Photochemical syn-anti isomerization reaction in N4-hydroxycytosine. An experimental matrix isolation and theoretical study

Photoisomerizations of N4-Hydroxycytosines

A series of N4-hydroxycytosines, unsubstituted or substituted with methyl groups at N3 or C5 atoms of the heterocyclic ring, was studied using the matrix-isolation method. Depending on the absence or presence of the methyl substituent at N3 or C5 atoms (or at both of them) the syn or anti form of the compounds (or a mixture of both forms) was trapped from the gas phase into a low-temperature matrix. Upon UV (lambda > 295 nm) irradiation of the matrixes the syn --> anti as well as the anti --> syn photoisomerization reactions were observed. The syn and anti isomers of N4-hydroxycytosines were identified by comparing their experimental IR spectra with the theoretical spectra calculated at the DFT(B3LYP)/6-31G(d,p) level. For the majority of the studied compounds, the UV induced reactions led to a photostationary state. The position of the final photostationary state was found to be a sensitive function of weak interactions of a studied N4-hydroxycytosine with the matrix environment: solid argon or solid nitrogen. However, not all of the studied photoisomerizations led to a classical photostationary state. For some of the investigated N4-hydroxycytosines, the position of the photostationary state was shifted very strongly in favor of the photoproduct, whereas for some others the position was shifted so strongly in favor of the starting isomer that no photoisomerization was observed. These experimental findings were elucidated by theoretical investigations of the potential energy surfaces of the ground (S0) and first excited (S1) electronic states of N4-hydroxycytosine. The crucial result of these calculations (carried out at the CASSCF level) was the localization of a conical intersection between S0 and S1 at a structure with perpendicular orientation of the hydroxylimino group with respect to the heterocyclic ring.

Photochemical syn-anti isomerization reactions in N2-hydroxyisocytosines--an experimental matrix isolation and theoretical study

N2-hydroxyisocytosine and 1-methyl-N2-hydroxyisocytosine were studied using a matrix isolation technique combined with infrared absorption spectroscopy. For N2-hydroxyisocytosine isolated in an Ar matrix (at 10 K), two imino-oxo isomers, one with the hydroxyimino =N-OH group directed toward the N1-H group (the form called further anti) and the second with the =N-OH group directed toward N3-H (syn), were observed in the ratio 1.4:1. The syn isomer is converted totally to the anti form after UV (lambda > 295 nm) irradiation of the matrix. A small amount of the N(3)H-hydroxy-amino tautomer of N2-hydroxyisocytosine was also detected in the matrix. This form did not react photochemically. For 1-methyl-N2-hydroxyisocytosine, only the syn form of the imino-oxo tautomer was observed after deposition of the matrix. UV (lambda > 295 nm) irradiation induced a photoreaction converting this isomer into the anti form. After 15% of the starting material had been converted into the product, a photostationary state was achieved, and no further progress of the reaction was observed. Subsequent UV irradiation (lambda > 335 nm) caused a back reaction, leading to a disappearance of the anti form and to the recovery of the initial syn isomer. All isomers were identified by comparing their experimental IR spectra with the spectra theoretically calculated at the DFT(B3LYP)/6-31G(d,p) level, where DFT is the density functional theory. Good agreement between the observed and predicted patterns of the spectral lines allowed for reliable identification. The experimental IR spectra were interpreted and discussed. The relative energies of the 12 isomers of N2-hydroxyisocytosine were calculated at the MP2/6-31G(d,p) and MP4//MP2/6-31G(d,p) levels. For six isomers of 1-methyl-N2-hydroxyisocytosine, the calculations were carried out at the MP2/6-31G(d,p) level. The anti form of the imino-oxo tautomer of N-hydroxyisocytosine and the syn form of the imino-oxo tautomer of 1-methyl-N2-hydroxyisocytosine were predicted to be the most stable.