The B-Z conformational transition and aggregation of poly[d(G-C)] induced by moderate concentrations of Mg(ClO 4) 2

Mg(ClO 4) 2 induces the cooperative B-to-Z transition of poly[d(G-C)]; the salt concentration at the midpoint is 0.26 M. A comparison with previous data for NaCl, MgCl 2 and NaClO 4 (F.M. Pohl and T.M. Jovin, J. Mol. Biol. 67 (1972) 375) indicates that Mg(ClO 4) 2 is more effective than would be anticipated from the simple additive effects of the Mg 2+ and ClO 4 − ions (the ionic strengths of the respective transition points are: NaCl, 2.4; MgCl 2, 2.1; NaClO 4, 1.8 and Mg(ClO 4) 2, 0.78). These results suggest the importance of specific interactions involving ClO 4 −, particularly in the presence of Mg 2+. The B–Z transition of poly[d(G-C)] can be monitored spectroscopically via the large hyperchromic shift at 295 nm and the inversion in the CD spectrum. The reaction is fully reversible and can be fitted by a monoexponential function with half times varying between 8 and 150 min. The observed relaxation times are strongly dependent on the concentration of Mg(ClO 4) 2 with a distinct maximum at the transition point, in accordance with a concerted mechanism involving only the B and Z states. As the polymer assumes the Z conformation it progressively aggregates into a gel-like precipitate, which, however, redissolves rapidly upon lowering the salt concentration. The natural DNA from Micrococcus lysodeikticus which has a high GC content of 72% is also precipitated by Mg(ClO 4) 2 but we do not have direct spectroscopic evidence for the involvement of the Z conformation in this phenomenon. Neither calf thymus DNA (41% GC) nor poly[d(A–T)] (0% GC) aggregates under the same conditions.