title: Cis-trans isomerization of the proline residue in insulin studied by 13C NMR spectroscopy.
abstract: The natural abundance 13C NMR spectrum of bovine insulin contains two resonances at 49.6 and 48.9 ppm which have a 7:3 intensity ratio at 298 K. Use of the DEPT spectral editing technique shows them to be of CH2 multiplicity. On the basis of their chemical shifts, which are well-resolved from other peaks, they are assigned as the C delta carbon of proline in trans and cis forms respectively. Since insulin contains only a single proline residue, the site of the isomerization can be localized at the peptide bond linking Thr-27 and Pro-28. On heating, the two peaks broaden, coalesce at 308 K, and then sharpen to yield a single peak at higher temperatures. The barrier for this process was calculated to be 64 kJ mol-1 (at the coalesce temperature), which is at the lower end of the range observed for proline isomerization in small peptides. Computer-graphic studies based on the X-ray crystal structure of insulin were used to deduce the structural implications of the cis-trans isomerism in this globular protein.
authors.affiliation: School of Pharmaceutical Chemistry, Victorian College of Pharmacy Ltd., Parkville, Australia.
authors.forename: K A
authors.initials: KA
authors.lastname: Higgins
authors.forename: D J
authors.initials: DJ
authors.lastname: Craik
authors.forename: J G
authors.initials: JG
authors.lastname: Hall
authors.forename: P R
authors.initials: PR
authors.lastname: Andrews
mesh_terms: D007328:Insulin / Q000032:analysis*; D009682:Magnetic Resonance Spectroscopy; D011392:Proline / Q000032:analysis*; D011487:Protein Conformation; D013237:Stereoisomerism; D013696:Temperature; D013816:Thermodynamics
chemical_list: D007328:Insulin; D011392:Proline