Depending on the preparation method used, enzymes under low water conditions can exhibit differences in catalytic activity that vary by several orders of magnitude. Because this can make the difference between whether a particular biotransformation is practically useful or not, it is important to control enzyme history carefully. The basic problem is simple: how to transfer the enzyme from an aqueous environment to one where it is dehydrated while ensuring, first, that it remains in a native conformation and, second, that the active site residues are in the correct protonation state. Although the importance of retaining native structure is well recognized, potential detrimental changes to the protonation state are often not considered. Usually, it is assumed that the enzyme will exhibit “pH memory” of the previous aqueous solution. As discussed in detail in Chapter 19, this is not always a valid assumption.