Volumetric effects of ionization of amino and carboxyl termini of alpha,omega-aminocarboxylic acids.
We have determined the partial molar volumes and adiabatic compressibilities of a homologous series of six alpha,omega-aminocarboxylic acids over a broad pH range at 25 degrees C. We interpret the resulting data in terms of the changes in hydration associated with neutralization of amino and carboxyl termini. By combining our volumetric results with pH-dependent data on 1-anilinonaphthalene-8-sulfonic acid fluorescence we propose the following explanation to the long-standing observation that changes in volume and compressibility accompanying neutralization of a carboxyl group depend on the type of the solute in contrast to solute-independent changes in these parameters accompanying neutralization of an amino group. Unlike amino groups, neutralized carboxyl groups are capable of forming hydrogen-bonded structures stabilized by hydrogen bonds between the carbonyl oxygen of one solute molecule and the hydroxyl group of another molecule. Formation of such hydrogen-bonded structures causes an additional decrease in solute hydration with concomitant increases in volume and compressibility. Furthermore, solutes with large aliphatic moieties may form larger associates stabilized, in addition to intermolecular hydrogen bonds, by hydrophobic interactions which will result in further increases in volume and compressibility. In the aggregate, our results emphasize the need for further studies focused on developing an understanding of the role of electrostatic interactions in stabilizing/destabilizing proteins and protein complexes.
Taulier
Nicolas
N
Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, 19 Russell Street, Toronto, Ont., Canada M5S 2S2.
Chalikian
Tigran V
TV
D000599:Amino Acids, Diamino / Q000737:chemistry*; D000817:Anilino Naphthalenesulfonates; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005456:Fluorescent Dyes; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D011487:Protein Conformation; D011489:Protein Denaturation; D055672:Static Electricity; D014867:Water / Q000737:chemistry
D000599:Amino Acids, Diamino; D000817:Anilino Naphthalenesulfonates; D005456:Fluorescent Dyes; D014867:Water; C027132:1-anilino-8-naphthalenesulfonate