NMR AND COMPUTER-AIDED MODELING STUDIES OF THE INTERACTIONS BETWEEN ACYCLIC HEXAPEPTIDE AND THE 2 ENANTIOMERS OF SOME BOC-AMINO AND FMOC-AMINO ACIDS

The cyclic hexapeptide cyclo[-Pro1-Gly2-Glu3(OBzl)-Pro4-Phe5,-Leu6-] ( 1) was modeled and synthesized to be used for chiral discrimination st udies. Total correlated spectroscopy and nuclear Overhauser effect spe ctroscopy spectra of the cyclic hexapeptide 1 in CDCl3 showed the pres ence 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 domi nant stereoisomers, only 1a interacted specifically with t-butyloxycar bonyl (Boc-) and 9-flourenylmethyloxycarbonyl (Fmoc-) amino acids in C DCl3. The interaction site of 1a when complexing with the derivatized amino acids was the chain segment Phe5-Leu6. The Phe5 NH and Leu6 NH p rotons are contiguous and solvent exposed. Their nmr signals shifted s trongly downfield with the addition of Boc- or Fmoc- amino acids to th e peptide solution. Thus, both NH protons hydrogen bond to the amino a cids, forming a two-point hydrogen-bonding complex. The peptide stereo isomer 1b did not interact specifically with the Boc- and Fmoc-amino a cids because of the lack of two contiguous and solvent-exposed peptidi c NH protons that seem to be needed for specific interactions of the c yclic hexapeptide 1 with the Boc- and Fmoc- amino acids. A clear diffe rence in the interaction of 1a with D- and L-enantiomers of Boc-Trp an d Fmoc-Trp was observed with nmr spectroscopy. Docking models and mole cular mechanics calculations together with nmr observations showed tha t 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 hydro gen-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 a romatic ring currents. The indole NH of the D-enantiomer did not hydro gen bond to the Pro1 carbonyl group because the formation of such a th ree-point hydrogen-bonding complex was thermodynamically unfavorable.