Oriented synchrotron radiation circular dichroism and linear dichroism spectroscopy of peptides in model membranes

摘要

The orientation of membrane-associated alpha-helical peptides was investigated using novel methodologies of oriented Synchrotron Radiation Circular Dichroism (SRCD) and linear dichroism (SRLD) spectroscopies. Because of its enhanced signal-to-noise and a detector geometry that minimised optical artefacts associated with conventional CD studies of membrane suspensions, SRCD enabled the measurement of oriented CD spectra. To accomplish this a specially-designed sample cell holder was produced which would maintain constant humidity in hydrated film samples. Distinct spectra were obtained for peptides oriented parallel or normal to the direction of the beam, corresponding to the parallel and perpendicular pi to pi* and n to pi* electronic transitions. To provide similar information for peptides associated with lipid vesicles, SRLD was used to examine suspensions of vesicles in a coquette flow-cell. SRCD studies of the samples in the same couette enabled interpretation of the information. The model systems used in this study were peptides of the KALP family with a number of different phospholipids. In TFE solution and as well in lipid vesicle suspensions, KALP produced CD spectrum typical of an alpha helix in an isotropic solution, whilst in oriented samples different spectra associated with the different directional transitions of the peptide bonds were found for peptides oriented transmembrane or parallel to the membrane surface. The alignments of the peptides under the different conditions were compared with the results obtained by 15N solid state NMR of the peptide in oriented lipid multilayers. Thus, these new approaches to examining peptides in membranes can provide information that is complementary to the secondary structural information present in conventional CD spectra.