Nmr and computer‐aided modeling studies of the interactions between a cyclic hexapeptide and the two enantiomers of some Boc‐ and Fmoc‐amino acids

Abstract

The cyclic hexapeptide cyclo [-Pro1-Gly2-Glu3 (OBzl)-Pro4-Phe5-Leu6-] (1) was modeled and synthesized to be used for chiral discrimination studies. Total correlated spectroscopy and nuclear Overhauser effect spectroscopy spectra of the cyclic hexapeptide 1 in CDCl3 showed the presence of three stereoisomers: two dominant stereoisomers 1a and 1b that exchanged chemically with each other, and a minor stereoisomer 1c (4%) that exchanged exclusively with the stereoisomer 1b. Of the two dominant stereoisomers, only 1a interacted specifically with t-butyloxycarbonyl (Boc-) and 9-flourenylmethyloxycarbonyl (Fmoc-) amino acids in CDCl3. The interaction site of 1a when complexing with the derivatized amino acids was the chain segment Phe5-Leu6. The Phe5 NH and Leu6 NH protons are contiguous and solvent exposed. Their nmr signals shifted strongly downfield with the addition of Boc- or Fmoc- amino acids to the peptide solution. Thus, both NH protons hydrogen bond to the amino acids, forming a two-point hydrogen-bonding complex. The peptide stereoisomer 1b did not interact specifically with the Boc- and Fmoc-amino acids because of the lack of two contiguous and solvent-exposed peptidic NH protons that seem to be needed for specific interactions of the cyclic hexapeptide 1 with the Boc- and Fmoc-amino acids. A clear difference in the interaction of 1a with D- and L-enantiomers of Boc- Trp and Fmoc-Trp was observed with nmr spectroscopy. Docking models and molecular mechanics calculations together with nmr observations showed that the NH proton of the indole ring of the Boc-L-Trp and the Fmoc-L-Trp hydrogen bonded to the Pro1 carbonyl group. In this three-point hydrogen-bonding complex, the indole ring becomes locked underneath the Leu residue. The nmr signals of all the Leu6 protons (except for Leu NH) shifted strongly upfield owing to the shielding effect of the indole aromatic ring currents. The indole NH of the D-enantiomer did not hydrogen bond to the Pro1 carbonyl group because the formation of such a three-point hydrogen-bonding complex was thermodynamically unfavorable.