Anion Bridges Drive Salting Out of a Simple Amphiphile from Aqueous Solution

Abstract

Neutron diffraction with isotope substitution has been used to determine the structural changes that occur on the addition of a simple salting-out agent to a dilute aqueous alcohol solution. The striking results obtained demonstrate a relatively simple process occurs in which interamphiphile anionic salt bridges are formed between the polar groups of the alcohol molecules. These ion bridges drive an increase in the exposure of the alcohol molecule nonpolar surface to the solvent water and hence point the way to their eventual salting out by the hydrophobic effect. DOI: 10.1103/PhysRevLett.89.215508 PACS numbers: 61.20.Qg, 61.12.-q, 64.75.+g, 82.30.Nr Ever since the early studies of Hofmeister [1] the effect of salts on the solubility of amphiphilic molecules has been of intense interest. Despite its regular use for the concentration and crystallization of biological molecules such as proteins, the phenomenon is not well understood. The ''classical view'''(see, e.g., [2]) explains the effect in terms of competition between the salt and the protein for water of solvation. Arakawa and Timasheff 's experiments [3] indicate that proteins are preferentially hydrated in the presence of salting-out agents, while Chick and Martin [4] suggest that the salt is excluded from the protein precipitate. These latter views argue that the saltingout agents do not interact with the protein and hence question the classical view. Recent NMR studies have argued in favor of direct salt-nonpolar group interactions [5,6], while other views favor an indirect interaction mediated by the hydration shells of the ions and the nonpolar solutes [7].Recent advances in the application of isotope substitution neutron scattering measurements have given us detailed insight into the structures of aqueous twocomponent amphiphile systems, showing us how amphiphiles interact in aqueous solution as temperature and concentration are varied [8][9][10][11]. We present here a significant extension of these techniques to an aqueous solution of tertiary butanol to which a salting-out agent has been added.To extract the structural correlations in this system a series of isotopically labeled solutions were prepared without and with sodium chloride, a known and simple salting-out agent. The stoichiometric ratio of alcohol to water for all solutions was 1:50, corresponding to a mole fraction concentration of 0:02. This concentration is close to the minimum in the relative partial molar volume of t-butanol (TBA) in t-butanol-water mixtures [12], a region that is traditionally associated with the maximum in hydrophobic effects observed in these solutions [13]. In the solutions containing NaCl, the NaCl:water ratio was fixed at 1:100, so that the concentration of ions (irrespective of type) is then equal to that of the alcohol molecules.To allow extraction of information relating to the hydration of the alcohol molecules and the structure of the water solvent, five isotopic mixtures were prepared for each of the salt and no-salt cases, namely, (1) CD 3 3 COH in ...

The Hydration of the Neurotransmitter Acetylcholine in Aqueous Solution

“…Neutron diffraction with isotopic substitution is an excellent technique to study the structure of hydrogen-containing fluids, mixtures of fluids, and solutions (12,13,15,16,19,(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31). This is due to the difference in scattering intensity by hydrogen and deuterium, which have scattering intensities of b H ¼ ÿ3.73 fm and b D ¼ 6.671 fm, respectively.…”

Infrared Spectroscopy of NaCl(CH3OH)n Complexes in Helium Nanodroplets

“…On the basis of neutron scattering measurements on NaCl in t-butanol, Bowron and Finney proposed that the salting-out effect is driven by a salt bridge consisting of a halide ion linking two alcohol molecules through two IHBs. 10 However, Paschek et al have rejected this suggestion and have instead presented both experimental and theoretical evidence for an alternative mechanism in which a halide ion attaches to the methyl groups in t-butanol and thereby enhances the attractive interactions between the hydrophobic methyl groups in different t-butanol molecules. 11 These two explanations of the salting-out effect are clearly very different.…”

“…The ν 3 mode is the symmetric CH stretch of the CH 3 group, while ν 9 and ν 2 are both antisymmetric CH stretches. These mode descriptions are approximate because multiple anharmonic couplings with overtone and combination states, such as 2ν 10 and ν 4 + ν 10 , are well-known in this region. 21 Indeed, a broad feature to the immediate red of the ν 9 band, which is most apparent in Figure 2b,c, probably reflects this additional vibrational complexity.…”

“…On the basis of neutron scattering measurements on NaCl in t-butanol, Bowron and Finney proposed that the salting-out effect is driven by a salt bridge consisting of a halide ion linking two alcohol molecules through two IHBs. 10 However, Paschek et al have rejected this suggestion and have instead presented both …”

Determination of γ-hydroxybutyrate (GHB) and its precursors in blood and urine samples: A salting-out approach

“…Structural changes that occur along addition of a simple salting-out agent to a dilute aqueous alcohol solution have been studied using neutron diffraction with isotope substitution [22]. It seems that a relatively simple process occurs, in which interamphiphile anionic salt bridges are formed between the polar groups of the alcohol molecules.…”