ABSTRACT
An induced axial chirality in the biphenyl core of the 2',1':1,2;1'',2'':3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid (Bip) residue, a conformationally labile, atropoisomeric, C(alpha)-tetrasubstituted alpha-amino acid, was observed by CD and (1)H NMR spectroscopic techniques in the linear dipeptides Boc-Bip-Xaa*-OMe where Boc=tert-butoxycarbonyl, OMe=methoxy, and Xaa*=D- and/or L-Ala, -Val, -Leu, -Phe, -(alphaMe)Val and -(alphaMe)Leu. Chiral induction was significantly lower in the isomeric dipeptides Boc-Xaa*-Bip-OMe, with the Xaa* residue located at the N-terminus of Bip, as well as in the cyclic dipeptide cyclo-[Bip-L-Ala]. The results obtained in solution were confirmed by X-ray diffraction analysis of a crystalline sample of Boc-(R)-Bip-D-Ala-OMe.
Subject(s)
Amino Acids/chemistry , Peptides/chemistry , Circular Dichroism , Crystallography, X-Ray , Nuclear Magnetic Resonance, Biomolecular , StereoisomerismABSTRACT
In the dipeptides Boc-Bip-L-Val-OMe and Boc-Bip-D-Val-OMe, an induced axial chirality in the biphenyl core of the Bip residue, a conformationally labile, proatropoisomeric C(alpha,alpha)-disubstituted glycine, was observed by electronic CD and (1)H NMR. Chiral induction is significantly higher when the Val residue is located at the C-terminal position of Bip. An outstanding phenomenon of propagation of chirality was demonstrated to occur in the related 3(10)-helical -(Bip)n-L-Val (n = 2-6) oligopeptides by CD and vibrational CD techniques.
