Oligonucleotides containing G.A pairs: effect of flanking sequences on structure and stability.

Sixteen oligodeoxyribonucleotides, 5'd(GXGAYC)3', X and Y = G, A, C, or T, have been synthesized and studied by UV melting and 1H and 31P NMR methods. By varying X and Y, the sixteen resulting oligonucleotides can theoretically form 10 duplexes with all possible Watson-Crick base pairs flanking the center two G.A base pairs. Two-dimensional 1H NMR data on 5'd(GCGAGC)3' revealed that the center bases G and A pair through G amino hydrogen bonding and that the two consecutive G.A pairs form excellent purine-purine stacks. The concurrent appearance of one or more upfield-shifted imino proton peaks (approximately 10.5 ppm) and both upfield- and downfield- shifted 31P signals (approximately -2 and approximately -5.1 ppm) was a unique characteristic in imino 1H and 31P NMR spectra and was used as a conformational probe for this type of G.A pairs. Using this probe, seven out of 10 duplexes of 5'GXGAYC3' were found to adopt the G.A base pairing with G amino proton bonding and G to G and A to A base stacking. Three were in the group comprising 5'pyrimidine-GA-purine3', and four were in the group comprising 5'purine-GA-purine3'/5'pyrimidine-GA-pyrimidine3'. The G.A pairs in 5'purine-GA-pyrimidine3' adopted a totally different conformation. Thermodynamic analysis indicated that duplexes in the 5'pyrimidine-GA-purine3' group were more stable than the duplexes in the 5'purine-GA-pyrimidine3' group. Overall, G.C base pairs were preferred as neighbors to this type of G.A pairs.