High pressure Raman study of isobutyramide.

Isobutyramide (IBA) has attracted considerable attention due to its expansive prospects for practical applications in the synthesis of drugs, dyes and other organic compounds. Herein we perform the high-pressure studies of IBA crystal by Raman spectral measurements at room temperature from ambient pressure to 30 GPa by using diamond anvil cells (DACs) to gain comprehensive insights into its structure and stability. Raman vibrational modes of IBA crystal at ambient pressure are resolved based on the experimental results and the first-principles theoretical calculations. High-pressure Raman scattering results show the Raman bands splitting, emergence/disappearance of the Raman bands and discontinuous wavenumber shifts at 1, 2 and 10 GPa, which indicate that IBA crystal undergoes three structural phase transitions at corresponding pressure. In addition, softening of the C = O and N-H stretching vibrational modes of IBA with increasing pressure can be interpreted by the reorganization of the hydrogen bond network of IBA molecules due to phase transition.

Temperature-dependent study of Fermi resonance of CH3CN and CH3CN---Li+ complex in CH3CN-LiClO4 mixture by Raman spectroscopy.

The Raman spectra of acetonitrile-LiClO4 mixture solution have been measured in the temperature range 20 to -196 °C at ambient pressure. Detailed Raman spectroscopy analysis revealed that, in acetonitrile-LiClO4 mixture solution, the liquid CH3CN transformed into solid phase ß at approximately -50 °C, and then into solid phase α at approximately -60 °C. Besides, the Fermi resonance parameters of CH3CN and CH3CN---Li+ complex at different temperatures were calculated by using the Bertran's equations, respectively. It was found that the Fermi resonance coefficient W of CH3CN---Li+ complex was not sensitive to the variation of temperature from 20 to -45 °C. In the case of CH3CN, however, the Fermi resonance coefficient W decreased from the temperature of 20 to -196 °C during which a sudden increase was observed at the temperature of -50 °C coinciding with the temperature of phase transition from liquid to solid phase ß. Finally, the temperature induced precipitation behavior of LiClO4 and the structural evolution of CH3CN on the Fermi resonance have been analyzed.

Role of helicity of α-helical antimicrobial peptides to improve specificity

A major barrier to the use of antimicrobial peptides as antibiotics is the toxicity or ability to lyse eukaryotic cells. In this study, a 26-residue amphipathic α-helical antimicrobial peptide A12L/A20L (Ac-KWKSFLKTFKSLKKTVLHTLLKAISS-amide) was used as the framework to design a series of D- and L-diastereomeric peptides and study the relationships of helicity and biological activities of α-helical antimicrobial peptides. Peptide helicity was measured by circular dichroism spectroscopy and demonstrated to correlate with the hydrophobicity of peptides and the numbers of D-amino acid substitutions. Therapeutic index was used to evaluate the selectivity of peptides against prokaryotic cells. By introducing D-amino acids to replace the original L-amino acids on the non-polar face or the polar face of the helix, the hemolytic activity of peptide analogs have been significantly reduced. Compared to the parent peptide, the therapeutic indices were improved of 44-fold and 22-fold against Gram-negative and Gram-positive bacteria, respectively. In addition, D- and L-diastereomeric peptides exhibited lower interaction with zwitterionic eukaryotic membrane and showed the significant membrane damaging effect to bacterial cells. Helicity was proved to play a crucial role on peptide specificity and biological activities. By simply replacing the hydrophobic or the hydrophilic amino acid residues on the non-polar or the polar face of these amphipathic derivatives of the parent peptide with D-amino acids, we demonstrated that this method could have excellent potential for the rational design of antimicrobial peptides with enhanced specificity.