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.
| HA | CA | CB | CO | |
| Ala | 4.35 | 52.5 | 19.0 | 177.1 |
| Cys | 4.65 | 58.8 | 28.6 | 174.8 |
| Asp | 4.76 | 54.1 | 40.8 | 177.2 |
| Glu | 4.29 | 56.7 | 29.7 | 176.1 |
| Phe | 4.66 | 57.9 | 39.3 | 175.8 |
| Gly | 3.97 | 45.0 | - | 173.6 |
| His | 4.63 | 55.8 | 32.0 | 175.1 |
| Ile | 3.95 | 62.6 | 37.5 | 176.8 |
| Lys | 4.36 | 56.7 | 32.3 | 176.5 |
| Leu | 4.17 | 55.7 | 41.9 | 177.1 |
| Met | 4.52 | 56.6 | 32.8 | 175.5 |
| Asn | 4.75 | 53.6 | 39.0 | 175.5 |
| Pro | 4.44 | 62.9 | 31.7 | 176.0 |
| Gln | 4.37 | 56.2 | 30.1 | 176.3 |
| Arg | 4.38 | 56.3 | 30.3 | 176.5 |
| Ser | 4.50 | 58.3 | 62.7 | 173.7 |
| Thr | 4.35 | 63.1 | 68.1 | 175.2 |
| Val | 3.95 | 63.0 | 31.7 | 177.1 |
| Trp | 4.70 | 57.8 | 28.3 | 175.8 |
| Tyr | 4.60 | 58.6 | 38.7 | 175.7 |
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