ShiftX

NMR

secondary structure and nearest neighbor effects. These tables were derived using data mining techniques from a large database of reference-corrected protein chemical shifts called RefDB.^[3] These sequence/structure dependencies on chemical shifts, which cannot easily be converted to analytical formulae, are combined with standard classical or semi-classical equations (for ring current effects and hydrogen bond effects) to further improve the 1H, 13C and 15N chemical shift calculations. ShiftX differs from other protein chemical shift calculation techniques in that it blends both empirical observations with classical or semi-quantum mechanical approaches. Most other protein chemical shift calculation methods use either empirical (such as SPARTA^[4]) or quantum mechanical (such as ShiftS^[5]) approaches, exclusively. ShiftX is both fast and accurate. It has a correlation coefficient (r) between measured and calculated shifts of 0.91(1HA), 0.98 (13CA), 0.99 (13CB), 0.86 (13CO), 0.91 (15N), 0.74 (1HN), and 0.907 (side chain 1H) with RMS errors of 0.23, 0.98, 1.10, 1.16, 2.43, 0.49, and 0.30 ppm. ShiftX is used in several programs or web servers including ShiftCor. It is also used in the generation and updating of the re-referenced chemical shift database known as RefDB