Infrared Dichroism of Isotope-edited α-Helices and β-Sheets

Isotope editing of amide infrared bands not only localises secondary structural elements within the protein but also yields conformational information that is not available from the linear dichroism of aligned samples without isotope editing. The additional information that can be derived on the orientational distribution of α-helices in membranes by the combined use of different amide bands and several positions of labelling is presented here. Also, the relationship between the azimuthal orientation of the transition moment and the protein structure is treated explicitly. A comprehensive analysis of the infrared dichroism for β-sheets and β-barrels is given here, for the first time. The orientation of the individual transition moments in a β-sheet that is essential for this analysis is derived for the different amide bands.