Abstract
We have previously suggested that variations in the 31P chemical shifts of individual phosphates in duplex oligonucleotides are attributable to torsional angle changes in the deoxyribose phosphate backbone. This hypothesis is not directly supported by analysis of the 1H/31P two-dimensional J-resolved spectra of a number of mismatch dodecamer oligonucleotide duplexes including the following sequences: d-(CGTGAATTCGCG), d(CGUGAATTCGCG), d(CGGGAATTCGCG), d(CGAGAATTCGCG), and d(CGCGAATTCACG). The 31P NMR signals of the dodecamer mismatch duplexes were assigned by 2D 1H/31P pure absorption phase constant time (PAC) heteronuclear correlation spectra. From the assigned H3' and H4' signals, the 31P signals of the base-pair mismatch dodecamers were identified. JH3'-P coupling constants for each of the phosphates of the dodecamers were obtained from 1H/31P J-resolved selective proton flip 2D spectra. By use of a modified Karplus relationship, the C4'-C3'-O3'-P torsional angles (epsilon) were obtained. JH3'-P coupling constants were measured for many of the oligonucleotides as a function of temperature. There exists a good linear correlation between 31P chemical shifts and the epsilon torsional angle. This correlation can be further extended to the C3'-O3'-P-O5' torsional angle (zeta) by using a linear relationship between epsilon and zeta obtained from crystal structure studies. The 31P chemical shifts follow the general observation that the more internally the phosphate is located within the oligonucleotide sequence, the more upfield the 31P resonance occurs. In addition, 31P chemical shifts show sequence- and site-specific variations. Analysis of the backbone torsional angle variations from the coupling constant analysis has provided additional information regarding the origin of these variations in 31P chemical shifts.
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