Zwei ineinander umwandelbare Strukturen erklären die spektroskopischen Eigenschaften des Wasser oxidierenden Enzyms des Photosystems II im S₂‐Zustand

Abstract

Aus der Kristallstruktur des Photosystems II abgeleitete DFT‐Modelle zeigen, dass der Sauerstoff produzierende Komplex im S2‐Zustand in zwei Formen existiert, die energetisch ähnlich und ineinander umwandelbar sind. Damit wird eine seit langem offene Frage zur Spektroskopie dieses Zustands beantwortet: Eine Form ergibt das Multilinien‐EPR‐Signal bei g=2.0 (rechts im Bild; rot O, violett Mn, gelb Ca), die andere das g≥4.1‐Signal (links).

The S2 State of the Oxygen‐Evolving Complex of Photosystem II Explored by QM/MM Dynamics: Spin Surfaces and Metastable States Suggest a Reaction Path Towards the S3 State

“…In the QM/MM optimized structures the spin ground-state for S 2 A corresponds to low spin (LS) S = 1/2 and for S 2 B corresponds to high spin (HS) S = 5/2, in agreement with previous results in the gas phase [11] (details concerning the broken symmetry states are described in Table S1). In particular we found that in S 2 A the LS (ground) state is 0.5 kcal mol À1 more stable than the HS state.…”

“…The endergonicity of the S 2 A to S 2 B transition and the height of the kinetic barrier between the two states have important consequences for the interpretation of previous experiments observing the transition between the EPR multi-line signal, associated with S 2 A , and the g % 4.1 signal, assigned to S 2 B . [11,15] According to our free-energy landscape, at room temperature (and in general for T ! 200 K) both S 2 A and S 2 B states are appreciably populated.…”

“…DG # < 9.0 kcal mol À1 [11] ). Assuming that the relation between the Gibbs energy of activation and the reaction rate constant can be described by the Eyring-Polanyi equation, a barrier of 10.6 kcal mol À1 would correspond at room temperature to an half-life time t AB % 1-10 ms, indicating that in these conditions the interconversion is thermally activated.…”

“…Beside the large number of experiments performed on PSII in the past years, theoretical studies have provided new insights into the structural, electronic, and magnetic properties of PSII. [6,7,10] In a recent contribution Pantazis et al [11] proposed the existence of two interconvertible structures consistent with the S 2 state and generating the two EPR signals. The two structures differ mainly in the position of the oxygen atom O5 (see inset in Figure 1), which is, in one case (Model A), bound to Mn4 to form a S = 1/2 spin state responsible for the MLS, and in the second case (Model B), to Mn1 in a S = 5/2 state associated with the g % 4.1 signal.…”

“…Convergence tests on quantum regions of similar sizes recently confirmed the reliability of our model. [14] As starting positions for both S 2 A and S 2 B models of the {Mn 4 CaO 5 } cluster we used the coordinates reported by Pantazis et al [11] Further details on methods, setup, and computational methods are provided in Supporting Information.…”