title: "Glass transition" near 200 K in the bacterial photosynthetic reaction center protein detected by studying the distances in the transient P+Q(A)- radical pair.
abstract: The transient radical pair P(+)Q(A)(-) in the photosynthetic reaction center from Rhodobacter sphaeroides R26 was studied over a wide temperature range using out-of-phase electron spin-echo envelope modulation (ESEEM) spectroscopy. This method is sensitive to the magnetic dipole-dipole interaction between the two electron spins of the pair and allows precise determination of the distance in the pair P(+)Q(A)(-). The out-of-phase data were complemented by normal in-phase ESEEM spectra from the two stable radicals of P(+) and Q(A)(-). The results seem to indicate that the radical pair undergoes a noticeable molecular motion around 200 K that may be characterized by a change in the distance in the pair by approximately 0.3 nm. As the two cofactors, P(+) and Q(A)(-), are held in a well-defined relative position by the reaction center protein, this means that the protein becomes flexible at 200 K. This effect may be ascribed to a dynamic glass transition around 200 K. The relation with the temperature dependence of the back reaction of P(+)Q(A)(-) is discussed.
authors.affiliation: Department of Biophysics, Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands.
authors.forename: Igor V
authors.initials: IV
authors.lastname: Borovykh
authors.forename: Peter
authors.initials: P
authors.lastname: Gast
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Dzuba
mesh_terms: D001671:Biochemistry / Q000379:methods; D002627:Chemistry, Physical / Q000379:methods*; D003198:Computer Simulation; D004583:Electrons; D005583:Fourier Analysis; D005609:Free Radicals; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011506:Proteins / Q000737:chemistry; D012242:Rhodobacter sphaeroides / Q000378:metabolism / Q000502:physiology*; D013057:Spectrum Analysis; D013696:Temperature
chemical_list: D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D011506:Proteins