G.G. Sheina

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.

The rotamerization of conformers of glycine isolated in inert gas matrices. An infrared spectroscopic study

Abstract The infrared spectra of glycine isolated in Ne, Ar and Kr matrices have been measured. The matrix-isolated glycine is shown to be in the molecular form. The spectral manifestations, both conformational and site splitting, are separated. Three different conformers of glycine have been identified experimentally for the first time. It is shown that during the deposition of the samples the substrate temperature must be lowered to 13 K — this is a decisive factor permitting fixation of a complete set of glycine conformers in an inert matrix. The relative energies of the three glycine conformers are estimated to be 0, 1.3–1.6 and 0.9–1.5 kcal/mol.

FTIR spectroscopic study of the UV-induced rotamerization of glycine in the low temperature matrices (Kr, Ar, Ne)

Using UV irradiation, the spectral bands of three glycine conformers in the FTIR matrix spectra were separated in the range 4000–180 cm−1. The absolute infrared intensities of the spectral bands of these conformers were measured. The spectral manifestations of the weak intramolecular H-bonds NH2---O in conformers I, III and the strong H-bond N---H-O-C in conformer II were found. The influence of the matrix cage on the efficiency of UV isomerization was demonstrated. The thermodynamic parameters of glycine conformers at T=358 K were established as: KI–II=0.13(±0.025), ΔHI–II=1000 (±600) J mol−1, ΔSI–II=−14.2(±2) J mol−1 K−1; and KI–III=0.16(±0.02), ΔHI–III=5800(±600) J mol−1, ΔSI–III=1(±2) J mol−1 K−1.

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.

FTIR spectroscopy of uracil derivatives isolated in Kr, Ar and Ne matrices: matrix effect and Fermi resonance

Abstract The vibrational spectra of uracil (2,4-dioxopyrimidine), 1,3-dideuterouracil and 5-methyluracil (thymine) in low temperature Kr, Ar and Ne matrices have been studied by Fourier-transform infrared (FTIR) spectroscopy. The absolute infrared intensities and frequencies of the absorption bands in the infrared (IR) spectra of uracil have been measured. The hypotheses of non-planar conformations of uracil and 5-methyluracil has been tested experimentally. It is shown that the increasing number of spectral bands can be attributed to the Fermi resonance and the matrix splitting. The matrix (Kr, Ar, Ne) effect on fine structure of the absorption bands and Fermi resonance in the IR spectra of uracil, 1,3-dideuterouracil, 5-methyluracil has been studied experimentally.

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.

FTIR investigation of the effect of matrices (Kr, Ar, Ne) on the UV-induced isomerization of the monomeric links of biopolymers

Abstract UV-induced isomerization of monomeric glycine and 2-hydroxypyrimidine in Kr, Ar and Ne matrices was investigated using Fourier transform infrared (FTIR) spectroscopy. The spectral bands of the three conformers of glycine and two tautomers of 2-hydroxypyrimidine were detected unambiguously. UV irradiation caused the partial conversion of the hydroxy form of 2-hydroxypyrimidine into the oxo form. The efficiency of the isomerization of glycine and 2-hydroxypyrimidine was reduced in the Ne matrix in comparison with the Ar and Kr matrices.