependent generation of reactive oxygen
species in cell culture media," Free Radical Biology and Medicine 2001, 30, 1418-1425.
(15) Reusser, P. "Etude sur l'inactivation photochimique de l'acide folique en presence de
riboflavine et de son inhibition par l'acide ascorbique," J. Internat. Vitaminol. 1970, 39,
64-72.
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