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.

AOT direct and reverse micelles as a reaction media for anisotropic silver nanoparticles functionalized with folic acid as a photothermal agent on HeLa cells

“…5 and 6. The band around 365 nm is assigned to the π-π* transition localized on the pterin ring of FA [32]. The other bands at 255 and 280 nm are also from π-π* transitions of the pterin [32].…”

Direct Observation and Control of Ultrafast Photoinduced Twisted Intramolecular Charge Transfer (TICT) in Triphenyl-Methane Dyes

“…The details of the setup have been described recently. 27 Briefly, the TRIR experiments were carried out in the following way: 800 nm laser pulses (3.6 W, 1 kHz, 30 fs) were generated with a femtosecond Ti:sapphire regenerative amplifier (Legend Elite, Coherent) seeded by a femtosecond Ti:sapphire mode-locked oscillator (Mantis, Coherent). The regenerative amplifier output was split in a 1:1 ratio and sent into two OPA systems (OPerA Solo, Coherent) used to generate the UV/vis pump pulses and IR probe pulses, respectively.…”

Anisotropic energy flow and allosteric ligand binding in albumin

“…Visible light excitation of MP or MG bound to the protein results in rapid energy flow to the protein due to internal conversion and subsequent relaxation processes 37 . The dynamics of energy flow through the secondary structure of BSA was probed using ultrafast time-resolved infrared spectroscopy 37,38 . We observe anisotropic energy flow through the protein structure following input of thermal energy into the flexible ligand binding sites, without local heating of the rigid helix bundles that connect these sites.…”