Photosynthetic water oxidation performed at the MnCaO cluster in photosystem II plays a crucial role in energy production as electron and proton sources necessary for CO fixation. Molecular oxygen, a byproduct, is a source of the oxygenic atmosphere that sustains life on earth. However, the molecular mechanism of water oxidation is not yet well-understood. In the reaction cycle of intermediates called S states, the S → S transition is particularly important; it consists of multiple processes of electron transfer, proton release, and water insertion, and generates an intermediate leading to O-O bond formation. In this study, we monitored the reaction process during the S → S transition using time-resolved infrared spectroscopy to clarify its molecular mechanism. A change in the hydrogen-bond interaction of the oxidized Y radical, an immediate electron acceptor of the MnCaO cluster, was clearly observed as a ∼100 μs phase before the electron-transfer phase with a time constant of ∼350 μs. This observation provides strong experimental evidence that rearrangement of the hydrogen-bond network around Y, possibly due to the movement of a water molecule located near Y to the Mn site, takes place before the electron transfer. The electron transfer was coupled with proton release, as revealed by a relatively high deuterium kinetic isotope effect of 1.9. This proton release, which decreases the redox potential of the MnCaO cluster to facilitate electron transfer to Y, was proposed to determine, as a rate-limiting step, the relatively slow electron-transfer rate of the S → S transition.

Verification problems of programs written in various paradigms (such as imperative, logic, concurrent, functional, and objectoriented ones) can be reduced to problems of solving Horn clause constraints on predicate variables that represent unknown inductive invariants. This paper presents a novel Horn constraint solving method based on inductive theorem proving: the method reduces Horn constraint solving to validity checking of first-order formulas with inductively defined predicates, which are then checked by induction on the derivation of the predicates. To automate inductive proofs, we introduce a novel proof system tailored to Horn constraint solving and use an SMT solver to discharge proof obligations arising in the proof search. The main advantage of the proposed method is that it can verify relational specifications across programs in various paradigms where multiple function calls need to be analyzed simultaneously. The class of specifications includes practically important ones such as functional equivalence, associativity, commutativity, distributivity, monotonicity, idempotency, and non-interference. Furthermore, our novel combination of Horn clause constraints with inductive theorem proving enables us to naturally and automatically axiomatize recursive functions that are possibly non-terminating, non-deterministic, higher-order, exception-raising, and over non-inductively defined data types. We have implemented a relational verification tool for the OCaml functional language based on the proposed method and obtained promising results in preliminary experiments.

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We investigated nitrite and nitrate ion formation during single-bubble cavitation by realizing stable light emission for more than 30 h. Production yields of nitrite and nitrate ions in water were obtained as a function of irradiation time. The concentrations of nitrate and nitrite ions increased linearly with irradiation time within experimental error, and the molar ratio of nitrate to nitrite ions was about 1:1. The formation rate for these two ions was 3.8 × 10-10 mol·dm-3·min-1, and the number of nitrite or nitrate ions per cycle was estimated as 7.1 × 106.