Complete proton and carbon-13 resonance assignments of the cyclodecapeptide of elastin by combined use of multiple and selective proton-decoupled 13C and 1H spectra

Multiple and selective ¹H irradiation techniques were used to assign all the peptide ¹H and ¹³C resonances to their respective residues for the cyclodecapeptide, cyclo-( L·Val ₁- L·Pro ₂-Gly ₃- L·Val ₄-Gly ₅) ₂, analog of the polypentapeptide of elastin. As the structure of interest has twofold symmetry on the NMR time scale, the primary problems are to delineate the resonances of Val, from Val, and those of Gly ₃ from Gly ₅ and to assign the five peptide CO resonances. Irradiation of the Pro ₂ α CH while observing the carbonyl carbon spectrum allows identification of the pro ₂CO resonance, which is selectively intensified. The Gly NH which on selective irradiation also causes the Pro ₂CO to become selectively intensified is the Gly ₃ N H. The GIy ₃ N H can be irradiated to identify the Gly ₃ αC H₂ protons in the PMR spectrum, etc. With this combined use of selective proton irradiation of a peptide N H and an αC H proton while observing the CMR spectrum and with the usual proton homonuclear decoupling, it becomes possible to assign all of the resonances. This approach obviates the expensive and time-consuming process of achieving difficult assignments by synthetic isotopic enrichments.