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.
An induced axial chirality of the biphenyl core of the Bip (2',1':1,2;1'',2'':3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid) residue in the terminally protected dipeptides Boc-Bip-beta-Xaa*-OMe (beta-Xaa* = L-beta(3)-HAla, L-beta(3)-HVal, L-beta(3)-HLeu, L-beta(3)-HPro, trans-(1S,2S)-ACHC, trans-(1R,2R)-ACHC, trans-(1S,2S)-ACPC, trans-(1R,2R)-ACPC) resulted in an induced circular dichroism, revealing the usefulness of the Bip method for a reliable and fast assignment of the absolute configuration of chiral beta-amino acids. Remarkably, the Bip method was also applied to the unique spin-labeled, cyclic, beta-amino acids cis/trans-beta-TOAC and trans-POAC. In particular, this study allowed the assignment of the unknown absolute configurations of the enantiomers of the latter compound.
Abstract:The results of X-ray diffraction analysis, fluorescence experiments and molecular mechanics calculations on the terminally protected hexapeptide -(S)Bin-Ala-Aib-TOAC-(Ala) 2 -are presented. This is the first peptide investigated photophysically that is characterized by a) a rigid, binaphthyl-based aamino acid (Bin) fluorophore, b) a rigid interchromophore bridge, the -Ala-Aibsequence, and c) a rigid, nitroxide-based a-amino acid quencher (TOAC). In the crystal state the backbone of the spectroscopically critical 1 ± 4 segment of both independent molecules in the asymmetric unit of the hexapeptide is folded in a regular, left-handed 3 10 -helix. The steady-state fluorescence spectra show a remarkable quenching of Bin emission by the TOAC residue located one complete turn of the helix apart. Time-resolved fluorescence measurements exhibit a biexponential decay with solvent-dependent lifetime components ranging from 0.5 to 1.5 ns and from 3 to 5 ns. Time-decay data combined with molecular mechanics calculations allowed us to assign these lifetimes to two left-handed 3 10 -helical conformers in which an intramolecular electronic energy transfer from excited Bin to TOAC takes place. For a given solvent the difference between the two lifetimes primarily depends on a different relative orientation of the two chromophores in the conformers, which is in turn related to a different puckering of the TOAC cyclic system.
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