Wednesday, October 20, 2010

Journal updates

J. Am. Chem. Soc., 2010, 132 (40), pp 13984–13987

High Resolution Measurement of Methyl 13Cm−13C and 1Hm−13Cm Residual Dipolar Couplings in Large Proteins
Chenyun Guo, Raquel Godoy-Ruiz, and Vitali Tugarinov*

NMR methodology is developed for high-resolution, accurate measurements of methyl 1Hm−13Cm (1DCH) and 13Cm−13C (1DCC) residual dipolar couplings (RDCs) in ILV-methyl-protonated high-molecular-weight proteins. Both types of RDCs are measured in a three-dimensional (3D) mode that allows dispersion of correlations to the third (13Cβ/γ) dimension, alleviating the problem of overlap of methyl resonances in highly complex and methyl-abundant protein structures. The methodology is applied to selectively ILV-protonated 82-kDa monomeric enzyme malate synthase G (MSG) that contains 273 ILV methyl groups with substantial overlap of methyl resonances in 2D methyl 1H−13C correlation maps. A good agreement is observed between the measured RDCs of both types and those calculated from the crystallographic coordinates of MSG for the residues with low-amplitude internal dynamics. Although the measurement of 1DCH RDCs from the acquisition dimension of NMR spectra imposes certain limitations on the accuracy of obtained 1DCH values, 1DCH couplings can be approximately corrected for cross-correlated relaxation effects. The ratios of 1DCH and 1DCC
couplings (1DCH/1DCC) are independent of methyl axis dynamics and the details of residual alignment [Ottiger, M.; Bax, A. J. Am. Chem. Soc. 1999, 121, 4690.].
The 1DCH/1DCC ratios obtained in MSG can therefore validate the employed correction scheme.

J. Am. Chem. Soc., 2010, 132 (40), pp 14015–14017

The Structure of Formaldehyde-Inhibited Xanthine Oxidase Determined by 35 GHz 2H ENDOR Spectroscopy

Muralidharan Shanmugam†, Bo Zhang‡, Rebecca L. McNaughton†, R. Adam Kinney†, Russ Hille*‡, and Brian M. Hoffman*†

The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) has been studied for four decades, yet it has not proven possible to distinguish unequivocally among the several structures proposed for this form. The uniquely large isotropic hyperfine coupling for 13C from CH2O led to the intriguing suggestion of a direct Mo−C bond for the active site of I. This suggestion was supported by the recent crystal structures of glycol- and glycerol-inhibited forms of aldehyde oxidoreductase, a member of the xanthine oxidase family. 1H and 2H ENDOR spectra of I(C1,2H2O) in H2O/D2O buffer now have unambiguously revealed that the active-site structure of I contains a CH2O adduct of Mo(V) in the form of a four-membered ring with S and O linking the C to Mo and have ruled out a direct Mo−C bond. Density functional theory computations are consistent with this conclusion. We interpret the large 13C coupling as resulting from a “transannular hyperfine interaction”.

J. Phys. Chem. A, 2010, 114 (24), pp 6622–6629
Solid-State NMR Spectra and Long, Intra-Dimer Bonding in the π-[TTF]22+ (TTF = Tetrathiafulvalene) Dication
Merrill D. Halling, Joshua D. Bell, Ronald J. Pugmire, David M. Grant* and Joel S. Miller*


The 13C chemical-shift tensor principal values for TTF and π-[TTF]22+ (TTF = tetrathiafulvalene) dimer dications have been measured in order to better understand the electronic structure and long intradimer bonding of these TTF-based dimer structures. The structure of π-[TTF]22+ is abnormal due to its two C−C and four S−S ca. 3.4 Å intradimer separations, which is less than the sum of the sulfur van der Waals radii, and has a singlet 1A1g electronic ground state. This study of TTF and [TTF]22+ was conducted to determine how the NMR chemical-shift tensor principal values change as a function of electronic structure. This study also establishes a better understanding of the interactions that lead to spin-pairing of the monomeric radical units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. The embedded ion method (EIM) was used to investigate the electrostatic lattice potential in [TTF]22+. These theoretical methods provide information on the tensor magnitudes and orientations of their tensor principal values with respect to the molecular frame. The experimental chemical-shift principal values agree with the calculated quantum mechanical chemical-shielding principal values, within typical errors commonly seen for this class of molecular system. Relatively weak Wiberg bond orders between the two [TTF]+ components of the dimer dication correlate with the long bonds linking the two [TTF]+ monomers and substantiate the claim that there is weak multicenter bonding present.

Inorg. Chem., 2010, 49 (12), pp 5522–5529
Incorporation of Phosphorus Guest Ions in the Calcium Silicate Phases of Portland Cement from 31P MAS NMR SpectroscopySøren L. Poulsen, Hans J. Jakobsen and Jørgen Skibsted*

Portland cements may contain small quantities of phosphorus (typically below 0.5 wt % P2O5), originating from either the raw materials or alternative sources of fuel used to heat the cement kilns. This work reports the first 31P MAS NMR study of anhydrous and hydrated Portland cements that focuses on the phase and site preferences of the (PO4)3− guest ions in the main clinker phases and hydration products. The observed 31P chemical shifts (10 to −2 ppm), the 31P chemical shift anisotropy, and the resemblance of the lineshapes in the 31P and 29Si MAS NMR spectra strongly suggest that (PO4)3− units are incorporated in the calcium silicate
phases, alite (Ca3SiO5) and belite (Ca2SiO4), by substitution for (SiO4)4−tetrahedra. This assignment is further supported by a determination of the spin−lattice relaxation times for 31P in alite and belite, which exhibit the same ratio as observed for the corresponding 29Si relaxation times. From simulations of the intensities, observed in inversion−recovery spectra for a white Portland cement, it is deduced that 1.3% and 2.1% of the Si sites in alite and belite, respectively, are replaced by phosphorus. Charge balance may potentially be achieved to some extent by a coupled substitution mechanism where Ca2+ is replaced by Fe3+ ions, which may account for the interaction of the 31P spins with paramagnetic Fe3+ ions as observed for the ordinary Portland cements. A minor fraction of phosphorus may also be present in the separate phase Ca3(PO4)2, as indicated by the observation of a narrow resonance at δ(31P) = 3.0 ppm for two of the studied cements. 31P{1H} CP/MAS NMR spectra following the hydration of a white Portland cement show that the resonances from the hydrous phosphate species fall in the same spectral range as observed for (PO4)3− incorporated in alite. This similarity and the absence of a large 31P chemical shift ansitropy indicate that the hydrous (PO4)3− species are incorporated in the interlayers of the calcium−silicate−hydrate (C−S−H) phase, the principal phase formed upon hydration of alite and belite.

Inorg. Chem., 2010, 49 (12), pp 5573–5583
Basic Coordination Chemistry Relevant to DNA Adducts Formed by the Cisplatin Anticancer Drug. NMR Studies on Compounds with Sterically Crowded Chiral LigandsJamil S. Saad*†§, Michele Benedetti‡, Giovanni Natile‡ and Luigi G. Marzilli*†

Me4DABPtG2 adducts with the bulky C2-symmetric chiral diamine, Me4DAB (N,N,N′,N′-tetramethyl-2,3-diaminobutane with R,R and S,S configurations at the chelate ring C atom, G = guanine derivative), exhibit slow conformer interchange and are amenable to characterization by NMR methods. The investigation of the cis-PtA2G2 adducts formed by clinically widely used anticancer drugs [A2 = diaminocyclohexane, (NH3)2] is impeded by the rapid conformer interchange permitted by the low A2 bulk near the inner coordination sphere. Me4DABPtG2 adducts exist as a mixture of exclusively head-to-tail (HT) conformers. No head-to-head (HH) conformer was observed. The Me4DAB chirality significantly influences which HT chirality is favored (ΔHT for S,S and ΛHT for R,R). For simple G ligands, the ratio of favored HT conformer to less favored HT conformer is 2:1. For guanosine monophosphate (GMP) ligands, the phosphate group cis G N1H hydrogen bonding favors the ΛHT and the ΔHT conformers for 5′-GMP and 3′-GMP adducts, respectively. For both HT conformers of cis-PtA2G2 adducts, the G nucleobase plane normally cants with respect to the coordination plane in the same direction, left or right, for a given A2 chirality. In contrast, the results for Me4DABPtG2 adducts provide the first examples of a change in the canting direction between the two HT conformers; this unusual behavior is attributed to the fact that canting always gives long G O6 to N−Me distances and that these Me4DAB ligands have bulk both above and below the coordination plane. These results and ongoing preliminary studies of Me4DABPt adducts with G residues linked by a phosphodiester backbone, which normally favors HH conformers, all indicate that a high percentage of HT conformer is present. Collectively, these findings advance fundamental concepts in Pt-DNA chemistry and may eventually help define the role of the carrier-ligand steric effects on anticancer activity.

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