The backbone dynamics of the 28 residue N-15-labelled human atrial natriuretic peptide have been examined by N-15 NMR methods. N-15 R-1, R-2 and {H-1}-N-15 NOE values were determined for the oxidised and reduced forms of the peptide (ANP(ox) and ANP(rd), respectively), and analysed using reduced spectral density mapping and an extended model-free approach. The two forms possessed correlation times for overall molecular motion of 4.7 ns and were highly flexible, with substantial contributions to relaxation processes from internal motions on picosecond to nanosecond time scales. Reduction of the Cys7-Cys23 disulphide bond to form ANP(rd) produced a very dynamic linear peptide with a mean overall order parameter of 0.2; the intramolecular cross-link in ANP(ox) increased this to a mean value of 0.4. A simple model for segmental backbone motion accounted for the R-2 values of both species using only two variable parameters, indicating that relaxation is dominated by interactions with sites <7 residues distant in the covalent network and that changes in the conformation of the disulphide bond lead to significant chemical exchange broadening in ANP(ox). The contributions of backbone dynamics to configurational entropy were determined and accounted for the different receptor binding affinities of cyclised and linear natriuretic peptides. (C) 2004 Elsevier Inc. All rights reserved.