Abstract
The structure of the antimicrobial peptide NK‐2 has been studied at the air–water interface and in different solutions using spectroscopic methods such as circular dichroism (CD) and infrared reflection absorption spectroscopy (IRRAS) as well as specular X‐ray reflectivity (XR). NK‐2 adopts an unordered structure in water, buffer, and in the presence of monomeric cationic and noncharged amphiphiles. However, it forms a stable α‐helix in 2,2,2‐trifluoroethanol (TFE) and in micellar solutions of anionic, cationic as well as nonionic amphiphiles, whereas only in sodium dodecyl sulfonate solutions the α‐helical structure can also be found below the critical micellar concentration (cmc). The amphiphilic molecule NK‐2 is surface active and forms a Gibbs monolayer at the air–buffer interface. In contrast, no adsorption was observed if NK‐2 is dissolved in water. During the adsorption process in buffer solutions, NK‐2 undergoes a conformational transition from random coil in bulk to α‐helix at the interface. This change of the peptide's secondary structure is known to be associated with its antimicrobial activity. A comparison of the experimental IRRA spectra with the simulated spectra indicates that the adsorbed NK‐2 α‐helix lies flat at the interface. This is confirmed by XR measurements which show that the thickness of the NK‐2 layer is ∼17 Å, which is the average diameter of a α‐helix, indicating that only a monomolecular adsorption layer is formed. Copyright © 2007 European Peptide Society and John Wiley & Sons, Ltd.