Conformation transition in the protein of a photosynthetic reaction center observed at the nanometer range of distances at cryogenic temperatures

Abstract

Electron spin echo (ESE) spectroscopy is applied to study magnetic dipolar interaction between electron spins in the transient P + Q A − radical pairs (P is the primary donor, a bacteriochlorophyll dimer, and Q A is the primary quinone acceptor) and the 3 PQ A − triplet-radical pairs in bacterial photosynthetic reaction centers of Rhodobacter sphaeroides R26. Distance separation in both pairs is about 29 Å. A well-resolved reversible conformational transition of the reaction center protein holding the P and Q A cofactors was observed between 13 and 20 K. This transition results in a narrowing of the distribution of protein conformations with decreasing temperature, with the width of distance distribution between P and Q A dropping from ca. 4 Å at 20 K to ca. 1 Å at 13 K. This transition implies the existence of low barriers in the protein energy landscape and the presence of cooperatively rearranging domains of the size of several nm both in the protein and in the surrounding glassy environment.