Conformational Switching Between Acids and their Anions by Hydrogen Bonding

Abstract

In general, pK a shifts for acids, such as thiols, carboxylic acids, phenols and phosphoric acid monoanions, are considered to occur under homogeneous conditions and change with dielectric constant. Theoretical calculations have been performed for homogeneous environments [1][2][3][4], and some improved theoretical studies have considered anisotropic and heterogeneous environments [4][5][6][7][8] to deal with the particularly large shift for thiols, phenols and carboxylic acids located inside proteins. This paper discusses pK a shifts for acids having neighbouring amide NHs.A relationship between the pK a shift and the formation constant is observed for metal complexes in hydrophobic environments. The influence of intramolecular NHÁ Á ÁX hydrogen bonds on the metal-X bond character is also observed for metal complexes. The presence of NHÁ Á ÁS hydrogen bonds in iron-sulfur proteins observed by crystallographic studies has been suggested by Adman and coworkers [9]. Various synthetic metal-thiolate complexes having chelating Cys-containing oligopeptides and synthetic simple intramolecularly NHÁ Á ÁS hydrogen-bonded thiolate ligands exist. NHÁ Á ÁS hydrogen bonds in metal-thiolate complexes play an important role in the following functions.Intramolecular hydrogen bonds contribute to the following: increase in the stabilization constant, e.g. Fe(II), Zn(II), Cd(II), Hg(II) and Ca(II) complexes; positive shift of the redox potential, e.g. Fe(II), Fe(III), Mo(IV) and W(IV) complexes; and increase in air stabilization, e.g. Fe(II) and Fe(III) complexes. In addition, the NHÁ Á ÁS hydrogen bond increases the formation constant for metal-sulfur complexes formed by substitution reaction between non-hydrogen-bonded and hydrogen-bonded thiolates [10]. The large formation constant is due not only to the pK a -lowering shift of thiol but also to a pp-dp interaction by the NHÁ Á ÁS hydrogen bond.

Contribution of Intramolecular NH···O Hydrogen Bonds to Magnesium–Carboxylate Bonds

“…Interestingly, an NH···O hydrogen bond stabilized a Ca–O (carboxylate, phosphate) bond, which was believed to be ionic, by significant contribution of dπ–pπ interaction. This has been established both experimentally and theoretically, even though the d-orbital of the Ca 2+ ion is formally vacant. , In the case of magnesium carboxylate, the d-orbital has an energy level that is too high to be used, making it unnecessary to consider the dπ–pπ interaction.…”

“…Systematic investigations of NH···X (X = O, S, Se) hydrogen bonds to the coordinated X to metal ion (M) have revealed significant stabilization of the M–X bond . At first, the effect of NH···S hydrogen bond to the covalent M–S (thiolate) bond was described. , This bond was seen in iron–sulfur proteins by crystallographic studies .…”