Conformational analysis of the octamer d(G‐G‐C‐C‐G‐G‐C‐C) in aqueous solution

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Abstract
Proton NMR studies in H2O and 2H2O were carried out on the self‐complementary DNA octamer d(G‐G‐C‐C‐G‐G‐C‐C) and compared with similar studies on the dimethylated analogue d(G‐G‐m5C‐m5C‐G‐G‐C‐C) [Sanderson, M. R., Mellema, J.‐R., van der Marel, G. A., Wille, G., van Boom, J. H. & Altona, C. (1983) Nucleic Acids Res. 11, 3333–3346]. Base, H1′, H2′ and H2” resonances were assigned by means of two‐dimensional nuclear Overhauser spectroscopy (NOESY) and correlated spectroscopy (COSY) experiments. Chemical shift vs temperature profiles were used to analyze the temperature‐dependent conformational behaviour and to extract thermodynamic parameters for the duplex‐to‐coil transition. Analysis of proton‐proton couplings were used to discriminate between J1′2′ and J1′2″ and between the H2′ and H2″; resonances as well as to obtain conformational parameters of the sugar rings. From the NOESY and COSY experiments, the temperature profiles and the analysis of the coupling data it is concluded that: (a) the compound adopts a B‐DNA‐type helix in solution; (b) the sugar rings in the intact duplex display limited conformational freedom; (c) methylation of the cytosine bases at the C5 position has no measurable effect on the conformational behaviour of the octamer, nor on the conformation of the sugar rings; however, methylation does increase the stability of the duplex in aqueous solution under conditions of low salt concentration.

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