pterine-6-carboxylic-acid and 4-aminobenzoylglutamic-acid

Conformational control of excited-state intramolecular electron transfer (ET) in folic acid (FA) has been investigated using femtosecond time-resolved infrared (TRIR) spectroscopy. Ultrafast excited-state ET between the pterin and the 4-aminobenzoyl subunits of FA is observed for the anionic form (at pH 10.0). An ET lifetime of 2.5 ps is estimated from Marcus theory for FA in the "U" conformation, in close agreement with the observed lifetime of 2.0 ps. Return to the ground state through the reverse ET reaction happens almost as rapidly, within 5 ps, resulting in rapid quenching of the singlet excited state. In mixed water:dimethyl sulfoxide solvent, ET becomes more unfavorable as FA adopts a more open conformation, thereby increasing the effective donor-acceptor distance and reducing the coupling energy. In contrast, no ET is observed for the cationic form of FA at low pH (6.0). In this case, the initial singlet excited state is localized on the pterin moiety of FA, and the excited-state charge distribution evolves with time. The charge redistribution in the pterin that occurs with intersystem crossing to the triplet state is characterized by changes in the transient IR spectrum. The excited-state lifetime is much longer in the absence of an ET quenching pathway. These results provide new insight into the mechanism of photodegradation and toxicity of FA. Ultrafast intramolecular ET in closed conformations of FA rapidly quenches the excited state and prevents efficient triplet state formation. Thus, conformations of FA that allow ultrafast intra-ET and rapid quenching of the singlet excited state play a key role in inhibiting pathological pathways following photoexcitation of FA.

Topics: Anions; Dimethyl Sulfoxide; Electron Transport; Folic Acid; Glutamates; Hydrogen-Ion Concentration; Pteridines; Spectrophotometry, Ultraviolet; Spectroscopy, Fourier Transform Infrared; Thermodynamics; Water

A study of the photodegradation of folic acid by ultraviolet radiation in aqueous solution has been made. Folic acid is photolysed by an apparent first-order kinetics and the log k-pH profile shows a gradual decrease in rate in the pH range 2.0-10.0. The profile indicates the appearance of three steps which reflect the participation of different ionic species of folic acid (pKa(1) 2.3, pKa(2) 8.3) in the photolysis reaction. The rate of photodegradation varies from 0.1550 x 10(-3) min(-1) (pH 10.0) to 5.04 x 10(-3) min(-1) (pH 2.5) in the pH range studied. The photolysis of folic acid shows that it is degraded to pterine-6-carboxylic acid and p-amino-benzoyl-L-glutamic acid. A maximum yield of these products is obtained at 3-8 h, depending upon the pH. An HPLC method has been used for the assay of folic acid and its degradation products.

Topics: Chromatography, High Pressure Liquid; Folic Acid; Glutamates; Hydrogen-Ion Concentration; Kinetics; Photolysis; Pteridines; Solutions; Ultraviolet Rays; Water