A server (CheShift) has been developed to predict 13 C ␣ chemical shifts of protein structures. It is based on the generation of 696,916 conformations as a function of the , , , 1 and 2 torsional angles for all 20 naturally occurring amino acids. Their 13 C ␣ chemical shifts were computed at the DFT level of theory with a small basis set and extrapolated, with an empirically-determined linear regression formula, to reproduce the values obtained with a larger basis set. Analysis of the accuracy and sensitivity of the CheShift predictions, in terms of both the correlation coefficient R and the conformational-averaged rmsd between the observed and predicted 13 C ␣ chemical shifts, was carried out for 3 sets of conformations: (i) 36 x-ray-derived protein structures solved at 2.3 Å or better resolution, for which sets of 13 C ␣ chemical shifts were available; (ii) 15 pairs of x-ray and NMR-derived sets of protein conformations; and (iii) a set of decoys for 3 proteins showing an rmsd with respect to the x-ray structure from which they were derived of up to 3 Å. Comparative analysis carried out with 4 popular servers, namely SHIFTS, SHIFTX, SPARTA, and PROSHIFT, for these 3 sets of conformations demonstrated that CheShift is the most sensitive server with which to detect subtle differences between protein models and, hence, to validate protein structures determined by either x-ray or NMR methods, if the observed 13 C ␣ chemical shifts are available. CheShift is available as a web server. chemical shifts prediction ͉ DFT calculations ͉ validation server A ccurate and fast validation of protein structures constitutes a long-standing problem in NMR spectroscopy (1-3). Investigators have proposed a plethora of methods to determine the accuracy and reliability of protein structures in recent years (4-8). Despite this progress, there is a growing need for more sophisticated, physics-based and fast structure-validation methods (1, 2, 7). With these goals in mind, we recently proposed a new, physics-based solution of this important problem (9), viz., a methodology that makes use of observed and computed 13 C ␣ chemical shifts (at the DFT level of theory) for an accurate validation of protein structures in solution (9) and in a crystal (10). Assessment of the ability of computed 13 C ␣ chemical shifts to reproduce observed values for a single or an ensemble of structures in solution and in a crystal was accomplished by using the conformationally-averaged root-mean-square-deviation (ca-rmsd) as a scoring function (9). While computationally intensive, this methodology has several advantages: (i) it makes use of the 13 C ␣ chemical shifts, not shielding, that are ubiquitous to proteins; (ii) it can be computed accurately from the , , and torsional angles; (iii) there is no need for a priori knowledge of the oligomeric state of the protein; and (iv) no knowledgebased information or additional NMR data are required.However, the primary and the most serious limitation of the method is the computational cost of such calculations, whi...