K. Szczepaniak

Matrix isolation infrared studies of nucleic acid constituents: Part 4. Guanine and 9-methylguanine monomers and their keto—enol tautomerism☆

Abstract Infrared absorption spectra have been studied for matrix isolated guanine and 9-methylguanine, which is a formal analogue of the natural nucleoside found in DNA and RNA. Three related compounds (isocytosine, 2-amino-5-chloropyrimidine and 2-dimethyl-amino-6-hydroxypurine) have also been examined. The spectra provide evidence for the existence of guanine and of 9-methylguanine as mixtures of the enol-amino and keto-amino tautomers, although the keto-amino tautomer is the only tautomeric form found in solution and in solid. The estimated enol—keto equilibrium constant fund in nitrogen matrices K = [E]/[K] is about 3.6 for guanine and 5.9 for 9-methylguanine. The significance of these results is evaluated in relation to the types of tautomers found in natural nucleic acids and to the concept of spontaneous and induced mutations caused by mis-pairing of the bases in the nucleic acids.

Tautomeric equilibria of 2(4)-monooxopyrimidines in the gas phase, in low-temperature matrices and in solution

Abstract IR absorption spectra, including the NH, OH and CO stretching regions, have been recorded for 4-oxo-6-methyl- and 2-oxo-4,6-dimethyl pyrimidines and several related derivatives, in the gas phase, in low-temperature inert matrices, and in several liquid solvents. All the 4-oxopyrimidines in the gas phase, and 4-oxo-6-methylpyrimidine in low-temperature matrices, exhibit comparable populations of the keto and enol forms. By contrast the 2-oxopyrimidines are predominantly in the enol forms. Both classes of com pounds are predominantly in the keto form in liquid solvent systems. The tautomeric equilibrium constant ( K T ) in the vapour phase for 4-oxo-2,6-dimethylpyrimidine is about 2, and for the other 4-oxopyrimidines is about 1. For 4-oxo-6-methylpyrimidine, the equilibrium constant in inert matrices varies slightly with the activity of the matrix gas, with the keto tautomer favoured in the more active matrix. From the temperature-dependence of K T the free energy difference between the two tautomeric forms of 4-oxo-6-methylpyrimidine in the vapour phase has been calculated. Heats of vaporization have also been calculated for the various compounds and related to their abilities to associate by hydrogen bonding in the condensed phase. The UV absorption spectra of some of the foregoing have also been recorded in the gas phase, but these were of only limited value in studies of tautomeric equilibria, as connpared to the IR spectra.

Matrix isolation studies of nucleic acid constituents—III. 1-Methyluracil, 3-methyluracil and 1,3-dimethyluracil monomers

Abstract Infrared spectral studies are reported for 1-methyluracil and its N 3 -deuterated homologue and also for 3-methyl- and 1,3-dimethyluracil, all isolated in argon matrices. The effects of N 3 -deuteration, and of methylation at N 1 and N 3 , on the spectrum of the parent uracil molecule are profound and have been investigated in detail. It is shown that the frequencies and relative intensities of the vibrational bands in the spectrum of the monomeric molecules exhibit a complicated dependence on N -deuteration or N -methylation. The spectra are interpreted by comparison with the predicted spectra using force constants and intensity parameters (APTs) from quantum mechanical ab initio calculations with STO-3G (force constants) and 4–31G (intensities) basis sets of the i.r. spectrum for the in-plane modes of uracil. Appropriate corrections were taken into account so that these predictions apply to the deuterated and methylated uracils.

Effect of intermolecular interactions on the infrared spectrum of 1-methyluracil

Abstract Results of studies of the effect of hydrogen bonding (in crystalline amorphous solid phases at 10 and 300 K) and of the effect of weak intermolecular interactions with different matrix materials (Ar,Xe,N2) on the i.r. spectra of 1-methyluracil (1-MeU) are reported and discussed. Comparison of the spectral properties, particularly the frequencies of the normal modes of vibration by hydrogen-bonded molecules of 1-MeU in different solid phases and at different temperatures (10, 300K) with the corresponding properties of monomeric molecules isolated in argon, zenon and in nitrogen matrices allows us to determine the effect of intermolecular interactions on each normal mode of the 1-MeU molecule, and to obtain a quantitative measure of this effect expressed by the frequency shifts of each normal mode.

Infrared matrix isolation studies on tautomerism of cytosine and isocytosine methyl-derivatives

Abstract IR N 2 matrix isolation spectra have been recorded in the OH, NH and CO stretching region. The spectra were interpreted to determine the structure of investigated compounds: 1-methylcytosine predominates in the aminooxo form I; 3-methylcytosine and 1-methylisocytosine - iminooxo III and IV; 6-methylisocytosine and N(2)-monomethylaminoisocytosine - aminohydroxy forms VII and VIII.

Autoassociates and tautomerism of 2-oxo-5-halogenopyrimidines: theoretical and experimental investigations

Abstract Theoretical calculations (on a semi-empirical level) of energy and geometry of the autoassociates of two tautomeric forms of 2-oxo-5X-pyrimidines (X  H, Cl, Br) are presented. On the basis of the calculated energies of autoassociates it is possible to explain the enol—keto tautomeric transition between the gas phase and condensed phases. We propose a double-proton transfer reaction as a possible mechanism for the tautomeric transition. Infrared absorption spectra of 2-oxo-5X-pyrimidines in the v (NH …) and v (CO) regions in solid phases and in low-temperature argon matrices are also presented and discussed. Comparison of IR spectra and results of the theoretical calculations with known crystallographic structures of 2-oxo-5X-pyrimidines (X  H, F, Cl) lead to the conclusion that the crystal structure of the 5-bromo derivative should be similar to that of the 5-chloro derivative.

Spectroscopic Studies on Vapour Phase Tautomerism of Natural Bases Found in Nucleic Acids

Infrared absorption spectra, in the vapour phase, have been recorded in the regions of NH, OH and carbonyl stretching frequencies, for a series of 1-substituted uracils and 9-substituted adenine and hypoxanthine, formal analogues of the natural nucleosides of uracil, adenine and hypoxanthine found in DNA and/or RNA. A number of related analogues was also examined, including the N-methyl derivatives of the known mutagen and chemotherapeutic agent, 5-fluorouracil. The infrared absorption spectra provide unequivocal evidence for the existence of 1-substituted uracils as the 2,4-diketo tautomer, of 9-substituted hypoxanthine as the 6-keto tautomer, and of 9-substituted adenine as the 6-amino tautomer. These are the known predominant tautomeric forms in solution, so that the gas phase results are in sharp contrast with those for other nitrogen heterocycles, where the tautomeric equilibrium constants may differ by several orders of magnitude in going from solution to the vapour phase. The significance of these results is evaluated in relation to the types of heterocyclic bases found in natural nucleic acids, and to concepts of spontaneous and induced mutations in terms of mis-pairing. The ultraviolet absorption spectra of the various compounds have also been examined in the gas phase, but proved of relatively limited use in studies on tautomeric equilibria under these conditions. Heats of vaporization have been determined for most of the compounds examined. In particular the heat of vaporization for 1,3-dimethyluracil, which is incapable of association by hydrogen bonding in the condensed phase, is much lower than for uracil and 1 (3) -methyluracils which can associate by hydrogen bonding.

Measurement of the absolute infrared intensity of the fundamental vibration of HCl in low temperature matrices

Abstract Using the concentration of HCl in the argon mixture in the gas phase (1:2000), the pathlength of a film (estimated by monitoring interference fringes) and the measured integrated absorbance of the HCl band, we have obtained the absolute intensity, A, for monomeric HCl isolated in the argon matrix. The average value is A = 40±5 km mole −1 , comparing very closely with the intensity of A=39km mole −1 of HCl in the gas phase. These results are compared with the studies of the intensity of HCl in other environments and correlated with the proton affinities of the matrix molecules or atoms.

A modulation of the anaphylactic basophil histamine release by selective H2 histamine agonists.

Two selective H2-histamine agonists, dimaprit and impromidine, have been tested for their action on histamine release from human basophils and rat mast cells.IgE-mediated basophil histamine release was inhibited by stimulation of histamine H2-receptors. However, differences between the actions of both dimaprit and impromidine were noticed. Both impromidine and dimaprit had no specific effect on 48/80-induced histamine release from rat mast cells, although the latter in higher concentrations either slightly increased spontaneous histamine release or non-specifically inhibited compound 48/80-induced release.The results are consistent with the view that activation of adenylate cyclase via H2-histamine receptors might be an important regulatory mechanism of histamine release from human basophils but not from rat mast cells.