FORMATION OF SEVERAL STABLE COMPLEXES BETWEEN THE MINOR COMPONENTS OFTHE CYCLIC TETRAPEPTIDE CYCLO-(-PRO(1)-ALA(2)-D-PHE(3)-LEU(4)-) AND SOME SPECIFIC BOC-AMINO ACIDS

The cyclic tetrapeptide cyclo(-Pro1-Ala2-D-Phe3-Leu4-) was modeled and synthesized to be used for molecular interactions and chiral discrimi nation studies in CDCl3. Total correlation spectroscopy and nuclear Ov erhauser effect spectroscopy spectra of the cyclic tetrapeptide showed the presence of one dominant stereoisomer-1-and three minor ones-2a, 2b, and 2c-in a relationship of 92 : 6 : 1 : 1. They formed three- to five-hydrogen bond complexes with Boc-D-Ser, Boc-L-Ser and Boc-L-Thr ( Boc: t-butyloxylcarbonyl). These three Boc-amino acids interact more s trongly with 2a, 2b, and 2c than with 1, altering their relative conce ntrations to 48 : 40 : 6 : 6. In the complex between the cyclic tetrap eptide and Boc-D-Ser, the stereoisomer 2a exchanged chemically with 1, 2b, and 2c, while 1 did not exchange with either 2b or 2c. This chemi cal exchange is due to cis-trans isomerization of the peptide bonds. T he chiral discrimination of 2a, 2b, and 2c was stronger than that of 1 . No complexation occurred with Boc-L-Ala or Boc-L-Trp. (C) 1993 John Wiley & Sons, Inc.