There is an empirical correlation between the protein backbone conformation and the secondary HA, CA, CB, and CO chemical shifts (the difference between observed shifts and randon coil shifts). Such chemical shifts are readily measured for all residues and they are used together with the coupling constants and other NMR constraints for structure refinement. Several methods have been proposed to identify secondary structure elements from the analysis of chemical shifts in proteins:

• Analysis of the figerprint region (HN-HA) of a 2D COSY map reforded for H₂O solution ( 91FEBS72-293 ) .
• Analysis of the ¹³CO region considering two different regions: 170.0-174.5 for beta regions and 177-185 ppm for alpha regions.
• The Chemical Shift Index (CSI) protocol. ( 04JB143-30 )
• Comparing differences of CA and CB chemical shifts with respect to amino acid sequence ( 93BIO13818 ).

 Aspects to consider in such deviations:

• Effect of backbone dihedral angle ( 91JACS5490 and 94METH363 )
• Effect of side-chain geometry ( 97JACS11941 and 97JACS11951 )
• Effect of hydrogen bonding ( 92JACS4320 ).
• Effect of amino acid sequence ( 94JACS8466 and 97FEBS285-419 ).
• Effect of aromatic ring currents ( 97JB389 ).