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Solid‐state synthesis of cyclo LD‐diphenylalanine: A chiral phase built from achiral subunits
Author(s) -
PérezMellor Ariel,
Le BarbuDebus Katia,
Zehnacker Anne
Publication year - 2020
Publication title -
chirality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.43
H-Index - 77
eISSN - 1520-636X
pISSN - 0899-0042
DOI - 10.1002/chir.23195
Subject(s) - homochirality , chemistry , zwitterion , chirality (physics) , enantiomer , crystallography , molecule , circular dichroism , infrared spectroscopy , monomer , dipeptide , stereochemistry , absolute configuration , phase (matter) , chiral symmetry , organic chemistry , peptide , polymer , nambu–jona lasinio model , physics , biochemistry , quantum mechanics , quark
The solid‐state structure of LL/DD or LD/DL diphenylalanine diluted in KBr pellets is studied by infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy. The structure depends on the absolute configuration of the residues. The natural LL diphenylalanine exists as a mixture of neutral and zwitterionic structures, depending on the humidity of the sample, while mostly the zwitterion is observed for LD diphenylalanine whatever the experimental conditions. The system undergoes spontaneous cyclization upon heating at 125°C, resulting to the formation of a diketopiperazine (DKP) dipeptide as the only product. The reaction is faster for LD than for LL diphenylalanine. As expected, LL and DD diphenylalanine react to form the LL and DD enantiomers of cyclo diphenylalanine. Interestingly, the DKP dipeptides formed from the LD or DL diphenylalanine show unexpected optical activity, with opposite VCD spectra for the products formed from the LD and DL reagents. This is explained in terms of chirality synchronization between the monomers within the crystal, which retain the symmetry of the reagent, resulting to the formation of a new chiral phase made from transiently chiral molecules.