PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Possible mechanisms for photosynthetic O₂ formation based on the higher resolution crystal structure, synthetic complexes and time-dependent electronic structure calculations. Rogier van Willigen¹, Jyotishman Dasgupta¹, Jian-Zhong Wu¹, Filippo De Angeles², Roberto Car¹, Charles Dismukes^*,1, ¹ Princeton University, USA² Princeton University, USA

ABSTRACT- Recent x-ray diffraction data(Ferreira et al., 2004)resolved the inorganic core as Mn₄Ca₁O₄, a cubical Mn₃Ca₁ core with three bridging 3-oxos and one exo-cubical Mn bound to a 4-oxo. This core exhibits intimate electronic interaction between all Mn atoms that determines its unique reactivity. Current debates concern the chemical identity of the two substrate molecules (-oxos derived from water or -oxo/carbonate), their location within the cluster and the mechanisms of S-state transitions (H+/e- removal) and O-O bond formation. Comparison of the kinetic parameters (rate constants, H/D isotope effect, entropies and enthalpies of activation) for S-state transitions to those for H atom transfer to synthetic cubane model complexes: L₆Mn₄O₄ → L₆Mn₄O₃OH, reveals that the S-state transitions do not likely involve pure H atom transfer to Yz but rather are pcet steps involving proton transfer to another base or distributed bases. We will also contrast two classes of possible mechanisms for the final O-O bond formation step involving heterolytic vs homolytic coupling. Aqueous phase enthalpies and gas phase ionization energies shows that CO₂ activates water upon forming H₂CO₃ by decreasing the electron affinity. Thus (bi)carbonate is a thermodynamically favored nucleophile vs water in a heterolytic pathway and, if kinetically accessible, will be the preferred precursor to O₂ We shall present kinetic data supporting the possibility that Ca²⁺ could serve as a rapid carbonic anhydrase site to enable CO₂ activation of substrate water during catalytic turnover. Carbonate may participate directly in a nucleophilic attack on the 4-oxo or on a putative manganyl oxo (Mn=O) which is as yet unidentified. Alternatively, a homolytic pathway involving coupling of two core -oxos triggered by carbonate-Ca²⁺ coordination change is energetically feasible for O-O bond formation. A homolytic pathway has been experimentally verified in the L₆Mn₄O₄ model cubanes and computationally described using dynamic density functional theory which indicates a low barrier pathway to O₂ via a transient peroxide intermediate.

KEY WORDS: oxygen evolution, photosystem II, water splitting, manganese