Chemical Shift Index
The chemical shift index or CSI is a simple graphical method that can be used to display and identify the type, location, and extent of secondary structures (helices, beta strands, and random coil regions) in polypeptides using only backbone chemical shift information. Originally developed for interpreting 1Hα chemical shifts, the chemical shift index exploits the well-known tendency of 1Hα shifts to be shifted upfield of their random coil values in helices and downfield of their random coil values in beta strands. To facilitate the identification of secondary structures in polypeptides from these shift tendencies, the chemical shift index employs a residue-specific “noise filter” to generate a visually pleasing, three-state or ternary (−1, 0, +1) index of chemical shift values. More specifically, if the upfield 1Hα shift (relative to a residue-specific random coil value) of a given residue is greater than 0.1 ppm, then that residue is given a value of −1....