Solvent Effects on Processes in Aqueous Solutions

Abstract

Most of the processes treated in Chapters 2 and 3 and part of 4 were assumed to take place in the vacuum. In reality these processes are usually carried out in a solvent. This is certainly true for biochemical processes such as the binding of oxygen to hemoglobin, the binding of substrate to enzymes, the helix-coil transition, etc. In a simple solvent it is often assumed that the theory itself may be retained except for an appropriate rescaling of the parameters involved. This, in general, is not true for complex solvents such as water. Here, the presence of a solvent might change not only the parameters involved in a specific theory, but also the theory itself. For instance, the PF of a simple Ising model with two-state units and nearest-neighbor interactions can be represented by a 2 × 2 matrix. The same model in a solvent could be affected in two ways. First, if the assumptions of the model can be retained, then only the parameters of the model are changed, e.g., the energies of each state or the interaction energies between nearest neighbors. Second, when the assumptions of the model cannot be retained, then the theory itself must be modified. For instance, if the solvent introduces next-nearest-neighbor correlations then the 2 × 2 matrix will not be sufficient to represent the PF of the system. Higher-order matrices would be necessary to treat the same model in a solvent. In more extreme cases, the 1-D model itself must be abandoned.