Ronald M. Scott

Models of intermolecular interactions involving proton transfer in biopolymers

The results of spectrophotometric and conductimetric studies of the ionization of several phenol derivatives in the presence of amines in dioxane and aqueous dioxane solutions, were used to delineate the ranges of solvent polarity over which 1:1 proton transfer complexes are formed. Increasing polarity favors the formation of proton transfer complexes up to a point determined by the nature of the acid-base pair. Proton transfer complexes display a measurable stability over an appreciable range of solvent polarity varying from low (D < 11) to medium (11 < D < 45). Their formation involves a free energy change ranging from −2.0 to −7.1 kcal · mole−1 and a decrease in enthalpy ranging from 6.1 to 16.8 kcal · mole−1. The effects of proton transfer complex formation between side chain groups of biopolymers are subdivided into structural, chemical and kinetic, and the relevance of the results obtained from model studies to biopolymers is discussed. It is suggested that formation of proton transfer complexes can affect the direction of conformational change in macromolecules and play an important role in the formation of oligomeric structures. The numerical inferiority of proton transfer complexes in biopolymers, relative to hydrophobic and hydrogen bonding interactions should be partially offset by the greater contribution of the former to the total free energy and enthalpy of formation of specific biopolymer conformations.

Hydrogen bonding interactions of p-nitrophenol

Abstract The thermodynamic parameters of the formation of the 1:1 hydrogen bonded complex of p-nitrophenol with dioxane, triethylamine and n-butylamine in cyclohexane, were determined by ultraviolet absorption spectroscopy: −ΔH = 7.03, 10.1, 9.21 kcal.mole−1, −ΔG = 2.41, 4.18, 4.44 kcaLmole−1 and −ΔS = 15.5, 19.8, 16.0 e.u., respectively. Virtually all of the p-nitrophenol is hydrogen bonded when the concentration of the proton acceptor reaches 0.1–0.5 M. At higher concentrations, additional bathochromic shifts are observed in the p-nitrophenol absorption spectrum. The solvation of the 1:1 p-nitrophenol complex in the concentrated dioxane and triethylamine solutions appears to follow 3:2 and 3:4 stoichiometries, respectively. The free energy, enthalpy and entropy of the dioxane solvation were determined to be 1.3 kcal.mole−1, −5.1 kcal.mole−1 and −21 e.u., respectively. In the case of n-butylamine, a new band appears at ~370 mμ, which continues to shift with increase in n-butylamine concentration, ending at 395 mμ in pure n-butylamine. This band was assigned to the absorption of the proton transfer complex NO2 ⋯H+-NH2Bu.

Solvent facilitated proton transfer in hydrogen bonded complexes of proton donor and acceptor groups in model protein systems.

Abstract The tautomeric equilibrium between a hydrogen bonded complex and a proton transfer complex (hydrogen bonded ion pair) in a number of phenol-amine systems, is dependent, among other factors, on the polarity of the solvent. The formation of a proton transfer complex is accompanied by a decrease in free energy of 2–6 kcal/mole. Based on spectrophotometric studies of the base dependent ionization of several phenol derivatives in aqueous dioxane solutions, it is likely that proton donor sidechain groups possessing a p K a in the range 3–10, can participate in proton transfer complexes with basic groups whose conjugate acids possess a greater p K a , in nonpolar and partially polar regions of a protein. It is suggested, that the decrease in free energy accompanying the formation of a proton transfer complex can affect the direction of a protein conformational transition.