ISSN:
0377-0486
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
The rates (kH) of deuterium exchange of 8-CH groups, as well as the rates (kD) of protium exchange of 8-CD groups in 5′-IMP and cIMP, were measured by laser Raman spectrophotometry over the temperature range 30-80°C. The kH, or ‘forward’ rates, were determined from exchange in D2O solutions, while the kD, or ‘reverse’ rates, were determined from exchange in H2O solutions of the nucleotides. The hypoxanthine nucleotides were found to exchange with rates intermediate between those of corresponding adenine and guanine nucleotides, reported previously. The values obtained for the rate constants as a function of temperature (k=A exp (-Ea/RT)) yield the following Arrhenius activation energies and frequency factors. For 5′-IMP: EaH=25.5±0.6 kcal mol-1, AH=2.61×1015h-1; EaD=24.9±0.6 kcal mol-1, AD=4.32×1014h-1. For cIMP: EaH=22.2±0.6 kcal mol-1, AH=2.52×1013h-1; EaD=21.4±0.6 kcal mol-1, AD=3.93×1012h-1. The lower activation energy for cIMP accounts for its more rapid exchange vis-à-vis 5′-IMP, confirming the trend observed also for adenine and guanine nucleotides. Moreover, as in the cases of adenine and guanine isomers, the differential exchange between cyclic and 5′-nucleotides of hypoxanthine is large at lower temperatures and small at higher temperatures. These results confirm that exchange at 8-C is influenced by the ribosyl phosphate group. The present data also provide evidence of small kinetic isotope effects in the 8-C exchange reactions. For 5′-IMP, kH/kD≈2.5, and for cIMP, kH/kD≈1.7. The isotope effects of 5′-IMP and cIMP are close to the values observed for guanine nucleotides. Finally, Raman vibrational spectra of four isotopic modifications of the hypoxanthine nucleotides are compared to provide specific assignments of the Raman bands to vibrations of the purine skeleton and its exocyclic substituents.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jrs.1250110616
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