NMR chemical shielding in cyclosarcosyl

NMR chemical shifts have a role to play when determining the structure of proteins by NMR methods through their dependence on the local conformation. NMR chemical shielding calculations are presented demonstrating that with an appropriate electronic model and basis set the CαH inequivalence in cyclosarcosyls can be rationalised. In the case of cyclotrisarcosyl the inequivalence is best understood in terms of a steric effect. While for cyclotetrasarcosyl, although significant distant, non-bonded, effects of a peptide group are seen, the vast majority of the large, greater than 2 ppm, inequivalence is attributable to the local conformation between adjacent sarcosine.