Structural cartoons of two modes of hydrogen bonding between the thiol moiety of dissolved cysteine and solvent water molecules.
(–, Violet) Sulfur K-edge XAS spectrum of cysteine in pH 5.1 citrate solution. Inset (a) (o), the rising K-edge energy region, and (–), the pseudo-Voigt fit. The dotted-dashed lines indicate the fit components including the arctangent representing the normalized ionization intensity. Inset (b) Second derivative of (o) the cysteine sulfur K-edge XAS spectrum, and (–, blue) of the pseudo-Voigt fit.
(a) (○) Sulfur K-edge XAS spectrum of cysteine dissolved in pH 5.1 aqueous citrate, and (–, blue) the MXAN fit to the spectrum (Rsq = 21.8). (b) The sulfur K-edge XAS as in (a) offset by unity, and (–, green) the MXAN fit using the theta function to exclude the rising edge energy region from the fit (Rsq = 1.46). (Inset) Expansion of the rising edge energy region of the XAS spectrum and the two MXAN fits to the spectrum. The Rsq values from (a) and (b) are not comparable because the fitted energy ranges are not identical.
The DFT geometry optimized structure for a single donor water-sulfur hydrogen bond associated with dissolved cysteine. The bulk water environment was approximated using the polarized continuum model.
(○), Sulfur K-edge XAS of cysteine in pH 5.1 citrate buffer. (a) (–, Red) The MXAN fit modeling one donor H-bonding water (mode I); and (b) (–, blue) the MXAN fit modeling one H-bond accepting water (mode II); see text. (Inset) Expanded display of the early continuum energy XAS region and the fits.
(-○-), The change in the MXAN goodness-of-fit Rsq value with sulfur-oxygen distance. The test for the dependence of Rsq with S–O distance began with one hydrogen-bonded water molecule at 2.49 Å (HO–H⋯SCys = 1.7 Å) and advanced stepwise through and beyond the DFT energy-optimized S–O distance of 3.49 Å.
(○) Sulfur K-edge XAS of cysteine in pH 5.1 citrate buffer and (–, red) the MXAN fit using the two-water model. Inset (a) (○) the early continuum energy region of the XAS spectrum and (–, red) the MXAN fit. Inset (b) Comparison of the fe(E) unfit residuals of (–, green) the one water model (see Figures 3 and 4 ) and (–, violet) the two water model.
The final MXAN structural model of the cysteine zwitterion with two waters. Water “1” represents the solvent molecule durably hydrogen-bonded to sulfur.
DFT calculated one-dimensional potential energy surface. (a) ±1 Å change in the S⋯H–OH hydrogen bond. The geometric structures from the two points shown in red have been used for the CCSD(T) energy calculations. (b) ±0.75 Å change in the S–C bond.
Contour plots of LUMO (33), LUMO+1 (34), LUMO+2 (35), and LUMO+3 (36). These orbitals have significant sulfur character and are expected to contribute to the sulfur K-edge XAS spectrum.
Comparison of (–), the experimental S K-edge XAS spectrum of cysteine, pH 5.1; (-ߙ-ߙ-, blue dashed) the TD-DFT simulated spectra of cysteine, broadened to the core-hole lifetime width of 0.6 eV FWHM, 44 showing the transitions that dominate the spectrum; and (–, red) the fully simulated XAS spectrum, broadened to the experimental resolution of 1 eV FWHM. (Top) 0-water model, (bottom) 1-water model.
(○) Sulfur K-edge XAS spectrum of dissolved cysteine, pH 5.1; (–, red) TD-DFT simulation of the rising edge energy region, broadened by 1 eV FWHM (see Figure 10 ); (–, green) individual transitions as calculated using TD-DFT; and (–, blue) the MXAN fit to the higher energy and continuum regions. The gray rectangle marks the energy region where the TD-DFT and MXAN simulations overlap.
Sulfur K-edge XAS spectrum of (-○-) cysteine in pH 5.1 citrate buffer; (–, red) the extended continuum theory MXAN fit to the full experimental XAS spectrum; and (–, blue) the three atom fragment also calculated using extended continuum theory. (Inset) Expansion of the XANES and near-edge continuum energy regions, also showing the structure of the minimal three-atom fragment.
MXAN H-bonding metrics.
Two-water MXAN fit metrics.
DFT Mulliken charges for cysteine with one S⋯Hw hydrogen bond modeled without and with a polarizable continuum.
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