Mechanism for Fluorescence Quenching of Tryptophan by Oxamate and Pyruvate: Conjugation and Solvation-Induced Photoinduced Electron Transfer

Abstract

Oxamate and pyruvate are isoelectronic molecules. They both quench tryptophan fluorescence with Stern-Volmer constants of 16 and 20 M, respectively, which are comparable to that of arcrylamide, a commonly used probe for protein structure. On the other hand, it is well known that neither the carboxylate group of these molecules nor the amide group is a good quencher. To find the mechanism of the quenching by oxamate and pyruvate, density functional theory computations with a polarizable continuum model, solvation based on density, and explicit waters, were performed. Results indicate that both molecules can be an electron acceptor via photoinduced electron transfer. There are two requirements. First, the carboxylate and amide moieties must be in direct contact to bring about noticeable quenching. The conjugation between the amide (or the keto) group and the carboxylate group leads to a lower π* orbital, which is the lowest unoccupied molecular orbital (LUMO), and can then accept an electron from the excited tryptophan. Second, since oxamate and pyruvate ions have high electron density, hydrogen bonds with waters, which can be simulated by an explicit water model, are essential. Their LUMO energies are strongly influenced by water in aqueous solution. The above findings demonstrate how tryptophan fluorescence gets quenched in aqueous solution. The findings may be important in dealing with those problems where frontier orbitals are considered, especially with molecules having high electron density.

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Incorporation of Complexation into a Coamorphous System Dramatically Enhances Dissolution and Eliminates Gelation of Amorphous Lurasidone Hydrochloride

“…In a general view, the quencher could be an electron acceptor in photoinduced electron transfer when its lowest unoccupied molecular orbital (LUMO) is lower than the fluorophore's LUMO or be an electron donor when its highest occupied molecular orbital is higher than that of the fluorophore. 66 The LUMO values of CYS and LH were calculated to be −5.276 and −3.850 eV, respectively, using Gaussian 6.0 software at the B3LYP/6-311++G(d,p) level. Electron transfer probably happened from LH to CYS within complex LH−CYS, which was responsible for the fluorescence quenching (Figure S3).…”

“…Therefore, it was considered that fluorescence quenching of LH by CYS was mainly due to the ground-state complex formation. In a general view, the quencher could be an electron acceptor in photoinduced electron transfer when its lowest unoccupied molecular orbital (LUMO) is lower than the fluorophore’s LUMO or be an electron donor when its highest occupied molecular orbital is higher than that of the fluorophore . The LUMO values of CYS and LH were calculated to be −5.276 and −3.850 eV, respectively, using Gaussian 6.0 software at the B3LYP/6-311++G­(d,p) level.…”