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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.Hofmeister effects ͉ ion interactions ͉ aqueous systems T he discovery of the selective interactions between simple ions and proteins dates back over a century to Hofmeister's studies with chicken egg protein; proteins could be selectively ''salted in'' or ''salted out'' by the addition of various salts to the solution (1). This ''Hofmeister effect'' has subsequently been observed with more salts and many more proteins, with relative magnitudes following the ''Hofmeister series'' ordering, as do various related phenomena (2, 3). Nevertheless, despite enormous effort, the origin of Hofmeister effects is not completely understood (4, 5).To rationalize biological ion specificity, such as the physiologically important preferential binding of sodium versus potassium with proteins, the Law of Matching Water Affinities was proposed by Collins (6, 7). Based on charge densities and electrostatic arguments, this law holds that ions with similar hydration free energies form the most stable (insoluble) contact ion pairs. In the case of proteins, the carboxylate group is considered to have a hydration energy much closer to that of sodium than to that of potassium, which is manifested in the sodium binding free energy being larger by 2.22 kcal/mol, as determined by Vrbka et al. (8) with simulations and conductivity measurements. The simulations indicated that the interaction is localized on the carboxylate groups of the protein. Conductivity measurements were performed on protein solutions for experimental support, revealing a larger relative decrease upon addition of sodium chloride compared with potassium chloride, indicative of sodium being more efficiently removed from solution than potassium. The rationalization of the preferential interaction was again that there was a closer match of hydration energy, which was reflected in more stable contact ion pairing between the protein carboxylate groups and sodium, versus potassium ions.In this article, we examine the selective interactions of alkali cations ...

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The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an "acceptor-only" configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s f σ* NH preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s f π* CN resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the R amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.

A computational approach is presented for prediction and interpretation of core-level spectra of complex molecules. Applications are presented for several isolated organic molecules, sampling a range of chemical bonding and structural motifs. Comparison with gas phase measurements indicate that spectral lineshapes are accurately reproduced both above and below the ionization potential, without resort to ad hoc broadening. Agreement with experiment is significantly improved upon inclusion of vibrations via molecular dynamics sampling. We isolate and characterize spectral features due to particular electronic transitions enabled by vibrations, noting that even zero-point motion is sufficient in some cases. PACS numbers: UnknownWhen applied to molecular systems, core level spectroscopies are powerful probes of both occupied and unoccupied electronic states, uniquely revealing intimate details of both intra-and inter-molecular interactions [1]. Methods involving x-ray absorption (XAS, NEXAFS, XANES) or x-ray photo-electron spectroscopy (XPS) are increasingly being applied to complex molecular systems, including nucleotides, peptides and large organic molecules [2]. However, a major limitation of this technology is the fact that extraction of molecular information from these experiments often depends explicitly on comparisons with theoretical calculations, which are extremely challenging to perform at experimental accuracy. In this Letter, we describe the extension of a recently developed method for predicting core-level spectra of condensed phases [3] to isolated organic molecules -pyrrole, s-triazine, pyrrolidine and glycine -which demonstrates qualitative improvements over existing methods [4][5][6] in comparison with experiment and provides new insights into the origins of particular spectral features in terms of coupling of electronic and vibrational degrees of freedom.The challenges for simulating gas phase core-level spectra are maintaining accuracy in the following areas: (1) description of the core-hole excited state; (2) representation of both bound excitonic states below the ionization potential (IP) and resonance states in the continuum above the IP; and (3) inclusion of vibrational effects, either due to experiments being performed near room temperature, or from intrinsic zero-point motion.Density functional theory (DFT) [7,8] has proved accurate in reproducing the excitation energies associated with core-level spectra via total energy differences (socalled ∆SCF or ∆KS) [9]. Accordingly, we model the lowest energy core-level excited state self-consistently using a full core-hole and excited electron (XCH) [3]. This is particularly important for molecular systems, where screening of the core-hole excitation is greatly enhanced by the presence of the excited electron, which can be strongly bound to the core-hole in the lowest energy excited state. In contrast, for non-molecular condensed phases, such as covalent and ionic crystalline solids, the inherent dielectric screening of the valence charge density o...

We report the effects of sampling nuclear quantum motion with path integral molecular dynamics (PIMD) on calculations of the nitrogen K-edge spectra of two isolated organic molecules. s-triazine, a prototypical aromatic molecule occupying primarily its vibrational ground state at room temperature, exhibits substantially improved spectral agreement when nuclear quantum effects are included via PIMD, as compared to the spectra obtained from either a single fixed-nuclei based calculation or from a series of configurations extracted from a classical molecular dynamics trajectory. Nuclear quantum dynamics can accurately explain the intrinsic broadening of certain features. Glycine, the simplest amino acid, is problematic due to large spectral variations associated with multiple energetically accessible conformations at the experimental temperature. This work highlights the sensitivity of near edge x-ray absorption fine structure (NEXAFS) to quantum nuclear motions in molecules, and the necessity of accurately sampling such quantum motion when simulating their NEXAFS spectra.

We report measurement of the valence-to-core (VTC) region of the K-shell x-ray emission spectra from several Zn and Fe inorganic compounds, and their critical comparison with several existing theoretical treatments. We find generally good agreement between the respective theories and experiment, and in particular find an important admixture of dipole and quadrupole character for Zn materials that is much weaker in Fe-based systems. These results on materials whose simple crystal structures should not, a prior, pose deep challenges to theory, will prove useful in guiding the further development of DFT and time-dependent DFT methods for VTC-XES predictions and their comparison to experiment.

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