Unusual contribution of 2-aminoadenine to the thermostability of DNA

The poly(dA-dU) and poly(dI-dC) duplexes have very similar thermostabilities (Tm). This similarity extends also to the pyrimidine 5-methyl group-containing poly(dA-dT) and poly(dI-m5dC). The differences between chemical structures of the A:U and I:C or the A:T and I:m5C base-pairs seem to be unimportant for the thermostability of the DNA. However, on the insertion of an amino group into position 2 of the purines the similarities disappear. Thermostabilities of poly(n2dA-dU) and poly(dG-dC) as well as the poly(n2dA-dT) and poly(dG-m5dC) are radically different. This is also the case with their other 5-substituted pyrimidine-containing derivatives, the 5-ethyl, 5-n-butyl and 5-bromo analogues. The G:C-based polynucleotides are more stable by an average of 40 degrees C than the n2A.U-based ones. Poly(dA,n2dA-dT)-s containing various proportions of A and n2A as well as the natural DNA of S-2L cyanophage that contains n2A bases instead of A were also studied. It was found that dependence of Tm on the n2A-content was non-linear and that the lower Tm is not the consequence of a particular nucleotide sequence. The possible structural reasons for the lower thermostabilization of these B-DNAs by the n2A:T base-pair as compared to the G:C are discussed.