Second order kinetics mean that two molecules must collide in the rate limiting step of the mechanism. This type of process is extremely common in chemical reaction dynamics. Bond forming reactions most often involve two molecules that must interact. Three-body processes are rare gy oomparison. First we examine the general theory of bimolecular reactions. We then consider some important specific examples, such as DNA hybridization and enzyme kinetics.
DNA hybridization is an example of second order kinetics. The concentratoin dependence of DNA hybridization is quite important. DNA hybridizatoin rates can vary over many orders of magnitude because of the fact that second order kinetics has such a strong dependence on concentration. DNA also has a large dynamic range of concentration since we can consider important examples where there is a single strand or only a few strands up to concnetrations in the micromolar range. DNA hybridization in intermediate concentration ratnges can be readily detected using UV-vis spectroscopy. This permits us to obtain good data on DNA hybridization rates.
Michaelis-Menten kinetics is the standard kinetic scheme used to dscribe enzyme kinetics. The model is a bimolecular one that invovlves and enzyme, E, and substrante, S, finding each other in solution to form an enzyme-substrate complex, ES. There are two important regimes of M-M kinetics. In the low [S] regime the diffusion of the subtrate is rate limiting. At high [S] the intrinsic catalytic rate cosntant of the enzyme, known as kcat, is rate limiting. Therefore, the M-M rate increases with [S] and then reaches a plateau at high [S]. The asumptotic rate is known as Vmax, which depends on the kcat as follows, Vmax = [E]0kcat. The other important parameter in the M-M model is Km, which is equal to the substrate concentration at which V0 = Vmax/2.
