UNUSUAL CONTRIBUTION OF 2-AMINOADENINE TO THE THERMOSTABILITY OF DNA

The poly(dA-dU) and poly(dI-dC) duplexes have very similar thermostabi lities (T-m). This similarity extends also to the pyrimidine 5-methyl group-containing poly(dA-dT) and poly(dI-m(5)dC). The differences betw een chemical structures of the A:U and I:C or the A:T and I:m(5)C base -pairs seem to be unimportant for the thermostability of the DNA. Howe ver, on the insertion of an amino group into position 2 of the purines the similarities disappear. Thermostabilities of poly(n(2)dA-dU) and poly(n(2)dG-dC) as well as the poly(n(2)dA-dT) and poly(dG-m(5)dC) are radically different. This is also the case with their other 5-substit uted pyrimidine-containing derivatives, the 5-ethyl, 5-n-butyl and 5-b romo analogues. The G:C-based polynucleotides are more stable by an av erage of 40 degrees C than the n(2)A.U-based ones. Poly(dA,n(2)dA-dT)- s containing various proportions of A and n(2)A as well as the natural DNA of S-2L cyanophage that contains n(2)A bases instead of A were al so studied. It was found that dependence of T-m on the n(2)A-content w as non-linear and that the lower T-m is not the consequence of a parti cular nucleotide sequence. The possible structural reasons for the low er thermostabilization of these B-DNAs by the n(2)A:T base-pair as com pared to the G:C are discussed.