@article {373,
	title = {An experimental and theoretical investigation of the chemical shielding tensors of C-13(alpha) of alanine, valine, and leucine residues in solid peptides and in proteins in solution},
	journal = {Journal of the American Chemical Society},
	volume = {123},
	year = {2001},
	note = {J Am Chem Soc486FTTimes Cited:47Cited References Count:31},
	month = {Oct 24},
	pages = {10362-10369},
	abstract = {<p>We have carried out a solid-state magic-angle sample-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic investigation of the C-13(alpha), chemical shielding tensors of alanine, valine, and leucine residues in a series of crystalline peptides of known structure. For alanine and leucine, which are not branched at the beta -carbon, the experimental chemical shift anisotropy (CSA) spans (Omega) are large, about 30 ppm, independent of whether the residues adopt helical or sheet geometries, and are in generally good accord with Omega values calculated by using ab initio Hartree-Fock quantum chemical methods. The experimental Omegas for valine C-alpha in two peptides (in sheet geometries) are also large and in good agreement with theoretical predictions. In contrast, the \&quot;CSAs\&quot; (Delta sigma*) obtained from solution NMR data for alanine, valine, and leucine residues in proteins show major differences, with helical residues having Delta sigma* values of similar to6 ppm while sheet residues have Delta sigma* approximate to 27 ppm. The origins of these differences are shown to be due to the different definitions of the CSA, When defined in terms of the solution NMR CSA, the solid-state results also show small helical but large sheet CSA values. These results are of interest since they lead to the idea that only the beta -branched amino acids threonine, valine, and isoleucine can have small (static) tensor spans, Omega (in helical creometries), and that the small helical \&quot;CSAs\&quot; seen in solution NMR are overwhelmingly dominated by changes in tensor orientation, from sheet to helix. These results have important implications for solid-state NMR structural studies which utilize the CSA span, Omega, to differentiate between helical and sheet residues. Specifically, there will be only a small degree of spectral editing possible in solid proteins since the spans, Omega, for the dominant nonbranched amino acids are quite similar. Editing on the basis of Omega will, however, be very effective for many Thr, Val, and Hen residues, which frequently have small (similar to 15-20 ppm) helical CSA (Omega) spans.</p>
},
	keywords = {threonine},
	isbn = {0002-7863},
	doi = {Doi 10.1021/Ja0115060},
	url = {<Go to ISI>://WOS:000171808300025},
	author = {Havlin, R. H. and Laws, D. D. and Bitter, H. M. L. and Sanders, L. K. and Sun, H. H. and Grimley, J. S. and Wemmer, D. E. and Pines, A. and Oldfield, E.}
}
@article {418,
	title = {Determination of dihedral angles in peptides through experimental and theoretical studies of alpha-carbon chemical shielding tensors},
	journal = {Journal of the American Chemical Society},
	volume = {119},
	year = {1997},
	note = {J Am Chem SocXt037Times Cited:90Cited References Count:57},
	month = {Aug 20},
	pages = {7827-7831},
	abstract = {<p>A simple method for the determination of backbone dihedral angles in peptides and proteins is presented. The chemical-shift anisotropies (CSA) of the central alanine alpha-carbon in powdered crystalline tripeptides, whose structures have been determined previously by X-ray crystallography, were measured by cross-polarization magic-angle-spinning nuclear magnetic resonance. The experimental CSA values were correlated with ab initio chemical-shielding calculations over Ramanchandran phi/psi space on an N-formyl-L-alanine amide fragment. Using this correlation, phi/psi probability surfaces for one of the tripeptides were calculated based only on the alpha-carbon CSA, allowing a prediction of backbone angles. Dihedral angles predicted by these calculations fall within +/-12 degrees of the values determined by crystallography. This approach should be useful in the determination of solid-slate protein structure.</p>
},
	keywords = {crystal-structure},
	isbn = {0002-7863},
	doi = {Doi 10.1021/Ja970124k},
	url = {<Go to ISI>://WOS:A1997XT03700025},
	author = {Heller, J. and Laws, D. D. and Tomaselli, M. and King, D. S. and Wemmer, D. E. and Pines, A. and Havlin, R. H. and Oldfield, E.}
}
