Formation of several stable complexes between the minor components of the cyclic tetrapeptide cyclo‐(‐Pro 1 ‐Ala 2 ‐ D ‐Phe 3 ‐Leu 4 ‐) and some specific Boc‐amino acids

The cyclic tetrapeptide cyclo(‐Pro 1 ‐Ala 2 ‐ D ‐Phe 3 ‐Leu 4 ‐) was modeled and synthesized to be used for molecular interactions and chiral discrimination studies in CDCl 3 . Total correlation spectroscopy and nuclear Overhauser 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 strongly with 2a , 2b , and 2c than with 1 , altering their relative concentrations to 48:40:6:6. In the complex between the cyclic tetrapeptide 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 chemical exchange is due to cis‐trans isomerization of the peptide bonds. The chiral discrimination of 2a , 2b , and 2c was stronger than that of 1 . No complexation occurred with Boc‐ L ‐Ala or Boc‐ L ‐Trp. © 1993 John Wiley & Sons, Inc.