Blurring the distinction between slow and intermediate chemical exchange

NMR is a vital tool for measuring the dynamics of biological macromolecules in solution. The chemical exchange observed is often divided into slow or intermediate exchange. Slow exchange affects principally the z magnetizatio ns of the system and is observed in modified spin-lattice relaxation experi ments. Intermediate exchange gives rise to broadening and coalescence in th e spectrum itself. This broadened spectrum is often considered as a whole, but we have shown that it is better to regard it as a sum of transitions, w ith a generalized picture of the transition probability. In this paper, we give explicit expressions for the lineshape for two sites and review some o f the recent applications of chemical exchange in biochemical NMR.