Chemical double mutant cycles for the quantification of cooperativity in H-bonded complexes.

Chemical double mutant cycles have been used in conjunction with new H-bonding motifs for the quantification of chelate cooperativity in multiply H-bonded complexes. The double mutant cycle approach specifically deals with the effects of substituents, secondary interactions, and allosteric cooperativity on the free energy contributions from individual H-bond sites and allows dissection of the free energy contribution due to chelate cooperativity associated with the formation of intramolecular noncovalent interactions. Two different doubly H-bonded motifs were investigated in carbon tetrachloride, chloroform, 1,1,2,2-tetrachloroethane, and cyclohexane, and the results were similar in all cases, with effective molarities of 3-33 M for formation of intramolecular H-bonds. This corresponds to a free energy penalty of 3-9 kJ mol(-1) for formation of a bimolecular complex in solution, which is consistent with previous estimates of 6 kJ mol(-1). This result can be used in conjunction with the H-bond parameters, alpha and beta, to make a reasonable estimate of the stability constant for formation of a multiply H-bonded complex between two perfectly complementary partners, or to place an upper limit on the stability constant expected for a less complementary system.