The x-ray absorption spectroscopy model of solvation about sulfur in aqueous L-cysteine

Abstract

The environment of sulfur in dissolved aqueous L-cysteine has been examined using K-edge x-ray absorption spectroscopy (XAS), extended continuum multiple scattering (ECMS) theory, and density functional theory (DFT). For the first time, bound-state and continuum transitions representing the entire XAS spectrum of L-cysteine sulfur are accurately reproduced by theory. Sulfur K-edge absorption features at 2473.3 eV and 2474.2 eV represent transitions to LUMOs that are mixtures of S-C and S-H σ * orbitals significantly delocalized over the entire L-cysteine molecule. Continuum features at 2479, 2489, and 2530 eV were successfully reproduced using extended continuum theory. The full L-cysteine sulfur K-edge XAS spectrum could not be reproduced without addition of a water-sulfur hydrogen bond. Density functional theory analysis shows that although the Cys(H)S· · · H-OH hydrogen bond is weak (∼2 kcal) the atomic charge on sulfur is significantly affected by this water. MXAN analysis of hydrogen-bonding structures for L-cysteine and water yielded a best fit model featuring a tandem of two water molecules, 2.9 Å and 5.8 Å from sulfur. The model included a S cys · · · H-O w1 H hydrogen-bond of 2.19 Å and of 2.16 Å for H 2 O w1 · · · H-O w2 H. One hydrogen-bonding water-sulfur interaction alone was insufficient to fully describe the continuum XAS spectrum. However, density functional theoretical results are convincing that the water-sulfur interaction is weak and should be only transient in water solution. The durable water-sulfur hydrogen bond in aqueous L-cysteine reported here therefore represents a break with theoretical studies indicating its absence. Reconciling the apparent disparity between theory and result remains the continuing challenge.

Using solution- and solid-state S K-edge X-ray absorption spectroscopy with density functional theory to evaluate M–S bonding for MS42−(M = Cr, Mo, W) dianions

“…Solution-phase S K-edge XAS experiments 35,51,53,61-68 were conducted on (PPh 4 ) 2 MS 4 salts dissolved in acetonitrile. To characterize the influence of self-absorption effects on the peak intensities, samples were analyzed using progressively decreasing analyte concentrations until normalized intensities remained constant.…”

Solvation structure of the halides from x-ray absorption spectroscopy

“…The raw spectra were processed and normalized as has been described previously. 71,72 Normalization was carried out using the application Pyspline. 69,73 EXAFS spectra were fit using the Feff8 code within the program suite EX-AFSPAK, 30,74,75 http://www-ssrl.slac.stanford.edu/∼george/ exafspak/exafs.htm.…”

NEXAFS spectroscopy of ionic liquids: experiments versus calculations

“…[23][24][25] The specificity of NEXAFS spectroscopy renders it an attractive technique for investigating elements that are essentially silent by NMR spectroscopy, for example, sulfur. [26][27][28][29] NEXAFS spectroscopy involves the study of the absorption of an X-ray photon by the element of interest, leading to promotion of an electron from a core orbital to a previously unoccupied molecular orbital (UMO). Absorption is generally indirectly monitored by measurement of either the fluorescence yield (FY) or the electron yield (EY), e.g.…”