Study of human calcitonin fibrillation by proton nuclear magnetic resonance spectroscopy

The fibrillation of human calcitonin (hCT) has been investigated by NMR in aqueous solution. The time course of proton one- and two-dimensional NMR spectra of hCT (80 mg/mL at pH 2.9) was measured during the fibrillation. It showed a gradual broadening of the peptide peaks, followed by a rapid broadening and subsequent disappearance of the peaks. The gradual broadening can be attributed to equilibrium between monomer and associated hCT, whereas the rapid broadening can be attributed to formation of aggregates and to gelation of the peptide solution. All the peptide peaks did not broaden and disappear simultaneously. Peaks of residues in the N-terminal (Cys1-Cys7) and central (Met 8-Pro23) regions broadened and disappeared faster during the gradual broadening than those in the C-terminal region (Gln24-Pro32). Moreover, in the N-terminal and central residues, peaks of Cys1, Leu4,9, Met 8, Tyr12, Asp15, and Phe16,19,22 disappeared faster than those of Asn3,17, Ser5, Cys7, Gln14, Lys18, and His20. Hydrogen-deuterium exchange of amide protons indicated the formation of hydrogen bonds caused by association of hCT molecules. The amphiphilicity of the peptide appears to be important for the hCT association.