Detection and monitoring of in vitro formation of salicylic acid from aspirin using fluorescence spectroscopic technique and DFT calculations.

Acetylsalicylic acid commonly termed as aspirin (AS) is a well known antipyretic and anti-inflammatory drug which can also be used to reduce death risks due to heart attack. In addition to this, it also exhibits some adverse effect such as gastrointestinal, tinnitus, Reye's syndrome. The side effects of AS such as gastrointestinal ulcer, tinnitus and Reye's syndrome are caused due to conversion of AS into its active metabolite salicylic acid (SAL). Conversion of AS into SAL has been investigated generally at basic pH. Since the pH of Gastrointestinal tract is on average neutral ranging from 6.5-7.4. Therefore in the present research work, in vitro conversion of AS to SAL was detected at neutral pH in both aqueous medium and human blood serum samples by time series fluorescence measurements and DFT study. The SAL obtained from AS at neutral pH was observed to be stable for ~ 6 and ~ 4 days in aqueous medium and blood serum, respectively. The mechanism of conversion of AS into SAL was investigated using the transition state theory employing density functional theory (DFT). On the basis of DFT calculation the in vitro formation of SAL from AS at neutral pH was found to involve two intermediate transition states.

Scavenging of hydroxyl, methoxy, and nitrogen dioxide free radicals by some methylated isoflavones.

Free radicals can be scavenged from biological systems by genistein, daidzein, and their methyl derivatives through hydrogen atom transfer (HAT), single-electron transfer (SET), and sequential proton-loss electron-transfer (SPLET) mechanisms. Reactions between these derivatives and the free radicals OH., OCH3., and NO2. via the HAT mechanism in the gas phase were studied using the transition state theory within the framework of DFT. Solvation of all the species and complexes involved in the HAT reactions in aqueous media was treated by performing single point energy calculations using the polarizable continuum model (PCM). The SET and SPLET mechanisms for the above reactions were also considered by applying the Marcus theory of electron transfer, and were found to be quite sensitive to geometry and solvation. Therefore, the geometries of all the species involved in the SET and SPLET mechanisms were fully optimized in aqueous media. The calculated barrier energies and rate constants of the HAT-based scavenging reactions showed that the OH group of the B ring in genistein, daidzein, and their methyl derivatives plays a major role in the scavenging of free radicals, and the role of this OH group in the HAT-based free-radical scavenging decreases in the following order: OH. > OCH3. > NO2.. The SPLET mechanism was found to be an important mechanism in these free-radical scavenging reactions, whereas the SET mechanism was not important in this context.

Detection of in Vitro Metabolite Formation of Leflunomide: A Fluorescence Dynamics and Electronic Structure Study.

The metabolic transformation of antirheumatic fluorescent drug leflunomide into its active metabolite teriflunomide through isoxazole ring opening has been monitored in vitro using steady state and time domain fluorescence spectroscopy and density functional theory. During metabolic reaction, absorption of leflunomide split into two bands resembling absorption spectra of teriflunomide. The fluorescence spectra reveal slow conversion of leflunomide to E and Z forms of teriflunomide in aqueous medium, which becomes faster at basic pH. The E form, which is more potent as a drug, becomes more stable with an increase in the basicity of the medium. Both molecules are associated with charge transfer due to twisting in the lowest singlet excited state. Excited state charge transfer followed by proton transfer was also observed in the Z form during the ring opening of leflunomide. Quantum yield and radiative decay rates have been observed to decrease for the metabolite because of an increase in nonradiative decay channels.

Modeling the scavenging activity of ellagic acid and its methyl derivatives towards hydroxyl, methoxy, and nitrogen dioxide radicals.

The reaction mechanisms involved in the scavenging of hydroxyl (OH(·)), methoxy (OCH3(·)), and nitrogen dioxide (NO2 (·)) radicals by ellagic acid and its monomethyl and dimethyl derivatives were investigated using the transition state theory and density functional theory. The calculated Gibbs barrier energies associated with the abstraction of hydrogen from the hydroxyl groups of ellagic acid and its monomethyl and dimethyl derivatives by an OH· radical in aqueous media were all found to be negative. When NO2(·) was the radical involved in hydrogen abstraction, the Gibbs barrier energies were much larger than those calculated when the OH(·) radical was involved. When OCH3(·) was the hydrogen-abstracting radical, the Gibbs barrier energies lay between those obtained with OH(·) and NO2(·) radicals. Therefore, the scavenging efficiencies of ellagic acid and its monomethyl and dimethyl derivatives towards the three radicals decrease in the order OH(·) >> OCH3(·) > NO2(·). Our calculated rate constants are broadly in agreement with those obtained experimentally for hydrogen abstraction reactions of ellagic acid with OH· and NO2· radicals.

Is FapyG mutagenic?: Evidence from the DFT study.

2,6-diamino-4-oxo-5-formamidopyrimidine (FapyG) is an oxidatively damaged product of guanine (G), which is mainly formed through metabolic processes that produce OH radicals. It has been proposed that in bacterial cells, FapyG retains the coding properties of G, and is, therefore, not mutagenic. However, in mammalian cells, FapyG induces G to thymine (T) mutation more dominantly than another ubiquitous oxidative lesion, that is, 8-oxoguanine (8-oxoG). The exact reasons for these coding properties of FapyG are not properly understood. In order to rationalize the cause of FapyG-mediated mutagenesis, all of the possible base-pair interactions of FapyG with cytosine (C), adenine (A), and T, in both anti- and syn- conformations, are studied in detail by using density functional theory (DFT). The effects of solvation on the coding properties of FapyG are also evaluated. We demonstrate that the anti-FapyG:C base pair has the highest binding energy, and that the base-pair alignment is similar to that of the normal G:C base pair. Therefore, insertion of C opposite anti-FapyG is preferred over the other DNA bases. This could be the reason for the non-mutagenic behavior of FapyG in bacterial cells. However, as the binding patterns and energies of anti-FpyG:A and syn-FapyG:A base pairs are similar, and these are also similar to those of the T:A base pair, mammalian polymerases may not distinguish between FapyG and T. As a result, A would be mistakenly inserted opposite either anti-FapyG or syn-FapyG, resulting in G to T transverse mutation.

A quantum chemical study of repair of O6-methylguanine to guanine by tyrosine: evaluation of the winged helix-turn-helix model.

The winged helix-turn-helix model for the repair of O6-MeG to guanine involving the reaction of O6-MeG with a tyrosine residue of the protein O6-alkylguanine-DNA alkyltransferase (AGT) was examined by studying the reaction mechanism and barrier energies. Molecular geometries of the species and complexes involved in the reaction, i.e. the reactant, intermediate and product complexes as well as transition states, were optimized employing density functional theory in gas phase. It was followed by single point energy calculations using density functional theory along with a higher basis set and second order M(phi)ller-Plesset perturbation theory (MP2) along with two different basis sets in gas phase and aqueous media. For the solvation calculations in aqueous media, the integral equation formalism of the polarizable continuum model (IEF-PCM) was employed. Vibrational frequency analysis was performed for each optimized structure and genuineness of transition states was ensured by visualizing the vibrational modes. It is found that tyrosine can repair O6-MeG to guanine by a two-step reaction. The present results have been compared with those obtained considering the helix-turn-helix model where the repair reaction primarily involves cysteine and occurs in a single-step. It is concluded that the repair through tyrosine envisaged in the winged helix-turn-helix model would be less efficient than that through cysteine envisaged in the helix-turn-helix model.