NMR studies on the conformation of cyclodipeptides with two identical L-aromatic amino acid residues in solutions - cyclo[-l-5(OH)Trp-l-5(OH)Trp] and cyclo[-l-Phe-l-Phe]

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Abstract
NMR studies on the conformations of cyclodipeptides containing two identical L-aromatic amino acid residues cyclo[-L-5(OH)Trp-L-5(OH)Trp]8 and cyclo[-L-Phe-L-Phe]9 in DMSO solutions are reported here. The 1H chemical shifts, the spin-spin coupling constants, and the NOE results show that in these two cyclodipeptides one residue occupies the folded antiperpendicular and the second the extended to nitrogen perpendicular conformers respectively. The results indicate that each of the identical residues is in a fast conformational equilibrium between the above two conformers so that one set of 1H and 13C resonances is observed for the two identical residues in the above two cyclodipeptides.