The formation of the chemical shift index is a method to determine the secondary structure of proteins from the chemical shifts of backbone atoms. It is based on the chemical shifts listed in the following table (+- 0.7ppm).
The following simple rules are applied:
1 is assigned if the measured chemical shift is greater than the CSI value range.
-1 is assigned, if the measured chemical shift is smaller than the CSI value range.
0 is assigned if the measured chemical shift lies within the value range.
To determine the secondary structure, proceed as follows:
The alpha helix is defined if four or more “-1” HA and/or “1” CA/CO are found one after the other.
A beta strand is defined when three or more “1” HA and/or “-1” CA/CO are found one after the other.
All other ranges are called coils.