Influences of urea and guanidine hydrochloride on the interaction of 6-thioguanine with bovine serum albumin

Presenting a new fluorescent probe, methyl(10-phenylphenanthren-9-yl)sulfane sensitive to the polarity and rigidity of the microenvironment: applications toward microheterogeneous systems

A molecule, methyl(10-phenylphenanthren-9-yl)sulfane (MPPS), with a straightforward structure, has been synthesized, characterized, and explored as a new fluorescent probe for microheterogeneous systems. The photophysical properties of MPPS have been studied through experimental and theoretical calculations using the range-separated hybrid functional CAM-B3LYP in conjunction with a 6-311++g(d,p) basis set. Theoretical calculations show that the freely rotating phenyl ring forms a 94° dihedral angle with the phenanthrene ring in the ground state. Experimentally found two absorption bands correspond to the n → π* and π → π* transitions supported by the frontier molecular orbital calculations. Two excited singlet states, E-1 and E-2 (the former being more stable than the latter in the gas phase), exist with dihedral angles between the phenyl and phenanthrene rings as 142° and 133°, respectively, in the gas phase. Two emitting states in a condensed medium of varying polarities are supported by the steady-state fluorescence and fluorescence intensity decay data. Emission energies, fluorescence intensities, and excited singlet state lifetimes change with the polarity of the solvents. To support that the free rotation in the molecule is responsible for these changes, the fluorescence properties of another molecule, methyl(10-(o-tolyl)phenanthren-9-yl)sulfane (MTPS), with restricted rotation of the substituted benzene, i.e., o-tolyl ring have been studied. The fast-intensity decay component of MPPS is ascribed to the conformer in the E-1 state. The molecule has proved to be an excellent polarity probe explored to determine the critical micelle concentrations (cmc) values of different surfactants, which agree well with the literature reports. Different regions of binding isotherm (specific, non-cooperative, cooperative, and massive binding) of a gemini surfactant, 12-6-12,2Br- with bovine serum albumin (BSA) have been successfully demonstrated by the steady-state and time-resolved fluorescence and fluorescence anisotropic properties of MPPS. Docking results show that MPPS resides in the hydrophobic pocket of BSA. The fluorescence quenching of BSA by MPPS reveals the location of Trp residues of BSA. Thus, a polarity and molecular rigidity-sensitive fluorescent molecule, MPPS has been presented here that can potentially be used to monitor the changes in the microenvironment of biomolecules in different processes.

Characteristics of the excited triplet states of thiolated guanosine derivatives and singlet oxygen generation

Thioguanine is sensitive to UVA light and generates singlet molecular oxygen (1O2*) when exposed to UVA. Three thioguanosine derivatives, 2',3',5'-tri-O-acetyl-6-thioguanosine (ta6TGuo), 2',3',5'-tri-O-acetyl-8-thioguanosine (ta8TGuo), and 2',3',5'-tri-O-acetyl-6,8-dithioguanosine (taDTGuo) were explored photophysically and photochemically. Nanosecond transient absorption and time-resolved near-infrared emission measurements were carried out to investigate the characteristics of their excited triplet states in acetonitrile solution. The quantum yield of intersystem crossing (ΦISC), the intrinsic decay rate constant (k0), the quenching rate constant by 3O2 (kq) and the self-quenching rate constant (kSQ) of their triplet states were all determined. From the precise analysis of the quantum yield of 1O2* generation (ΦΔ) against the concentration of dissolved molecular oxygen, the fraction of the triplet states quenched by dissolved oxygen which gives rise to 1O2* formation (SΔ) was successfully obtained with high accuracy. The ΦΔ values at low oxygen concentrations reveal that these thioguanosines, particularly taDTGuo, can still effectively generate 1O2* at low molecular oxygen concentrations like carcinomatous microenvironments. These findings indicate that taDTGuo would perform well as a potential agent for photo-induced cancer therapies.