When a polypeptide is in a so-called ‘random coil’ conformation, i.e. it is highly dynamic with no secondary or tertiary structural preference, each amino acid type adopts very characteristic chemical shifts values, referred to as ‘random coil chemical shifts’. These shifts are essentially just averages over all possible conformations which the amino acid can adopt in the random coil. Wishart et al. have published a complete set of 1H, 13C and 15N random coil chemical shifts measured on Gly-Gly-Xxx-Ala-Gly-Gly peptides in 1M urea. Measurements on Gly-Gly-Xxx-Pro-Gly-Gly peptides take account of the effect which proline has on restricting the conformation of neighbouring amino acids.
Usually random coil chemical shifts are used as reference values in order to detect regions of the protein which are not in a random coil conformation (see the section on secondary structure ).
D.S. Wishart, C.G. Bigam, A. Holm, R.S. Hodges and B.D. Sykes (1995) J. Biomol. NMR 5 67-81. (Link to Article)