Characterization of selective binding of alkali cations with carboxylate by x-ray absorption spectroscopy of liquid microjets
- Janel S. Uejio,
- Craig P. Schwartz,
- Andrew M. Duffin,
- Walter S. Drisdell,
- Ronald C. Cohen, and
- Richard J. Saykally†
- Department of Chemistry, University of California, Berkeley, CA 94720-1460; and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94618
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Edited by Barry Ninham, Australian National University, Canberra, Australia, and accepted by the Editorial Board February 27, 2008 (received for review January 8, 2008)
Abstract
We describe an approach for characterizing selective binding between oppositely charged ionic functional groups under biologically relevant conditions. Relative shifts in K-shell x-ray absorption spectra of aqueous cations and carboxylate anions indicate the corresponding binding strengths via perturbations of carbonyl antibonding orbitals. XAS spectra measured for aqueous formate and acetate solutions containing lithium, sodium, and potassium cations reveal monotonically stronger binding of the lighter metals, supporting recent results from simulations and other experiments. The carbon K-edge spectra of the acetate carbonyl feature centered near 290 eV clearly indicate a preferential interaction of sodium versus potassium, which was less apparent with formate. These results are in accord with the Law of Matching Water Affinities, relating relative hydration strengths of ions to their respective tendencies to form contact ion pairs. Density functional theory calculations of K-shell spectra support the experimental findings.
Footnotes
- †To whom correspondence should be addressed. E-mail: saykally{at}berkeley.edu
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Author contributions: R.C.C. and R.J.S. designed research; J.S.U., C.P.S., A.M.D., and W.S.D. performed research; J.S.U. analyzed data; and J.S.U. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission. B.N. is a guest editor invited by the Editorial Board.
- © 2008 by The National Academy of Sciences of the USA
