Infrared Linear Dichroism of Retinal Proteins   

Detecting preferential orientation within a protein

Retinal proteins are membrane proteins and are therefore oriented in the embedding membrane matrix. This intrinsic microscopic orientation can be largely maintained in macroscopic samples suitable for UV-visible and infrared spectroscopy. These sample types typically have a very high degree of order and allow the study of the spatial orientation of groups within the protein.

Groups with a preferential orientation within the protein also have a preferential orientation of their vibrational modes and therefore of the resulting absorption coefficient of the corresponding infrared absorption band. The resulting infrared properties of the sample are termed Linear Dichroism. There are several experimental setups to measure this dichroism of absorbing groups experimentally. One possibility is to measure absorption spectra of the samples under different orientations to the polarized infrared beam and to calculate from these the spatial orientation of the vibrational mode.

We focus on the spatial re-orientation of the retinal chromophore in the retinal proteins rhodopsin and bacteriorhodopsin immediately after light-induced isomerization of the retinal chromophore. Our aim is to understand how the photon energy absorbed by the retinal is stored in the initially twisted retinal and in localized retinal-protein interactions and how it is subsequently released globally to induce conformational changes of the entire protein.