Abstract
The determination of the degree of folding (β) of the diketopiperazine ring (DKP) of cyclic dipeptides from 3J(HNCH) and 5J(HH), the homoallylic coupling across peptide bonds, is discussed. From measurements of the 100 MHz 1H n.m.r. spectra of cyclic dipeptides containing glycine or sarcosine[i.e. cyclo-Gly(Sar)-X where X = amino acid] it is concluded that β increases with the bulkiness of amino-acid X for both D2O and DMSO solutions. The results are in agreement with those determined from the c.d. spectra of the same cyclic dipeptides in aqueous solution. A correlation is observed between the magnetic non-equivalence of the Gly (or Sar) methylene group and the degree fo folding of cyclic dipeptides containing amino-acids not substituted by aromatic rings at the β-carbon atom.
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