E.D. Radchenko

IR spectra of guanine and hypoxanthine isolated molecules

High resolution spectra of guanine, hypoxanthine, isocytosine, 2-aminopyrimidine and their deutero- and methyl derivatives obtained in Ar matrices by the low temperature IR spectroscopy method are reported. Normal modes of enol tautomers of guanine, 9-CH3-guanine, hypoxanthine and 2-aminopurine are calculated. Force fields are the same as for purine. Results calculated are used to interpret the experimental spectra. Keto—enol tautomerism is shown to exist in guanine and hypoxanthine, the proportions of enol tautomer being 50 and 5%, respectively. Possible biological applications of the results obtained are discussed.

Theoretical and experimental studies of adenine, purine and pyrimidine isolated molecule structure

Abstract High-resolution spectra of adenine isolated molecules and its deutero- and methyl- derivatives, and of purine were obtained using low-temperature IR spectroscopy in inert gas matrices. Normal modes of adenine and purine tautomers were calculated by the valence-force field method to interpret data obtained. Force constants of model compounds —imidazole and pyrimidice—were used in calculations as zero parameters or adenine and purine. Results obtained lead to the conclusion that the imidazole ring tautomerization influences IR spectra band splitting.

Experimental and theoretical studies of molecular structure features of cytosine

Abstract High-resolution vibrational spectra of the heterocyclic nitrogen compounds cytosine, its deutero analogue and pyrimidine, have been obtained by IR spectroscopy in an Ar matrix. On the basis of calculation results, the normal vibrations of isolated molecules are assigned and the forces constants found. It is shown that an isolated molecule of cytosine exists mainly in the enol form, which agrees with theoretical predictions.

Infrared spectra of benzoic acid monomers and dimers in argon matrix

Abstract The infrared spectra of benzoic and deuterobenzoic acids isolated in Ar matrices were measured using the matrix-to-sample ( M / S ) ratios 750 and 250. The spectra were interpreted both by AMl semiempirical and variational methods. The calculation of the potential energy surface of benzoic acid monomer shows that only the syn conformer of benzoic acid must be present in the matrix, which is in complete agreement with experimental data obtained previously. Matrix annealing mainly favours the formation of cyclic symmetrical dimers with two intermolecular H-bonds. The frequency shifts of some vibrations of the COOH group on association are measured. More complex aggregates were revealed in the matrix isolation measurements.

Infrared matrix isolation studies of amino acids. Molecular structure of proline

Abstract IR spectra of proline and deuteroproline isolated in low temperature Ar matrixes have been obtained. It is shown that in the isolated state proline exists in the molecular form. The spectra are interpreted using normal coordinate analysis. It is found that band splitting in the spectra is caused by the occurrence of two proline conformations. The structures of these conformation differ in the position of the COOH group with respect to pyrrolidine ring. The conformations are found to be stabilized by the intramolecular hydrogen bond. It has been shown that conformational equilibrium results in the splitting of most bands in the IR spectra of proline and deuteroproline. This splitting is maximal for CO stretching bands (23 cm −1 ). The structure and relative energies of the conformations are determined by the AM1 quantum chemical method.

The IR spectrum of formic acid in an argon matrix

Abstract A new method of preparation of matrix-isolated molecules of formic acid at low temperatures has been developed. A set of experimental frequencies and intensities of the IR spectrum of monomer molecules of formic acid isolated in a low temperature argon matrix has been obtained. It is shown on the basis of analysis of the IR spectrum that the new method eliminates association and decomposition of molecules of the acid during preparation of the sample.

MNDO and IR spectroscopic studies of tautomerism of isocytosine

Abstract The relative energies of isocytosine tautomeric forms and their dipole moments and ionization potentials have been calculated by the semiempirical MNDO method with complete geometrical optimization. MNDO calculations are shown to predict the most stable amino-enol tautomer. Low-temperature matrix IR spectroscopy gives high resolution spectra of isolated molecules of isocytosine and model compounds. The observed spectra are interpreted in the frame of the model suggesting the existence of two tautomeric forms of isocytosine, enol and ketone, in the frozen gas phase. Theoretical calculations of tautomer vibration spectra, taking into account the empirical force field, indicate that the observed spectrum of isocytosine is the superposition of the spectra of enol and ketone tautomers. The keto-enol equilibrium of isocytosine in the gas phase is shown to shift to the enolic form, which correlates with MNDO calculations.

Vibrational spectra and molecular structure of nucleic acid bases and their complexes in low temperature matrices

Abstract New results of investigations on the molecular structure of canonic bases of uracil and thymine are given. Using theoretical calculations it is shown that increase of band numbers in the range of CO stretching vibrations of the high-resolution IR spectra and uracil and thymine may be due to the existence of the different conformations of these compounds. IR spectra and the structure of hydrates and selfassociates of cytosine isolated in Ar matrices are also studied. Keto-enolic equilibrium is shown to shift to the keto-form on the intermolecular interactions in the matrix.

Proton transfer in autoassociates and hydrates of pyrimidine bases: an MNDO/M study

Abstract The effects of hydration and self-association on the tautomeric equilibria of the pyrimidine bases cytosine and isocytosine have been studied theoretically. Energy and structural characteristics of intermolecular complexes are calculated by the semiempirical quantum chemical MNDO/M method. Reaction coordinate calculations are carried out for three different mechanisms of proton transfer for the keto-enol and amine-imine systems. Activation energy values for these reactions are shown to correlate with the relative content of various tautomeric forms of the compounds studied in low-temperature matrices.

Studies of vibrational spectra of 6-fluorothymine and its methyl derivatives

Abstract The solid and the argon-matrix isolated samples of 6-fluorothymine, 1-methyl-6-fluorothymine, 1,3-dimethyl-6-fluorothymine and 3-methylfluorothymine are studied at low temperatures by i.r. spectroscopy. The spectra obtained are interpreted using the calculation of planar normal vibration frequencies. The electronegative fluorine atom in the 6-position is shown to have a strong influence on the normal vibration frequency changes of crystalline and isolated thymine. NH ⋯ F hydrogen bonds are evidenced to exist in crystalline samples.