Assignment of the 1H‐nmr resonances of the four rotamers of β‐casomorphin‐5 in DMSO

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Abstract
We report the complete assignment of the 1H‐nmr spectrum of β–casomorphin‐5 in DMSO‐d6 solution. With a combination of one‐dimensional, double quantum filtered correlated spectroscopy, homonuclear Hartmann–Hahn, and rotating frame nuclear Overhauser enhancement spectroscopy (ROESY) spectra, we were able to differentiate the four conformers originating from two Xxx‐Pro bonds present in the sequence. Exchange peaks in the ROESY spectra confirmed the presence of four interchanging conformational isomers. Based on integrations, the relative populations of the four species were estimated, while characteristic sequential nuclear Overhauser enhancements (NOEs) were used to determine the orientation of the Xxx‐Pro bonds. This orientation was also shown to correlate with the chemical shift changes for the α protons of both the Xxx and Pro residues. Finally, interresidue NOEs indicate conformational preferences for the aromatic side chains, especially in the all‐trans conformer.

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