title: 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.
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA. ritis@slac.stanford.edu
authors.forename: Ritimukta
authors.initials: R
authors.lastname: Sarangi
authors.forename: Patrick
authors.initials: P
authors.lastname: Frank
authors.forename: Maurizio
authors.initials: M
authors.lastname: Benfatto
authors.forename: Silvia
authors.initials: S
authors.lastname: Morante
authors.forename: Velia
authors.initials: V
authors.lastname: Minicozzi
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
mesh_terms: D003545:Cysteine / Q000737:chemistry*; D008954:Models, Biological; D008958:Models, Molecular; D011789:Quantum Theory; D012997:Solvents; D013455:Sulfur / Q000737:chemistry*; D014867:Water / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D012997:Solvents; D014867:Water; D013455:Sulfur; D003545:Cysteine