"Asymmetric" opening reaction mechanism of Z-DNA base pairs: a hydrogen exchange study.

With the tritium-Sephadex method, the hydrogen-exchange kinetics of the five NH protons of guanine and cytosine residues in Z-form poly(dG-dC) X poly (dG-dC) were measured as a function of temperature and catalyst concentration. Over the measured temperature range from 0 to 34 degrees C, two classes of protons with constant amplitudes are found. The three protons of the fast class, which were assigned to the guanine amino and imino protons, have an exchange half-time in the minute time range (at 20 degrees C the half-time is 2.5 min) and an activation energy of 18 kcal mol-1. Since these two types of protons exchange at the same rate in spite of their grossly different pK values, the exchange of these protons must be limited by the same nucleic acid conformational change. The two cytosine amino protons of the slow class are especially slow with exchange half-times in the hour time range (at 20 degrees C the exchange half-time is 1 h) and the activation energy is 20 kcal mol-1. The exchange of these two protons is not limited by some nucleic acid conformational change as shown by the marked exchange acceleration of these protons upon addition of 0.2 M imidazole. In addition, we have also reexamined the hydrogen-deuterium exchange kinetics of the amino protons of guanosine cyclic 2',3'-monophosphate by a spectral difference method using a stopped-flow spectrophotometer. The measured kinetic process is monophasic with a rate constant of 3 s-1 at 20 degrees C, which is in the same range as the predicted rate constant of the guanine amino protons.(ABSTRACT TRUNCATED AT 250 WORDS)