Vibrational and chiroptical spectroscopic characterization of gamma-turn model cyclic tetrapeptides containing two beta-Ala residues.
The optical spectroscopic characterization of gamma-turns in solution is uncertain and their distinction from beta-turns is often difficult. This work reports systematic ECD and vibrational circular dichroism (VCD) spectroscopic studies on gamma-turn model cyclic tetrapeptides cyclo(Ala-beta-Ala-Pro-beta-Ala) (1), cyclo(Pro-beta-Ala-Pro-beta-Ala) (2) and cyclo(Ala-beta-Ala-Ala-beta-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 gamma-turns. The ECD spectra in ACN of 1 and 2 are characterized by a negative n-->pi* band near 230 nm and a positive pi-->pi* 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-d(3) 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-d(2). 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 approximately 1630 cm(-1) or below) are indicative of a C(7) H-bonded inverse gamma-turns with Pro in position 2, while gamma-turns encompassing Ala absorb at higher frequency (above 1645 cm(-1)).
Vass
Elemér
E
Laboratory for Chiroptical Structure Analysis, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary.
Majer
Zsuzsa
Z
Kohalmy
Krisztina
K
Hollósi
Miklós
M
D000409:Alanine / Q000737:chemistry*; D000595:Amino Acid Sequence; D002942:Circular Dichroism; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009842:Oligopeptides / Q000737:chemistry*; D055620:Optical Phenomena; D010456:Peptides, Cyclic / Q000737:chemistry*; D011487:Protein Conformation; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared; D013057:Spectrum Analysis; D013237:Stereoisomerism; D014732:Vibration
D009842:Oligopeptides; D010456:Peptides, Cyclic; D000409:Alanine