Deconvolution of Raman spectroscopic signals for electrostatic, H-bonding, and inner-sphere interactions between ions...

Quantitative analysis of metal ion-phosphodiester interactions is a significant experimental challenge due to the complexities introduced by inner-sphere, outer-sphere (H-bonding with coordinated water), and electrostatic interactions that are difficult to isolate in solution studies. Here, we provide evidence that inner-sphere, H-bonding and electrostatic interactions between ions and dimethyl phosphate can be deconvoluted through peak fitting in the region of the Raman spectrum for the symmetric stretch of non-bridging phosphate oxygen (ν(s)PO(2)(-)). An approximation of the change in vibrational spectra due to different interaction modes is achieved using ions capable of all or a subset of the three forms of metal ion interaction. Contribution of electrostatic interactions to ion-induced changes to the Raman ν(s)PO(2)(-) signal could be modeled by monitoring attenuation of ν(s)PO(2)(-) in the presence of tetramethylammonium, while contribution of H-bonding and inner-sphere coordination could be approximated from the intensities of altered ν(s)PO(2)(-) vibrational modes created by an interaction with ammonia, monovalent or divalent ions. A model is proposed in which discrete spectroscopic signals for inner-sphere, H-bonding, and electrostatic interactions are sufficient to account for the total observed change in ν(s)PO(2)(-) signal due to interaction with a specific ion capable of all three modes of interaction. Importantly, the quantitative results are consistent with relative levels of coordination predicted from absolute electronegativity and absolute hardness of alkali and alkaline earth metals.