Copolymers of adenylic and 2-aminoadenylic acids. Effect of progressive changes in hydrogen bonding and stacking on interaction with poly(uridylic acid).

Random copolymers of adenylic acid and 2-aminoadenylic acid form both double and triple helices with poly(uridylic acid) [poly(U)]. Transition temperatures of two-stranded helices increase with 2NH2A content, exhibiting a slight positive departure from linearity and indicating that the contribution to helix stability arising from introduction of the 2-amino group does not significantly depend upon base sequence. We have shown previously that poly(2NH2A) . poly(U) does not undergo a disproportionation reaction (2 leads to 3 transition). Extrapolation from melting curves of 1:1 complexes between A,2NH2A copolymers and poly(U) indicates a Tm for the 2 leads to 3 transition of poly(2NH2A) . poly(U) which is too high to be observable under normal conditions. Addition of an organic solvent (50% ethylene glycol), however, lowers Tm by promoting unstacking of single-stranded poly(2NH2A) sufficiently to permit observation of the disproportionation of poly(2NH2A) . poly(U) for the first time. Transition breadths of 1:1 complexes of A,2NH2A copolymers with poly(U) are greater than those of either of the homopolymer complexes in the middle range of composition (67 and 48% A) but not at 25% A. These results are consistent with previous calculations on the effect of heterogeneity in base-pair stability on DNA transition breadths. In the poly(A), poly(U) system, Et4N+ counterion reduces the Tm of the double and triple helices by 26 and 41 degrees C, respectively. The larger depression in the latter case arises from the higher charge density of the triple helix and less effective counterion screening by Et4N+. In the poly(2NH2A), poly(U) system Tm,2 leads to 1 is reduced by 24 degrees C, but extrapolation of the copolymer results indicates a reduction of approximately 100 degrees C for Tm,3 leads to 2, accounting for previous failure to observe a triple helix in this system. CD spectra of A,2NH2A copolymers suggest that much of the spectral region can be regarded as a contribution of the CD spectra of the parent polymers poly(A) and poly(2NH2A) but that the region from 255 to 275 nm requires that contributions made by longer range interactions be taken into account.