Tuesday, May 11, 2010

Journal of Physical Chemistry B, vol. 114, Issues 18

Molecular Level Characterization of the Inorganic−Bioorganic Interface by Solid State NMR: Alanine on a Silica Surface, a Case Study

Ira Ben Shir†, Shifi Kababya†, Tal Amitay-Rosen‡, Yael S. Balazs† and Asher Schmidt*†

J. Phys. Chem. B, 2010, 114 (18), pp 5989–5996
DOI: 10.1021/jp100114v
Publication Date (Web): April 16, 2010

Abstract: The molecular interface between bioorganics and inorganics plays a key role in diverse scientific and technological research areas including nanoelectronics, biomimetics, biomineralization, and medical applications such as drug delivery systems and implant coatings. However, the physical/chemical basis of recognition of inorganic surfaces by biomolecules remains unclear. The molecular level elucidation of specific interfacial interactions and the structural and dynamical state of the surface bound molecules is of prime scientific importance. In this study, we demonstrate the ability of solid state NMR methods to accomplish these goals. l-[1-13C,15N]Alanine loaded onto SBA-15 mesoporous silica with a high surface area served as a model system. The interacting alanine moiety was identified as the −NH3+ functional group by 15N{1H}SLF NMR. 29Si{15N} and 15N{29Si}REDOR NMR revealed intermolecular interactions between the alanine −NH3+ and three to four surface Si species, predominantly Q3, with similar internuclear N···Si distances of 4.0−4.2 Å. Distinct dynamic states of the adsorbed biomolecules were identified by 15N{13C}REDOR NMR, indicating both bound and free alanine populations, depending on hydration level and temperature. In the bound populations, the −NH3+ group is surface anchored while the free carboxylate end undergoes librations, implying the carboxylate has small or no contributions to surface binding. When surface water clusters grow bigger with increased hydration, the libration amplitude of the carboxyl end amplifies, until onset of dissolution occurs. Our measurements provide the first direct, comprehensive, molecular-level identification of the bioorganic−inorganic interface, showing binding functional groups, geometric constraints, stoichiometry, and dynamics, both for the adsorbed amino acid and the silica surface.

Selective Chemical Shift Assignment of Bacteriochlorophyll a in Uniformly [13C−15N]-Labeled Light-Harvesting 1 Complexes by Solid-State NMR in Ultrahigh Magnetic Field

Anjali Pandit*, Francesco Buda, Adriaan J. van Gammeren†, Swapna Ganapathy and Huub J. M. de Groot
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
J. Phys. Chem. B, 2010, 114 (18), pp 6207–6215
DOI: 10.1021/jp100688u

Abstract: Magic-angle spinning (MAS) 13C−13C correlation NMR spectroscopy was used to resolve the electronic ground state characteristics of the bacteriochlorophyll a (BChl a) cofactors in light-harvesting 1 (LH1) complexes of Rhodopseudomonas acidophila (strain 10050). The BChl a 13C isotropic chemical shifts of the LH1 complexes are compared to the 13C chemical shifts for BChl a dissolved in acetone-d6 and to 13C NMR data that has been obtained for the B800 and B850 BChl molecules in Rps. acidophila peripheral light-harvesting complexes (LH2). Since both complexes contain BChl a cofactors, we can address the chemical shift variability for specific carbon responses between the two types of antennae. The global shift pattern of the LH1 BChl's resembles the shift patterns of the LH2 α- and β-B850 BChl's, while some carbon responses, in particular the C3 and C31, show significant deviations. A comparison with density functional theory (DFT) shift calculations provides insight into the BChl concomitant structural and electronic interactions in the ground state. The differences in the LH1 BChl observed chemical shifts relative to the 13C responses of BChl a in solution cannot be explained by local side chain interactions, such as hydrogen bonding or nonplanarity of the C3 acetyl, but appear to be dominated by protein-induced macrocycle distortion. Such shaping of the macrocycle will contribute significantly to the red shift of the BChl Qy absorbance band in purple bacterial light-harvesting complexes.

Solid-State 137Ba NMR Spectroscopy: An Experimental and Theoretical Investigation of

Hiyam Hamaed†, Eric Ye‡, Konstantin Udachin§ and Robert W. Schurko*†

J. Phys. Chem. B, 2010, 114 (18), pp 6014–6022
DOI: 10.1021/jp102026m

Abstract: Ultrawideline 137Ba SSNMR spectra of several barium-containing systems (barium nitrate, barium carbonate, barium chlorate monohydrate, barium chloride dihydrate, anhydrous barium chloride, and barium hydrogen phosphate) were acquired at two different magnetic field strengths (9.4 and 21.1 T) using frequency-stepped techniques. The recently reported WURST−QCPMG pulse sequence (O’Dell et al. Chem. Phys. Lett. 2008, 464, 97−102) is shown to be very useful for rapidly acquiring high signal-to-noise 137Ba SSNMR spectra. The breadths of the second-order quadrupolar-dominated spectra and experimental times are notably reduced for experiments conducted at 21.1 T. Analytical simulations of the 137Ba SSNMR spectra at both fields yield the quadrupolar parameters, and in select cases the barium chemical shift anisotropies (CSAs). Quadrupolar interactions dominate the 137Ba powder patterns, with quadrupolar coupling constants, CQ(137Ba), ranging from 7.0 to 28.8 MHz. The 137Ba electric field gradient (EFG) parameters extracted from these spectra are correlated to the local environments at the barium sites, via consideration of molecular symmetry and structure, and first principles calculations of 137Ba EFG tensors performed using CASTEP software. The rapidity with which 137Ba SSNMR spectra can be acquired using the WURST pulse sequence and/or at ultrahigh magnetic fields and the sensitivity of the 137Ba EFG tensor parameters to the changes in the barium environment suggest that 137Ba SSNMR has great potential for structural characterization of a variety of barium-containing materials.

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