Conformationally constrained amino phenylalanine

The P-tetralin amino acid induces the a-helical conformation by fixing the torsional angles along the peptide backbone at about -60° (< >) and -50° ( p).109 P-Tetralin amino acids may be regarded as cyclic-constrained phenylalanine analogues. As shown in Section II.A, this class of unnatural amino acids is known to stabilize distinct conformations in peptides since the two substituents at the a-cen-ter restrict the available conformational space. Cyclic a,a-dialkylated glycines and a-substituted alanines preferentially adopt a-helical conformations.205... 

Surely, the introduction of a covalent bond between the aromatic ring of an a-amino acid residue and the peptide backbone has proven to be a useful further conformation restriction. For example, 1,2,3,4-tetrahydroisoquinoline carboxylic acid (Tic) is a cyclic constrained analog of phenylalanine (Figure 5), in which a methylene bridge is placed between the a-nitrogen, and 2 -carbon of the aromatic ring (Kazmierski Hruby, 1988). 

---