In spite of the biological importance of the binding of Zn2+, Ca2+, and Mg2+ to the carboxylate group, cation-acetate binding affinities and binding modes remain actively debated. Here, we report the first use of Raman multivariate curve resolution (Raman-MCR) vibrational spectroscopy to obtain self-consistent free and bound metal acetate spectra and one-to-one binding constants, without the need to invoke any a priori assumptions regarding the shapes of the corresponding vibrational bands. The experimental results, combined with classical molecular dynamics simulations with a force field effectively accounting for electronic polarization via charge scaling and ab initio simulations, indicate that the measured binding constants pertain to direct (as opposed to water separated) ion pairing. The resulting binding constants do not scale with cation size, as the binding constant to Zn2+ is significantly larger than that to either Mg2+ or Ca2+, although Zn2+ and Mg2+ have similar radii that are about 25% smaller than Ca2+. Remaining uncertainties in the metal acetate binding free energies are linked to fundamental ambiguities associated with identifying the range of structures pertaining to non-covalently bound species.

CNRS Université de Paris UPR 9080 Laboratoire de Biochimie Théorique 13 Rue Pierre et Marie Curie 75005 Paris France and Institut de Biologie Physico Chimique Fondation Edmond de Rothschild PSL Research University Paris France

Department of Chemistry Purdue University West Lafayette Indiana 47907 USA

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic Flemingovo nám 2 16610 Prague 6 Czech Republic

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic Flemingovo nám 2 16610 Prague 6 Czech Republic and CNRS Université de Paris UPR 9080 Laboratoire de Biochimie Théorique 13 Rue Pierre et Marie Curie 75005 Paris France and Institut de Biologie Physico Chimique Fondation Edmond de Rothschild PSL Research University Paris France

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