Journal
JOURNAL OF RAMAN SPECTROSCOPY
Volume 41, Issue 3, Pages 288-302Publisher
WILEY
DOI: 10.1002/jrs.2467
Keywords
cyclic di-amino acid peptides; X-ray crystallography; ab initio calculations; vibrational spectra
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Funding
- EPSRC [GR/L85176]
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Cyclo(L-Glu-L-Glu) has been crystallised in two different polymorphic forms. Both polymorphs are monoclinic, but form 1 is in space group P2(1) and form 2 is in space group C2. Raman scattering and FT-IR spectroscopic studies have been conducted for the N,O-protonated and deuterated derivatives. Raman spectra of orientated single crystals, solid-state and aqueous solution samples have also been recorded. The different hydrogen-bonding patterns for the two polymorphs have the greatest effect on vibrational modes with N-H and C=O stretching character. DFT (B3-LYP/cc-pVDZ) calculations of the isolated cyclo(L-Glu-L-Glu) molecule predict that the minimum energy structure, assuming C-2 symmetry, has a boat conformation for the diketopiperazine ring with the two L-Glu side chains being folded above the ring. The calculated geometry is in good agreement with the X-ray crystallographic structures for both polymorphs. Normal coordinate analysis has facilitated the band assignments for the experimental vibrational spectra. Copyright (C) 2009 John Wiley & Sons, Ltd.
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