Effect of 5-alkyl substitution of uracil on the thermal stability of poly [d(A-r5U)] copolymers.

The thermal transition of polyd(A-r5U)] polydeoxynucleotides (where r was a hydrogen atom, or a methyl, ethyl, n-propyl, n-butyl or n-pentyl group) was studied by measuring the derivative melting profiles of the polymers in the range of 0.01--0.36 M K+, at pH 6.8. According to the Tm values, polydeoxynucleotide analogues show lower thermal stability than polyd(A-T)] at any counterion concentration applied. At a given salt concentration, Tm of the alkyl analogues decreased as the number of carbon atoms (n) in the r substituent of polyd(A-r5U)] increased. 1/Tm plotted against against 1/n yielded a linear relationship. Cooperativity of the melting of all polyd(A--U)] analogues decreased with the increase of salt concentration in the solution. This change depended again on 5-substitution of the uracil moiety of polyd(A-U)]. Smallest decrease was observed in the case of polyd(A--U)] whereas largest decrease was shown by polyd(A-pe5U)] (pe=pentyl group).