Binding of Biologically Relevant Divalent Cations to Aqueous Carboxylates: Molecular Simulations Guided by Raman Spectroscopy

In spite of the biological importance of the binding of Zn2+, Ca2+, and Mg2+ to carboxylate anions, previous experimental and computational studies have reached conflicting conclusions regarding the corresponding binding affinities. 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 non-covalently bound species.