Showing papers by "Maciej J. Nowak published in 2012"

Spontaneous tunneling and near-infrared-induced interconversion between the amino-hydroxy conformers of cytosine.

Abstract: Spontaneous and near-infrared/infrared (NIR/IR)-induced interconversions between two amino-hydroxy conformers of monomeric cytosine have been investigated for the compound isolated in a low-temperature argon matrix. Combined use of a laser source (which provides narrowband NIR radiation) and a broadband NIR/IR source of excitation light allowed a detailed investigation of mutual conversions of the two conformers in question. The experiments carried out within the current work demonstrated that upon broadband NIR/IR irradiation (with the IR source of FTIR spectrometer) the population ratio of the two amino-hydroxy conformers changes towards a ratio corresponding to a photostationary state. Evolution of the conformer population ratio towards the photostationary ratio occurred independent of the initial ratio of conformers, which could be prepared by a population shift (in favor of one of the forms) induced by narrowband NIR excitation. Moreover, spontaneous tunneling conversion of the higher-energy conformer into a lower-energy form was observed for cytosine isolated in a low-temperature argon matrix kept in the dark. This process is slow and occurs on a time scale of days. The tunneling process, studied for matrix-isolated cytosine, clearly follows a dispersive type of kinetics rather than the classical monoexponential kinetics.

UV-induced amino → imino hydrogen-atom transfer in 1-methylcytosine.

Abstract: Monomers of 1-methylcytosine, isolated in low-temperature argon matrixes, were excited with narrowband tunable UV light. Irradiation at λ = 314 nm resulted in syn-anti photoisomerizations between the two minor imino-oxo forms of the compound, while the dominating amino-oxo form stayed intact. Subsequent irradiations at 308 nm (as well as at shorter wavelengths) led to the amino → imino hydrogen-atom transfer converting the biologically relevant amino-oxo tautomer into the imino-oxo forms. This is the first report on the amino → imino phototautomerism in 1-methylcytosine. The observed UV-induced syn-anti photoisomerizations within the imino-oxo forms of 1-methylcytosine were found to lead to photostationary states. The photostationary [syn]/[anti] population ratio depended on the wavelength of the exciting UV light. This dependence was not monotonous. Spectral indications of the open-ring isocyanate product, generated from the amino-oxo tautomer upon UV (λ ≤ 308 nm) irradiation, were also observed.

Photochemical isomerizations of thiosemicarbazide, a matrix isolation study.

Abstract: Two thione conformers of monomeric thiosemicarbazide were trapped from the gas phase into a low-temperature Ar matrix. A phototransformation converting the less stable form of the compound into the most stable conformer was induced by irradiation with near-IR (λ = 1462 nm) or UV (λ > 320 nm) light. This photoeffect allowed separation of the IR spectra of the observed thione forms. The structures of both observed isomers were identified by comparison of the separated experimental IR spectra with the spectra theoretically predicted for two most stable forms of the compound. The population ratio of the two conformers in an Ar matrix, prior to any irradiation, was estimated to be equal ≈2:1. Irradiation of matrix-isolated thiosemicarbazide with shorter-wavelength UV (λ > 270 nm) light induced a phototautomeric reaction generating thiol forms of the compound.