Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem II

Since the end of the 1950s hydrogencarbonate (‘bicarbonate’) is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn₄O_xCa cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (≈ 0.3) HCO₃⁻ per PSII upon addition of formate. The same amount of HCO₃⁻ release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO₃⁻ from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO₃⁻ is corroborated by showing that a reductive destruction of the Mn₄O_xCa cluster inside the MIMS cell by NH₂OH addition does not lead to any CO₂/HCO₃⁻ release. We note that even after an essentially complete HCO₃⁻/CO₂ removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain ≥ 75% of their initial flash-induced O₂-evolving capacity. We therefore conclude that HCO₃⁻ has only ‘indirect’ effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.     

Big six of the individual adaptive ability of the sleep-wake cycle: Explanation and measurement

Research is directed at understanding the nature of differences between individuals in the adaptive abilities of their sleep-wake cycles. We proposed a three-dimensional model that reduces the subjectively assessed variation of the sleep-wake patterns to three underlying physiological variables. This model explains the results of factor analysis of chronotypological questionnaires and predicts the hierarchical structure of the sleep-wake adaptability with three most general abilities at the top level, six basic abilities at the middle level, and 18 primary (most specific) abilities at the bottom level. Any such subjectively assessed trait of any of these three hierarchical levels might be located in the space of three physiological parameters (namely, these are amplitude of circadian modulation of sleep-wake states, rate of accumulation of sleep debt and intensity of circadian arousal). The present study is aimed at generation and testing a particular prediction of the model. It points to the features of one basic and five primary abilities that are not accounted by the 52-item Sleep-Wake Pattern Assessment Questionnaire. Twenty new questionnaire items relevant to these predicted abilities were produced and corrected through a series of three questionnaire surveys of the residents of Novosibirsk (n = 150, 180 and 200). The results of factor analysis of the intermediate and final versions of the questionnaire provided evidence of applicability of the model for identification of the abilities that have not yet been measured by means of the questionnaire and for improvement of measurement of already identified abilities.