Unusual proton exchange properties of Z-form poly[d(G-C)].

Real-time solvent-exchange NMR has been used to study the proton exchange in Z-form poly[d(G-C)]. In 4.5 M NaCl the most slowly exchanging protons (about two orders of magnitude slower than in B-DNA) are identified as the guanine imino proton and the cytosine amino proton hydrogen bonded to the guanine carbonyl. Both protons exchange at the same rate and the exchange follows single-exponential kinetics and cannot be catalyzed, implying that the exchanges of the two protons both occur from the same transient solvent-exposed state. The exchange depends strongly on temperature and the activation energy for exchange ( approximately 22 kcal/mol) is the same as the activation energy for the B --> Z transition. The rate of proton exchange is identical with the B --> Z transition rate, both measured in 4.5 M NaCl. The correlation between the B --> Z kinetics and the proton exchange also extends to 3.25 M NaClO(4) solutions, in which both rates are 5 times faster. This unexpected parallelism between the B --> Z transition kinetics and the Z exchange kinetics indicates that the rate-limiting steps in the two processes are similar.