Conformational Properties of Arenicins: From the Bulk to the Air-Water Interface

The structures of two antimicrobial peptides (arenicin Ar-1 and its linear derivative C/S-Ar-1) are studied in different solutions and at the air water interface using spectroscopic methods such as circular dichroism (CD) and infrared reflection absorption spectroscopy (IRRAS) as well as grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity (XR). Both peptides exhibit similar structures in solution. In the buffer used for most of the experiments the main secondary structure elements are 22% beta-turn, 38% beta-sheet and 38% random coil. The amphiphilic peptides are surface-active and form a Gibbs monolayer at the air buffer interface. The surface activity is drastically increased by increasing the ionic strength of the subphase. The beta-sheet layer is quite stable and can be compressed to higher surface pressures. This adsorption layer is very crystalline. Bragg peaks corresponding to an interstrand distance of 4.78 angstrom and to an end-to-end distance have been observed. This end-to-end distance can be connected with the observed differences in the layer thickness leading to the assumption that the peptides form a hairpin which is bended depending on the interactions with the counterions.