Carbon-13 magnetic resonance. XVII. Pyrimidine and purine nucleosides

Abstract

The natural abundance carbon-13 nuclear magnetic resonance spectra of 29 nucleosides, including those naturally occurring, are described. The spectra may be divided into two regions: that due to the nitrogen base ( -37-33 ppm) and that due to the ribose moiety (37-89 ppm). The relative invariance of the ribose carbon shifts with nitrogen base has enabled their assignment independent of base. Total assignments are also given for the carbon atoms in the pyrimidine and purine bases. The observed shifts for the naturally occurring nucleosides correlate at least qualitatively with MO parameters derived for the parent bases. Consequently, it is apparent that the electronic structure of the nucleosides is reflected in the carbon-13 shifts. Furthermore, the shifts correlate reasonably well with the known chemical reactivity of these molecules. A cautionary note is added to prevent misconceptions relating to the simplicity of the factors contributing to the carbon-13 chemical shift.Recent advances in the theory and instrumentation of nuclear magnetic resonance have made this technique most promising for studies of bioorganic and even biopolymer molecules. Since the pioneering work of the Jardetzkys,2 much progress has been made in the area of proton magnetic resonance (pmr) studies on the pyrimidine and purine bases, nucleosides, and nucleotides.3•4 The few protons on the pyrimidine or purine rings generally lead to well-resolved and easily interpretable pmr spectra which are dependent upon concentration, pH, and temperature, as well as other factors.4The pmr technique is limited in this area due to the need for sites in the molecular framework to be proton bearing.Carbon-13 magnetic resonance spectroscopy (cmr) affords an opportunity to study in detail the structure of the molecular framework, with only the exception of the heteroatom sites. The carbon-13 spectra of the unsubstituted bases purine5 and pyrimidine6 have been determined. The spectrum of purine5 provided the first example of the analysis of carbon-13 spectra of biologically important molecules and the ordering of the chemical shifts C-2, C-8, and C-6 differed from that noted for the corresponding directly bonded protons (6,2,8).7 A gross correlation of the carbon-13 shift data with theoretical estimates of charge density was observed suggesting that pmr shifts are somewhat less reliable in considerations of the relation between shift and charge. More recently correlation of the carbon-13 shifts with total charge has been shown to be more reliable.8 Further, the variation in the ordering of the (1) (a)

Expression of mammalian glutathione S-transferase 5-5 in Salmonella typhimurium TA1535 leads to base-pair mutations upon exposure to dihalomethanes.

“…4B). (34)(35)(36), except the added methylene linking the dGuo and GSH. Attachment at the N2 atom was indicated by the lack of UV spectral properties associated with adducts formed at other sites on the guanine moiety and confirmed by the '5N splitting of the -CH2-protons (derived from the methylene moiety of S-(1-acetoxymethyl)GSHJ and especially the 13C signals in the product obtaned when the reaction was done using [N2-15N]dGuo (Fig.…”

Carbon‐13 Nuclear‐Magnetic‐Resonance Studies on Selected Amino Acids, Peptides, and Proteins

“…The sensitivity of 13C chemical shifts to pH provides a means of obtaining the pK values of individual groups and offers the possibility of determining the electron density distribution in biologically importanc compounds [54]. Further, the characteristic pH dependence of 13C chemical shifts may be useful in the assignment of resonances in more complex molecules, a first step in obtaining structural information.…”

Assignments in the Carbon-13 Nuclear Magnetic Resonance Spectra of Vitamin B 12 , Coenzyme B 12 , and Other Corrinoids: Application of Partially-Relaxed Fourier Transform Spectroscopy

“…Differentiation within the pairs was done for C-4,7 and C-8,9 by comparison of the spectra of dicyanocobalamin and cyanocobalamin, on the assumption that the resonances that shift appreciably upon coordination are closer to the coordinating nitrogen of the benziminazole ring than resonances that are not shifted. The base carbons of the 5'-deoxyadenosyl group in coenzyme B12 (Table 3) were assigned (17) by comparison of the spectra of cyanocobalamin, coenzyme B12, and adenosine. The methylene carbon directly attached to cobalt in coenzyme B12 was not detected, probably as a result of line broadening arising from interaction with 59Co (see above).…”