title: Photoinduced electron transfer in folic acid investigated by ultrafast infrared spectroscopy.
abstract: 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.
authors.affiliation: Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
authors.forename: Guifeng
authors.initials: G
authors.lastname: Li
authors.forename: Donny
authors.initials: D
authors.lastname: Magana
authors.forename: R Brian
authors.initials: RB
authors.lastname: Dyer
mesh_terms: D000838:Anions / Q000737:chemistry; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D004579:Electron Transport; D005492:Folic Acid / Q000737:chemistry*; D005971:Glutamates / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D011621:Pteridines / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D000838:Anions; D005971:Glutamates; D011621:Pteridines; C405202:pterine-6-carboxylic acid; D014867:Water; D005492:Folic Acid; C011398:4-aminobenzoylglutamic acid; D004121:Dimethyl Sulfoxide