Conformation and orientation of chlorophyll-proteins in photosystem I by circular dichroism and polarized infrared spectroscopies

The protein conformation and orientation of Photosystem I (PS I) particles have been investigated by a combination of ultraviolet circular dichroism and polarized infrared spectroscopies. These PS I particles have been studied before and after reconstitution in phosphatidylcholine vesicles. The native state of the pigments of PS I was characterized by monitoring the low-temperature fluorescence emission spectra as well as the visible CD and linear dichroism spectra at room temperature. Computed analysis of the ultraviolet CD spectra of PS I complex indicates that the secondary structure of the protein is largely α-helical (52 ± 4%) with a very low amount of β-structure. Polarized infrared difference spectra of oriented PS I show a significant orientation of these α-helical segments with the α-helix axes tilted on the average at approx. 35° from the membrane normal.