Dilute solutions of phenol and CH3(CH = CH)5CH = NC4H9 (compound 1) in hydrocarbon solvents are subjected to decreasing temperature and their electronic absorption spectra recorded. Initially only the spectrum of the Schiff base, compound 1, is present. As temperature is lowered, absorbance decreases and the spectrum of the H-bonded form of 1 appears. Continued lowering causes a decrease in absorption of the H-bonded form and a rise in absorption of the proton transferred form of 1. The absorbance, and hence the concentration, of the species are measured quantitatively as a function of temperature, and these data are converted into DELTA-H-degrees and DELTA-S-degrees for each reaction step. The constants are used in DELTA-G(T)-degrees = DELTA-H-degrees - T-DELTA-S-degrees to develop a description of events. In both reactions, values of both constants are negative. With respect to either equilibrium, the negative value of DELTA-H-degrees displaces it to the right, and the negative value of DELTA-S-degrees displaces it to the left. Hydrogen bonding is a specific reaction in which the intrinsic thermodynamic constants dominate. A large negative DELTA-S-degrees results from the bimolecular, 2 --> 1 reaction, but lowering temperature decreases the size of -T-DELTA-S-degrees, forcing equilibrium to the right. The proton transfer step is more complex, but the size of the negative DELTA-H-degrees appears to depend, in part, on the value of DELTA-S-degrees. This suggests that the sign and magnitude of the constants results from the action of the newly formed ion pair as it reorders its solvent cage. If in simple chemical systems, the value of the -T-DELTA-S-degrees can be decreased by decreasing temperature, it is deduced that in protein, -T-DELTA-S-degrees is decreased by decreasing the negative value of DELTA-S-degrees. This is accomplished when the protein folding process preorders amino acid side chains which are approximate to the reacting groups.
