Volker H. Hoffmann

The orientation of melittin in lipid membranes. A polarized infrared spectroscopy study

Polarized infrared spectra of melittin incorporated into macroscopically oriented lipid membranes are reported. From the linear dichroism of the amide I and amide II vibrational bands, the spatial orientation of the melittin helices was determined as being preferentially parallel to the membrane normal, under our experimental condition of low water content and an ordered lipid phase. Considering the various models for the orientation of melittin in lipid membranes proposed in the literature, we conclude that our data are in accord with an arrangement whereby the hydrophobic part of the polypeptide either spans the bilayer in the form of two bent helix segments, or is folded back within one monolayer in the form of a wedge.

DRIFT studies of thick film un-doped and Pd-doped SnO2 sensors: temperature changes effect and CO detection mechanism in the presence of water vapour

Diffuse reflectance infrared Fourier transform measurements were performed on tin oxide based thick film gas sensors operated in normal working conditions. We characterised SnO2 sensors at different temperatures between room temperature and 300 °C. The results show the presence of different surface OH groups as well as coordinated water on the SnO2 sensor surface. Their intensity changes with temperature. During the temperature cycles the bands’ peak positions are reversibly changed but their intensity is not. CO measurements were performed at 300 °C at different humidity levels (0 and 50% r.h.) on un-doped and Pd-doped sensors. In the presence of CO we observed in the spectra: a decrease of the OH groups on the SnO2 surfaces, the appearance of gaseous CO2 and CO in the pores of the sensitive layer and an increase of hydrated protons and of the free charge concentration. The effects are dramatically influenced by the water vapour concentration, temperature, dopands (Pd) and can be correlated with simultaneously performed sensor resistance measurements.

In situ diffuse reflectance infrared spectroscopy study of CO adsorption on SnO2

The surface interaction between differently prepared tin oxide samples and CO was studied by Diffuse Reflectance Infrared Fourier Transform (DRIFT)-spectroscopy at room temperature. As samples, commercial powders (Merck) and home-made powders obtained by a sol–gel preparation were used. Several bands corresponding to isolated and rooted OH groups, coordinated water and condensed water in two forms: ordered layers of water molecules and water in the liquid form, were found. Their appearance and intensity depended on the sample origin and storing conditions. Moreover, the spectra of all samples show bands corresponding to different hydrated protons (H3O+, H5O2+) and carbonate ions of different intensities. To get information about the interaction between SnO2 powders and CO, in-situ measurements were conducted. We found that differently bound water and specific surface OH-groups react with CO and that surface carbonate ions dissociate because of acidic intermediate products. As a result, different bands corresponding to physisorbed CO appear at approximately 2200 cm−1, together with changes in the region corresponding to covalent bound chelating and bridging bidentate carbonates and carboxylate (between 1700 and 1000 cm−1).

Small interacting peptides. Part I. Interaction of cyclohexapeptides with an unspecific SiOH surface: comparison of infrared investigations and force field calculations

The interaction of cyclohexapeptides c(X(1)(1)K(2)X(2)(3)K(4)X(3)(5)K(6)) in water with hydrolysed silicon surfaces were studied by attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy and by force field calculations. The band sequences (1800-1500 cm(-1)) for dissolved and adsorbed cyclohexapeptides were recorded and compared with those obtained after flushing with distilled water in order to eliminate the background signal of the peptides in solution. Band analyses and principal component analyses were carried out for the characteristic peptide vibrations in order to evaluate the spectra. In addition, force field calculations were performed to study the binding energies to the surface and to illustrate the possible structures of the cyclohexapeptides. The positively charged lysine side chains of the cyclohexapeptides interact with the OH groups of the surface, as indicated by band shifts. This also was verified by the force field calculations. The bonding stability increases with the number of interacting sites (lysine side chains and other peptide residues) to the surface. These sites are determined by structure and polarity of the cyclohexapeptides.