Charge density-dependent strength of hydration and biological structure

Abstract

Small ions of high charge density (kosmotropes) bind water molecules strongly, whereas large monovalent ions of low charge density (chaotropes) bind water molecules weakly relative to the strength of water-water interactions in bulk solution. The standard heat of solution of a crystalline alkali halide is shown here to be negative (exothermic) only when one ion is a kosmotrope and the ion of opposite charge is a chaotrope; this standard heat of solution is known to become proportionally more positive as the difference between the absolute heats of hydration of the corresponding gaseous anion and cation decreases. This suggests that inner sphere ion pairs are preferentially formed between oppositely charged ions with matching absolute enthalpies of hydration, and that biological organization arises from the noncovalent association of moieties with matching absolute free energies of solution, except where free energy is expended to keep them apart. The major intracellular anions (phosphates and carboxylates) are kosmotropes, whereas the major intracellular monovalent cations (K+; arg, his, and lys side chains) are chaotropes; together they form highly soluble, solvent-separated ion pairs that keep the contents of the cell in solution.

Ion Dynamics in Cationic Lipid Bilayer Systems in Saline Solutions

“…Until rather recently, Na + ions were thought to be rather indifferent cations with respect to phospholipids, 94,95 and ion binding in general to take place only in the headgroup region. 28,30 During the past decade, however, this view has been challenged.…”

SAXS/SANS on Supercharged Proteins Reveals Residue-Specific Modifications of the Hydration Shell

“…Since sodium ions contain fewer electrons than chloride ions, our SAXS results suggest that water molecules are ''contracting'' locally more around acidic than around basic surface residues and that protein HS properties are a combined result of a residue-specific ionic interaction and structural changes of water molecules in their vicinity. Indeed, an important factor not taken into account by most SAXS/ SANS programs is the contribution of solvent ions in the vicinity of proteins, which may display concentrations that vary significantly from those in the bulk as a function of the local physicochemical protein surface properties (24,25). Recent MD simulations suggest that ions may either bind specifically and locally to oppositely charged protein surface residues (67,68) or accumulate at nonpolar surface patches (66) as a function of type (i.e., anions/cations) and size, an observation supported by SANS experiments with varying salt concentrations and type (5,33).…”

“…Increased water density in the first hydration shell (HS), as well as surface-specific differences in the organization of the hydration water, have been reported by MD simulations (17)(18)(19)(20) and have been described thermodynamically in terms of electrorestriction (21). MD studies involving nucleic acids (7,22,23) or proteins (24,25), as well as anomalous x-ray solution studies on proteins and nucleic acids (26,27), have revealed the specific recruitment of ions into their HSs.…”

Salting-in with a Salting-out Agent: Explaining the Cation Specific Effects on the Aqueous Solubility of Amino Acids

“…In fact, it has been argued that ammonium and sodium ions cannot form cyclo-complexes with amino acids in the same way as magnesium and other divalent cations such as Ca 2+ , 78 Na + , and K + are often viewed as roughly neutral. 95 Density functional theory calculations of the relative stability of gasphase complexes between metal ions and amino acids 96−98 have shown that the Gibbs energies of (ion−amino acid) systems are much less positive in the case of divalent ions than of monovalent ions. For instance, for (ion−arginine) complexes, 96 the values of the gas-phase Gibbs energies follow the order Ni 2+ (−1544 kJ mol −1 ) < Cu 2+ (−1404 kJ mol −1 ) < Zn 2+ (−1280 kJ mol −1 ) < Mg 2+ (−1028 kJ mol −1 ) < Ca 2+ (−660 kJ mol −1 ) < Li + (−328 kJ mol −1 ) <Na + (−225 kJ mol −1 ) < K + (−152 kJ mol −1 ), at T = 298.15 K.…”