Conformational Properties of the Unnatural Amino Acid β-Methylphenylalanine In a Linear Octapeptide System; Correlations of ¹³C-NMR Chemical Shifts with the Side-Chain Stereochemistry of These Amino Acid Residues

Conformational properties of the four stereoisomers ([2S,3S], [2S,3R], [2R,3S], and [2R,3R]) of a synthetic amino acid, β-methylphenylanaline (β-MePhe), in a bioactive octapeptide sequence of CCK, H-Asp¹-Tyr²-β-MePhe³-Gly⁴-Trp⁵-Nle⁶-Asp⁷-Phe⁸-NH₂, have been studied by using and ¹H and ¹³C-2D NMR spectroscopy. β-MePhe³ residues introduce significant perturbations to the side-chain conformations. On the basis of the rotamer populations determined by a combination of homonuclear and heteronuclear vicinal coupling constants, each of the four different stereoisomers of β-MePhe residues virtually eliminates one of the three staggered side-chain conformations, trans for (2S,3S)- and (2R,3R)-β-MePhe, gauche(+) for (2S,3R)-β-MePhe, and gauche(-) for (2R,3S)-β-MePhe, respectively. It also was revealed that the side-chain rotamer populations of the Tyr² residue are influenced by different configurations of the β-carbon in the adjacent β-MePhe³ residues. An empirical correlation between the ¹³C chemical shifts of the β-CH₃ and the stereochemistry of β-methylphenylalanine side chains has been established, i.e., the δc of the β-MePhe in (2S,3S)- and (2R,3R)- isomers is at lower field by ca. 3 ppm relative to those in (2S,3R)- and (2R,3S)-isomers. This correlation can be rationalized on the basis of the γ-substituent effect in ¹³C-NMR chemical shift, and it may become a useful probe for side-chain conformations of similar molecules. Furthermore, these β-methylphenylalanine amino acids will provide useful side-chain conformational constraints in peptide and mimetic design.