Aqueous solvation effect on the prototropic tautomerism of 2-thiocytosine

The Effect of Sulphur Atom on the Structure of Biomolecule 2-Thiocytosine in the Gas-Phase, Solid-State, and Hydrated Forms and in DNA-DNA Microhelices as Compared to Canonical Ones

This study is focused on the effects of the sulphur atom in position 2 of the cytosine molecule, 2-thiocytosine (2TC), on the molecular structural parameters in the isolated state, as well as in the hydration, solid state arrangement, Watson-Crick pairs, and DNA-DNA microhelices, as compared to the canonical form. The main six tautomers were optimised at the MP2 and CCSD levels, and the sulphur atom does not show any effect on the stability trend of cytosine. The energy difference between T2b and T2a tautomers is twice as low in 2TC (1.15 kJ/mol) than in cytosine (2.69 kJ/mol). The IR and laser Raman spectra of 2TC were accurately assigned using DFT computations and solid-state simulations of the crystal unit cell through several tetramer forms. The results notably improve those previously published by other authors. The effect of explicit water molecules surrounding 2TC up to 30, corresponding to the first and second hydration shells, on geometries and tautomerism was analysed. The Watson-Crick base pairs' stability (ΔECP = -97.458 kJ/mol) was found to be less than with cytosine (-105.930 kJ/mol). The calculated dipole moment was also lower (4.205 D) than with cytosine (5.793 D). The effect of 2TC on the 5'-dG-dC-dG-3' and 5'-dA-dC-dA-3' DNA-DNA optimised microhelices was evaluated through their calculated helical parameters, which indicates a clear deformation of the helix formation. The radius (R) with 2TC appears considerably shorter (6.200 Å) in the 5'-dA-dC-dA-3' microhelix than that with cytosine (7.050 Å). Because of the special characteristics of the 2TC molecule, it can be used as an anticancer drug.

Resonance Raman spectroscopic and density functional theoretical study on microsolvated 2-Thiocytosine clusters with polar solvents

Microsolvation effects on the excited state deactivation dynamics of 2-thiocytosine (2tC) were studied in hydrogen-bonded 2tC clusters with protic solvents using resonance Raman, FT-IR, FT-Raman, UV-vis spectroscopy combining with density functional theoretical calculation. Two protic solvents, water (H₂O) and methanol (MeOH), and one aprotic solvent, acetonitrile (MeCN), were used to investigate the 2tC(H₂O)_1-5, 2tC(MeOH)_1-5, and 2tC(MeCN)_1-3 microsolvated clusters. In CH₃OH and H₂O solvents, most of the Raman shifts were due to the vibration modes of 2tC(solvent)_n (solvent = H₂O, CH₃OH; n = 1-4) clusters via intermolecular NH⋯O hydrogen bonds (HB). The intermolecular >NH⋯O hydrogen bond interactions, which are the key constituents of stable thione structure of 2tC, revealed the spectra difference of 2tC in CH₃CN, CH₃OH and H₂O. With the aid of electronic structural and vibration frequency calculations, the observed Raman spectra were assigned to the low energy isomers of 2tC(solvent)₂ (solvent = H₂O, CH₃OH) clusters in water and methanol and 2tC(CH₃CN) in acetonitrile solvents. 2tC(solvent)₂ clusters in water and methanol may prohibit or promote excited state proton transfer reaction from sulfur atom to neighbor nitrogen atom due to the hydrogen bonding chain between 2tC and protic solvent molecules. Our experimental and theoretical studies confirmed that the hydrogen bond sites were located on the specified functional group SCNH of 2tC with solvent molecules.

Solvatochromic Effects on the Absorption Spectrum of 2-Thiocytosine

The solvatochromic effects of six different solvents on the UV absorption spectrum of 2-thiocytosine have been studied by a combination of experimental and theoretical techniques. The steady-state absorption spectra show significant shifts of the absorption bands, where in more polar solvents the first absorption maximum shifts to higher transition energies and the second maximum to lower energies. The observed solvatochromic shifts have been rationalized using three popular solvatochromic scales and with high-level multireference quantum chemistry calculations including implicit and explicit solvent effects. It has been found that the dipole moments of the excited states account for some general shifts in the excitation energies, whereas the explicit solvent interactions explain the differences in the spectra recorded in the different solvents.

Modeling of purine derivatives transport across cell membranes based on their partition coefficient determination and quantum chemical calculations

Mercaptopurine (6-MP), thioguanine (6-TG), and azathioprine (AZA) are purine antimetabolites introduced as anticancer or immunosuppressive drugs decades ago. Methylated AZA, called MAZA, is among the investigational drugs. The present study compares MAZA to the widely recognized drugs AZA, 6-MP, and 6-TG with respect to the ability of being transported across cell membranes. The obtained octanol/water phases partition coefficients and results of quantum chemical calculations predict the following sequence of hydrophobicity: MAZA > AZA > 6-TG > 6-MP.