Influence of Cd~(2+) on the spin state of non-heme iron and on protein local motions in reactions centers from purple photosynthetic bacterium Rhodospirillum rubrum

摘要

Non-heme Fe is a conservative component of the Q-type photosynthetic reaction centers but its function remains unknown. Applying Mossbauer spectroscopy we show that in Rhodospirillum rubrum the non-heme Fe exists mostly in a ferrous low spin state. The binding of Cd~(2+) ions in the vicinity of the quinone-Fe complex changes the high spin state of the non-heme Fe into a low spin one characterized by hyperfine parameters similar to those obtained for the non-heme Fe low spin state in untreated reaction centers, as confirmed by Mossbauer measurements. The nuclear inelastic scattering of synchrotron radiation experiments show that the contribution of vibrations at low energies, between 3-15 meV, activated at 240 K are damped in the bacterial reaction centers treated with CdCl_2. No influence of Cd~(2+) ions is observed on the soft vibrational states at 60 K. These results suggest that binding of cadmium cations within the reaction centers may enhance decoupling of the non-heme Fe from the surrounding protein matrix at temperatures higher than 200 K, what can explain the slowing down of electron transfer between the Qa and Qb quinones by Cd~(2+).