Vibrational spectroscopy and crystal structure analysis of two polymorphs of the di-amino acid peptide cyclo(L-Glu-L-Glu)

Andrew P. Mendham, Rex A. Palmer, Brian S. Potter, Trevor J. Dines, Martin J. Snowden, Robert Withnall, Babur Z. Chowdhry

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    Abstract

    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.

    Original languageEnglish
    Pages (from-to)288-302
    Number of pages15
    JournalJournal of Raman Spectroscopy
    Volume41
    Issue number3
    DOIs
    Publication statusPublished - Mar 2010

    Keywords

    • cyclic di-amino acid peptides
    • X-ray crystallography
    • ab initio calculations
    • vibrational spectra
    • ALANYL-L-ALANYL
    • CYCLIC DIPEPTIDES
    • CONFORMATION
    • DIKETOPIPERAZINE
    • POLYPEPTIDES
    • PROTEINS
    • DENSITY

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