Chemical-shift Index (CSI)

The Chemical shift index (CSI) is a commonly accepted procedure to stablish the secondary structure of proteins based on chemical shift differences with respect to some predefined 'random coil' values. It can be applied from the measured HA, CA, CB and CO chemical shifts for each residue in a protein ( 92BIO1647 , 94METH363 , 94METH392 , 94JB171 , and 01JACS2970 ). The standard chemical shift values (+-0.7 ppm) used in the CSI method are: 

RESIDUE TYPE    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
 Simple rules are applied:
  • "1" if the measured chemical shift is greater than the specific CSI value.
  • "-1" if it's smaller.
  • "0" if it within the expected range.
    • The secondary structure is established following this: When several CSIs are available, a concensus CSI based on the majority rule approach allows to improve the prediction of the secondary structure.