ABSTRACT

Such simple reactions correspond to by far to the most common and also the practically most useful situation. In addition, even in situations where a single kinetic step does not strictly apply, it may represent an excellent approximation. For example, the annealing of two DNA strands S1 and S2 leading to a duplex (as studied by P. L. Privalov in Chapter 11 in this book) proceeds through two consecutive kinetic steps according to

S + S D*1 2 on

 →←  (15.1a)

D* Dzip unzip

k  →←  (15.1b)

where D* is an unstable short duplex (corresponding to an initial nucleus of three to four base pairs), which is eventually transformed into a full stable duplex through a monomolecular zipping process (typically an induced-t mechanism). is two-step mechanism was established long ago aer two remarkable studies (Craig et al. 1971; Porschke and Eigen 1971). According to these studies, it can nevertheless be accurately described kinetically by a global single-step mechanism S S D1 2

off +  →← 

ko(global) ≠ ko), because the zipping process (Equation 15.1b) is extremely fast in comparison to the formation of the initial nucleus (Equation 15.1a).