Article

Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-ray Absorption and ab initio Calculations

BESSY m.b.H., Albert-Einstein-Strasse 15, Berlin, Germany.
The Journal of Physical Chemistry B (Impact Factor: 3.3). 08/2008; 112(40):12567-70. DOI: 10.1021/jp805177v
Source: PubMed

ABSTRACT

Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.

Download full-text

Full-text

Available from: Robert Vácha
  • Source
    • "[75−80] In aqueous solutions, we may expect the order of ion-pairing—also important for in vivo processes—for freely dissolved dissociated PFCAs to be Na + >Li + >K + >NH + 4 based on analogous work with other carboxylates. [81] Overall, there remains a high level of uncertainty as to the importance of these possible carboxylate group acidity driven sorption and ion-pairing processes for PFCAs. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Acidity constants were calculated using the semiempirical PM6 pK(a) estimation method for all C(2) through C(9) perfluoroalkyl carboxylate (PFCA) congeners and the straight-chain C(10) through C(13) isomers. According to the PM6 estimates, the linear congeners within each PFCA homologue group have the highest pK(a) values by up to 6 units depending on the degree of branching in the perfluoroalkyl chain. In general, the higher the degree of branching in the perfluoroalkyl chain within a homologue group, the lower the estimated pK(a) value. When the branching is closest to the terminal carboxylate group, the effect on the calculated pK(a) is greatest. Although the PM6 calculated pK(a) values agree well with previously reported estimates for selected linear PFCA congeners using the SPARC and COSMOtherm approaches, all computational approaches only show good agreement with reported experimental values for short chain PFCAs (C(2) through C(5)). Increasing divergences are observed between calculated and experimental results by up to several pK(a) units as the perfluoroalkyl chain length increases beyond C(5). The findings demonstrate a need for additional experimental pK(a) measurements for an expanded set of both linear and branched PFCA congeners to confirm previous experimental reports that are potentially in error, and upon which to calibrate existing computational methods and environmental, toxicological, and waste treatment method models.
    Full-text · Article · Apr 2009 · Journal of Environmental Science and Health Part A
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Studies of aqueous interfaces and of the behavior of ions therein have been profiting from a recent remarkable progress in surface selective spectroscopies, as well as from developments in molecular simulations. Here, we summarize and place in context our investigations of ions at aqueous interfaces employing molecular dynamics simulations and electronic structure methods, performed in close contact with experiment. For the simplest of these interfaces, i.e., the open water surface we demonstrate that the traditional picture of an ion-free surface is not valid for large soft (polarizable) ions such as the heavier halides. Both simulations and spectroscopic measurements indicate that these ions can be present and even enhanced at surface of water. In addition we show that that ionic product of water exhibits a peculiar surface behavior with hydronium but not hydroxide accumulating at the air/water and alkane/water interfaces. This result is supported by surface selective spectroscopic experiments and surface tension measurements. However, it contradicts the interpretation of electrophoretic and titration experiments in terms of strong surface adsorption of hydroxide; an issue which is further discussed here. The applicability of the observed behavior of ions at the water surface to investigations of their affinity for the interface between proteins and aqueous solutions is explored. Simulations 2 show that for alkali cations the dominant mechanism of specific interactions with the surface of hydrated proteins is via ion pairing with negatively charged amino acid residues and with the backbone amide groups. As far as halide anions are concerned, the lighter ones tend to pair with positively charged amino acid residues, while heavier halides exhibit affinity to non-polar protein patches, which resembles their behavior at the air/water interface. These findings, together with results for more complex molecular ions, allow us to formulate a local model of interactions of ions with proteins with the aim to rationalize at the molecular level ion-specific Hofmeister effects, e.g., salting out of proteins.
    Preview · Article · Jan 2008
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Studies of aqueous interfaces and of the behavior of ions therein have been profiting from a recent remarkable progress in surface selective spectroscopies, as well as from developments in molecular simulations. Here, we summerize and place in context our investigations of ions at aqueous interfaces employing molecular dynamics simulations and electronic structure methods, performed in close contact with experiment. This result is supported by surface-selective spectroscopic experiments and surface tension measurements. However, it contradicts the interpretation of electrophoretic and titration experiments in terms of strong surgace adsorption of hydroxidie; Simulations show that for alkali cations the dominant mechanism of specific interactions with the surface of hydrate proteins is via ion pairing with negatively charged amino acid residues and with the back bone amide groups. These finding, together with results for more complex molecular ions, allow us to formulate a local model of interactions of ions with proteins with the aim to rationalize at the molecular level ion-specific. Hofmeister effects, e.g. the salting out of proteins.
    Preview · Article · Feb 2009 · Faraday Discussions
Show more