Vibrational and chiroptical spectroscopic characterization of γ-turn model cyclic tetrapeptides containing two β-Ala residues

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Abstract

The optical spectroscopic characterization of γ-turns in solution is uncertain and their distinction from β-turns is often difficult. This work reports systematic ECD and vibrational circular dichroism (VCD) spectroscopic studies on γ-turn model cyclic tetrapeptides cyclo(Ala-β-Ala-Pro- β-Ala) (1), cyclo(Pro-β-Ala-Pro-β-Ala) (2) and cyclo(Ala-β-Ala-Ala-β-Ala) (3). Conformational analysis performed at the 6-31G(d)/ B3LYP level of theory using an adequate PCM solvent model predicted one predominant conformer for 1-3, featuring two inverse γ-turns. The ECD spectra in ACN of 1 and 2 are characterized by a negative n→π* band near 230 nm and a positive π→π* band below 200 nm with a long wavelength shoulder. The ECD spectra in TFE of 1-3 show similar spectra with blue-shifted bands. The VCD spectra in ACN-d3 of 1 and 2 show a +/-/+/- amide I sign pattern resulting from four uncoupled vibrations in the case of 1 and a sequence of two positive couplets in the case of 2. A -/+/+/- amide I VCD pattern was measured for 3 in TFE-d2. All three peptides give a positive couplet or couplet-like feature (+/-) in the amide II region. VCD spectroscopy, in agreement with theoretical calculations revealed that low frequency amide I vibrations (at ∼1630 cm-1 or below) are indicative of a C7 H-bonded inverse g-turns with Pro in position 2, while γ-turns encompassing Ala absorb at higher frequency (above 1645 cm-1).

Original languageEnglish
Pages (from-to)762-771
Number of pages10
JournalChirality
Volume22
Issue number8
DOIs
Publication statusPublished - Aug 2010

Keywords

  • Conformational analysis
  • ECD
  • Electronic circular dichroism
  • FTIR
  • Infrared spectroscopy
  • VCD
  • Vibrational circular dichroism
  • β-amino acid

ASJC Scopus subject areas

  • Analytical Chemistry
  • Catalysis
  • Pharmacology
  • Drug Discovery
  • Spectroscopy
  • Organic Chemistry

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