It can be useful to describe the Gibbs free energy changes for the, binding to a protein of a molecule, A-B, and of its component parts, A and. B, in terms of the "intrinsic binding To reach a meaningful conclusion about this problem (and to avoid conclusions that depend on the sequence in which experiments are carried out), it is necessary to find a way of relating the observed binding of A and B to that of A-B (1-5). In one simple approach to this problem, the binding of A-B is considered to occur in two steps, in which the initial binding of the A or B moiety is followed by the binding of the rest of the molecule (Eq. 4). The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. KA

It is pointed out that translational and (overall) rotational motions provide the important entropic driving force for enzymic and intramolecular rate accelerations and the chelate effect; internal rotations and unusually severe orientational requirements are generally of secondary importance. desolvation. Unequivocal evidence that theories giving rate accelerations on the order of 55 AM from entropic factors are wrong or incomplete is provided by several intramolecular reactions, such as ring closures of succinate derivatives, in which the presence of one or more free rotations ensures that a reacting group can move out of an unfavorable conformation so that the fraction of the starting material in a high-energy, strained or desolvated form will be negligible. The comparison between intra-and intermolecular reactions may be based on the free energy of either the transition state, from rate measurements, or the product, from equilibrium measurements, relative to the starting material; the latter comparison has the advantage that the structure of the product is known. Thus, the equilibrium constant for succinic anhydride formation (Eq. 1) is3 X 105(1) more favorable than that for acetic anhydride formation from the free acids, which implies an "effective concentration" of 3 X 105 M of the carboxylic acid groups of succinic acid relative to each other (2, 9), and the "effective concentration" of the neighboring carboxylate group that determines the rate of anhydride formation from substituted monophenyl succinate anions (Eq. 2) is about 105 M (4,10,11

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