Hey ho, where'd the proton go? Final deprotonation of O6 within the S3 state of photosystem II.

The deprotonation of O6 within the S3 state marks the final deprotonation event before the formation of oxygen­oxygen bond interactions and eventual production and release of dioxygen. Gaining a thorough understanding of this event, from the proton acceptors involved, to the exfiltration pathways available, is key in determining the nature of the resulting oxygen species, influencing the mechanism through which the first oxygen­oxygen bond forms. Computational analysis, using BS-DFT methodologies, showed that proton abstraction by the local Glu189 residue provides consistent evidence against this being a viable mechanistic pathway due to the lack of a stable product structure. In contrast, abstraction via W3 shows an increasingly stable oxo-oxo product state between r[O5O6] = 2.1 Å & 1.9 Å. The resulting oxo-oxo state is stabilised through donation of ß electron character from O6 to Mn1 and α electron character from O6 to O5. This donation from the O6 lone pair is shown to be a key factor in stabilising the oxo-oxo state, in addition to showing the initiation of first O5-O6 bond.

A Reappraisal of the S2 State of Nature's Water Oxidizing Complex in Its Low and High Spin Forms.

Density functional theory calculated 14N hyperfine couplings are obtained for the Mn1 ligated π-N of residue His332 of the photosystem 2 water oxidizing complex. An open cubane, O4H, model closely matches the experimental coupling obtained for the high spin S = 5/2 form of the S2 state, supporting an open cubane structure for this state. We also investigate the unusual geometric features for the S2 state obtained by X-ray free electron laser structure determinations and rationalize it as an equilibrium occurring at room temperature between W1/O4 deprotonated and protonated forms of the open cubane structure.

A Rare Case of a Patient Being Alert and Communicative Despite Severe Hypothermia.

Hypothermia is defined as a significant drop in core body temperature below 35°C (95°F). It is traditionally staged as mild, moderate, severe, and profound at temperatures of 35°C to 32°C (95°F to 89.6°F), 32°C to 28°C (89.6°F to 82.4°F), <28°C (<82.4°F), and <24°C (75.2°F), respectively. It can also be classified into the same stages by clinical presentations. We present a patient that fits into two different stages based on core body temperature and clinical presentation.  A 58-year-old homeless male with a history of seizures and alcohol use presented via emergency medical services after spending the night outside and uncovered with a core body temperature of 25.1°C (77.1°F) via a urinary bladder thermometer, meeting criteria for severe, near profound, hypothermia. However, he was alert and communicating, shivering, with tachycardia, tachypnea, normal oxygen saturation, and elevated blood pressure, suggestive of mild hypothermia clinically. Passive and active external and internal rewarming were utilized to treat, with the removal of wet clothing, forced air patient warming system, warm blankets, and warm lactated ringers given intravenously. He was soon transferred to the intensive care unit and first returned to normothermic levels after approximately 10 hours from presentation. An electrocardiogram was obtained after resolution of shivering and revealed atrial fibrillation without Osborn waves. He remained in the hospital for the following week to treat his atrial fibrillation, hypothermia-induced rhabdomyolysis, and alcohol withdrawal. He was discharged without neurologic deficits and medically stable with appropriate resources.  This case demonstrates a unique presentation of severe hypothermia. To our knowledge, there has not been a reported case of severe hypothermia that does not involve severe central nervous system depression, severe slowing of vitals, and/or comatose status. These clinical symptoms normally begin during moderate hypothermic levels near 32°C (89.6°F), yet our patient presented without any central nervous system depression and with accelerated vitals that are more consistent with mild hypothermia yet had a core temperature of 25.1°C (77.1°F). Treatment was dictated by his core body temperature rather than clinical presentation. Because of this incongruence between symptoms and true severity of disease in hypothermia, we recommend diagnosis and treatment of hypothermia always be confirmed and based on core body temperature via a low-reading thermometer instead of clinical presentation alone.

A Pragmatic Approach to Identifying and Profiling Primary Care Clinicians and Primary Care Practices in the USA.

BACKGROUND: There are no consistent data on US primary care clinicians and primary care practices owing to the lack of standard methods to identify them, hampering efforts in primary care improvement. METHODS: We develop a pragmatic framework that identifies primary care clinicians and practices in the context of the US healthcare system, and applied the framework to the IQVIA OneKey Healthcare Professional database to identify and profile primary care clinicians and practices in the USA. RESULTS: Our framework prescribes sequential steps to identify primary care clinicians by cross-examining clinician specialties and organizational affiliations, and then identify primary care practices based on organization types and presence of primary care clinicians. Applying this framework to the 2021 IQVIA data, we identified 365,751 physicians with a primary specialty in primary care, and after excluding those who further specialized (24%), served as hospitalists (5%), or worked in non-primary care settings (41%), we determined that 179,369 (49%) of them were actually practicing primary care. We identified 287,506 nurse practitioners and 134,083 physician assistants and determined that 88,574 (31%) and 29,781 (22%), respectively, were delivering primary care. We identified 94,489 primary care practices, and found that 45% of them were with one primary care physician, 15% had two physicians, 12% employed nurse practitioners or physician assistants only, and 19% employed both primary care physicians and specialists. CONCLUSIONS: Our approach offers a pragmatic and consistent alternative to the diverse methods currently used to identify and profile primary care workforce and organizations in the USA.

Insights into PSII's S3YZ• State: An Electronic and Magnetic Analysis.

Using BS-DFT (broken-symmetry density functional theory), the electronic and magnetic properties of the S3YZ• state of photosystem II were investigated and compared to those of the S3 state. While the O5 oxo-O6 hydroxo species presents little difference between the two states, a previously identified [O5O6]3- exhibits reduced stabilization of the O5-O6 shared spin. This species is shown to have some coupling with the YZ• center through Mn1 and O6. Similarly, a peroxo species is found to exhibit significant exchange couplings between the YZ• center and the Mn cluster through Mn1. Mechanistic changes in O-O bond formation in S3YZ• are highlighted by analysis of IBOs (intrinsic bonding orbitals) showing deviation for Mn1 and O6 centered IBOs. This change in coupling interactions throughout the complex as a result of S3YZ• formation presents implications for the determination of the mechanism spanning the end of the S3 and the start of the S4 states, affecting both electron movement and oxygen bond formation.

Computational Study on the Influence of Mo/V Centers on the Electronic Structure and Hydrazine Reduction Capability of [MFe3S4]3+/2+ Complexes.

[MFe3S4] cubanes have for some time been of interest for their ability to mimic the electronic and geometric structure of the active site of nitrogenase, the enzyme responsible for fixing N2 to NH3. Nitrogenase naturally occurs in three forms, with the major difference being that the metal ion present in the cofactor active site is either molybdenum (FeMoco), vanadium (FeVco), or iron. The molybdenum and vanadium versions of these cofactors are more closely studied, owing to their larger abundance and rate of catalysis. In this study, we compare free energy profiles and electronic properties of the Mo/V cubanes at various stages during the reduction of N2H4 to NH3. Our findings highlight the differences in how the complexes facilitate the reaction, in particular, vanadium's comparatively weaker ability to interact with the Fe/S network and stabilize reducing electrons prior to N-N bond cleavage, which may have implications when considering the lower efficiency of the vanadium-dependent nitrogenase.

Mechanism of Nitrogen Reduction to Ammonia in a Diiron Model of Nitrogenase.

Nitrogenase is a fascinating enzyme in biology that reduces dinitrogen from air to ammonia through stepwise reduction and protonation. Despite it being studied in detail by experimental and computational groups, there are still many unknown factors in the catalytic cycle of nitrogenase, especially related to the addition of protons and electrons and their order. A recent biomimetic study characterized a potential dinitrogen-bridged diiron cluster as a synthetic model of nitrogenase. Using strong acid and reductants, the dinitrogen was converted into ammonia molecules, but details of the mechanism remains unknown. In particular, it was unclear from the experimental studies whether the proton and electron transfer steps are sequential or alternating. Moreover, the work failed to establish what the function of the diiron core is and whether it split into mononuclear iron fragments during the reaction. To understand the structure and reactivity of the biomimetic dinitrogen-bridged diiron complex [(P2P'PhFeH)2(µ-N2)] with triphenylphosphine ligands, we performed a density functional theory study. Our computational methods were validated against experimental crystal structure coordinates, Mössbauer parameters, and vibrational frequencies and show excellent agreement. Subsequently, we investigated the alternating and consecutive addition of electrons and protons to the system. The calculations identify a number of possible reaction channels, namely, same-site protonation, alternating protonation, and complex dissociation into mononuclear iron centers. The calculations show that the overall mechanism is not a pure sequential set of electron and proton transfers but a mixture of alternating and consecutive steps. In particular, the first reaction steps will start with double proton transfer followed by an electron transfer, while thereafter, there is another proton transfer and a second electron transfer to give a complex whereby ammonia can split off with a low energetic barrier. The second channel starts with alternating protonation of the two nitrogen atoms, whereafter the initial double proton transfer, electrons and protons are added sequentially to form a hydrazine-bound complex. The latter split off ammonia spontaneously after further protonation. The various reaction channels are analyzed with valence bond and orbital diagrams. We anticipate the nitrogenase enzyme to operate with mixed alternating and consecutive protonation and electron transfer steps.

Updating the Military Unique Curriculum for a Ready Medical Force.

INTRODUCTION: Previous conflicts have demonstrated the impact of physician readiness on early battlefield mortality rates. To prepare for the lethal nature of today's threat environment and the rapid speed with which conflict develops, our medical force needs to sustain a high level of readiness in order to be ready to "fight tonight." Previous approaches that have relied on on-the-job training, just-in-time predeployment training, or follow-on courses after residency are unlikely to satisfy these readiness requirements. Sustaining the successes in battlefield care achieved in Iraq and Afghanistan requires the introduction of effective combat casualty care earlier and more often in physician training. This needs assessment seeks to better understand the requirements, challenges, and opportunities to include the Military Unique Curriculum (MUC) during graduate medical education. MATERIALS AND METHODS: This needs assessment used a multifaceted methodology. First, a literature review was performed to assess how Military Unique Curricula have evolved since their initial conception in 1988. Next, to determine their current state, a needs-based assessment survey was designed for trainees and program directors (PDs), each consisting of 18 questions with a mixture of multiple choice, ranking, Likert scale, and free-text questions. Cognitive interviewing and expert review were employed to refine the survey before distribution. The Housestaff Survey was administered using an online format and deployed to Internal Medicine trainees at the Walter Reed National Military Medical Center (WRNMMC). The Program Director Survey was sent to all Army and Navy Internal Medicine Program Directors. This project was deemed to not meet the definition of research in accordance with 32 Code of Federal Regulation 219.102 and Department of Defense Instruction 3216.02 and was therefore registered with the WRNMMC Quality Management Division. RESULTS: Out of 64 Walter Reed Internal Medicine trainees who received the survey, 32 responses were received. Seven of nine PDs completed their survey. Only 12.5% of trainees felt significantly confident that they would be adequately prepared for a combat deployment upon graduation from residency with the current curriculum. Similarly, only 14.29% of PDs felt that no additional training was needed. A majority of trainees were not satisfied with the amount of training being received on any MUC topic. When incorporating additional training on MUC topics, respondents largely agreed that simulation and small group exercises were the most effective modalities to employ, with greater than 50% of both trainees and PDs rating these as most or second most preferred among seven options. Additionally, there was a consensus that training should be integrated into the existing curriculum/rotations as much as possible. CONCLUSIONS: Current Military Unique Curricula do not meet the expected requirements of future battlefields. Several solutions to incorporate more robust military unique training without creating any significant additional time burdens for trainees do exist. Despite the limitation of these results being limited to a single institution, this needs assessment provides a starting point for improvement to help ensure that we limit the impact of any "peacetime effect."

Outcomes Associated With Intensive Care and Organ Support Among Patients With COVID-19: A Systematic Review and Meta-Analysis.

BACKGROUND: Accurate accounting of coronavirus disease 2019 (COVID-19) critical care outcomes has important implications for health care delivery. RESEARCH QUESTION: We aimed to determine critical care and organ support outcomes of intensive care unit (ICU) COVID-19 patients and whether they varied depending on the completeness of study follow-up or admission time period. STUDY DESIGN AND METHODS: We conducted a systematic review and meta-analysis of reports describing ICU, mechanical ventilation (MV), renal replacement therapy (RRT), and extracorporeal membrane oxygenation (ECMO) mortality. A search was conducted using PubMed, Embase, and Cochrane databases.We included English language observational studies of COVID-19 patients, reporting ICU admission, MV, and ICU case fatality, published from December 1, 2019 to December 31, 2020. We excluded reports of less than 5 ICU patients and pediatric populations. Study characteristics, patient demographics, and outcomes were extracted from each article. Subgroup meta-analyses were performed based on the admission end date and the completeness of data. RESULTS: Of 6,778 generated articles, 145 were retained for inclusion (n = 60,357 patients). Case fatality rates across all studies were 34.0% (95% CI = 30.7%, 37.5%, P < 0.001) for ICU deaths, 47.9% (95% CI = 41.6%, 54.2%, P < 0.001) for MV deaths, 58.7% (95% CI = 50.0%, 67.2%, P < 0.001) for RRT deaths, and 43.3% (95% CI = 31.4%, 55.4%, P < 0.001) for extracorporeal membrane oxygenation deaths. There was no statistically significant difference in ICU and organ support outcomes between studies with complete follow-up versus studies without complete follow-up. Case fatality rates for ICU, MV, and RRT deaths were significantly higher in studies with patients admitted before April 31st 2020. INTERPRETATION: Coronavirus disease 2019 critical care outcomes have significantly improved since the start of the pandemic. Intensive care unit outcomes should be evaluated contextually (study quality, data completeness, and time) for the most accurate reporting and to effectively guide mortality predictions.

Magnetic and Electronic Structural Properties of the S3 State of Nature's Water Oxidizing Complex: A Combined Study in ELDOR-Detected Nuclear Magnetic Resonance Spectral Simulation and Broken-Symmetry Density Functional Theory.

ELDOR-detected nuclear magnetic resonance (EDNMR) spectral simulations combined with broken-symmetry density functional theory (BS-DFT) calculations are used to obtain and to assign the 55Mn hyperfine coupling constants (hfcs) for modified forms of the water oxidizing complex in the penultimate S3 state of the water oxidation cycle. The study shows that an open cubane form of the core Mn4CaO6 cluster explains the magnetic properties of the dominant S = 3 species in all cases studied experimentally with no need to invoke a closed cubane intermediate possessing a distorted pentacoordinate Mn4 ion as recently suggested. EDNMR simulations found that both the experimental bandwidth and multinuclear transitions may alter relative EDNMR peak intensities, potentially leading to incorrect assignment of hfcs. The implications of these findings for the water oxidation mechanism are discussed.

How Nature Makes O2: an Electronic Level Mechanism for Water Oxidation in Photosynthesis.

In this paper, we combine broken symmetry density functional calculations and electron paramagnetic resonance analysis to obtain the electronic structure of the penultimate S3 state of nature's water-oxidizing complex and determine the electronic pathway of O-O bond formation. Analysis of the electronic structure changes along the reaction path shows that two spin crossovers, facilitated by the geometry and magnetism of the water-oxidizing complex, are used to provide a unique low-energy pathway. The pathway is facilitated via the formation and stabilization of the [O2]3- ion. This ion is formed between ligated deprotonated substrate waters, O5 and O6, and is stabilized by antiferromagnetic interaction with the Mn ions of the complex. Combining the computational, crystallographic, and spectroscopic data, we show that an equilibrium exists between the O5 oxo and O6 hydroxo forms with an S = 3 spin state and a deprotonated O6 form containing a two-center one-electron bond in [O5O6]3- which we identify as the form detected using crystallography. This form corresponds to an S = 6 spin state which we demonstrate gives rise to a low-intensity EPR spectrum compared with the accompanying S = 3 state, making its detection via EPR difficult and overshadowed by the S = 3 form. Simulations using 70% of the S = 6 component give rise to a superior fit to the experimental W-band EPR spectral envelope compared with an S = 3 only form. Analyses of the most recent X-ray emission spectroscopy first moment changes for solution and time-resolved crystal data are also shown to support the model. The computational, crystallographic, and spectroscopic data are shown to coalesce to the same picture of a predominant S = 6 species containing the first one-electron oxidation product of two water molecules, that is, [O5O6]3-. Progression of this form to the two-electron-oxidized peroxo and three-electron-oxidized superoxo forms, leading eventually to the evolution of triplet O2, is proposed to be the pathway nature adopts to oxidize water. The study reveals the key electronic, magnetic, and structural design features of nature's catalyst which facilitates water oxidation to O2 under ambient conditions.

Characteristics and reasons for use associated with solitary alcohol and marijuana use among U.S. 12th Grade Students, 2015-2021.

BACKGROUND: Little is known regarding what sociodemographic characteristics and reasons for use are associated with adolescent solitary alcohol and marijuana use. METHODS: Data from 7845 12th grade students participating in the nationally-representative Monitoring the Future study from 2015 to 2021 were used to examine cross-sectional associations between sociodemographics, heavy drinking/marijuana use, reasons for use, and past 12-month solitary alcohol or marijuana use among past 12-month users. Historical trends and possible differences related to the COVID-19 pandemic also were examined. RESULTS: Solitary use prevalence increased from 2015 to 2021 with no evidence of significant COVID-19 deviations. In 2021, solitary alcohol use was reported by 32.1% (SE 3.01) and solitary marijuana use by 55.8% (4.72) of those reporting past 12-month use. Common and substance-specific sociodemographic risk factors were observed. Binge drinking was associated with solitary alcohol use; frequent marijuana use was associated with solitary marijuana use. Reasons for use related to coping with negative affect were associated with solitary use. Compulsive use reasons were more strongly associated with solitary alcohol than marijuana use. Drinking to have a good time with friends was negatively associated with solitary alcohol use but this association was not seen for solitary marijuana use. CONCLUSIONS: The percentage of adolescents who use alcohol or marijuana when they were alone has increased among those who report using each substance. Associations between solitary use and (a) higher levels of consumption and (b) coping with negative affect highlight the importance of solitary use as a risk indicator.

Tool to improve patient-provider interactions in adult primary care: Randomized controlled pilot study.

OBJECTIVE: To assess whether an intervention to help patients prioritize goals for their visit would improve patient-provider communication and clinical outcomes. DESIGN: Randomized controlled pilot study. SETTING: Primary care clinic. PARTICIPANTS: There were 120 adult hypertensive patients enrolled. INTERVENTION: Patients were randomized to receive either usual care or a previsit patient activation card developed through a series of focus groups that prompted patients to articulate their needs and set priorities for their clinic visit. Encounters were audiorecorded, transcribed, and assessed using duplicate ratings of patient activation and decision making. MAIN OUTCOME MEASURES: The primary outcome was change in medication adherence as measured by pill count at 4 and 12 weeks after the initial visit. Secondary outcomes evaluated patient-provider interaction quality (patient satisfaction, patient activation, shared decision making, patient trust, and physicians' perceived difficulty of the encounter), functional status, and blood pressure control. RESULTS: Of the 120 enrolled patients, 106 completed the baseline visit (mean age of 66 years, 53% women, 57% Black, 36% White). Participants had multiple comorbidities (median number of medications = 8). During the visit, there was greater patient activation in the intervention arm than in the control arm (4.4 vs 3.8, P = .047; ratings were based on a scale from 1 to 10). However, after the visit there were no differences in medication adherence (4 weeks: 45.8% vs 49.5%; 12 weeks: 49.4% vs 51.1%), blood pressure control (4 weeks: 133/78 mm Hg vs 131/77 mm Hg; 12 weeks: 129/77 mm Hg vs 129/76 mm Hg), or encounter satisfaction (78.6% vs 73.8% fully satisfied; P = .63). There were also no differences in shared decision making, patients' trust, or perceived difficulty of the encounter. CONCLUSION: A single previsit tool designed to prompt patients to set a prioritized agenda improved patient activation during the visit, but did not affect the quality of the interaction or postvisit patient-centred outcomes.

Building on a Sequential Mixed-Mode Research Design in the Monitoring the Future Study.

Given the promise of the web push plus e-mail survey design for providing cost-effective and high-quality data (Patrick et al. 2018, 2019) as an alternative to a paper-and-pencil mailed survey design for the longitudinal Monitoring the Future (MTF) study, the current study sought to further enhance the web push condition. The MTF sample is based on US nationally representative samples of 12th grade students surveyed annually. The MTF control group for the current study included participants who completed the in-school baseline survey in the 12th grade and were selected to participate in their first follow-up survey in 2017 via mailed surveys (N = 1,222). A supplementary sample (N = ∼2,450) was assigned to one of the two sequential mixed-mode conditions. Those in condition 1 (N = 1,198), or mail push, were invited to complete mailed surveys and later given a web survey option. Those in condition 2 (N = 1,173), or enhanced web push, were invited to complete a web survey (the same as in the 2014 study, but with the addition of text messages and quick response (QR) codes and the web survey was optimized for mobile devices) and then later given a mailed survey option. Research aims were to examine response rates across conditions, as well as how responses were distributed across mode (paper, web), devices (computer, smartphone, table), and method of accessing the web survey (hand-entered URL, QR code, e-mail link, SMS link). Response rates differed significantly: the MTF control group was 34.2 percent, mail push was 35.4 percent, and enhanced web push was 42.05 percent. The higher response rate in the enhanced web push condition suggests that the additional strategies were effective at bringing in more respondents. Key estimates produced by the enhanced web push condition did not differ from those of the MTF control group.

Self-reported perceived negative consequences of marijuana use among U.S. young adult users, 2008-2019.

PURPOSE: This study estimated self-reported perceived negative marijuana use consequences among a national sample of U.S. young adults, examining consequence prevalence differences by use frequency, college attendance, living situation, employment, sex, and race/ethnicity; and use frequency/sociodemographic characteristic interactions. METHODS: A subsample of 1,212 respondents from the 2004-2018 class cohorts of 12th grade students participating in the nationally-representative Monitoring the Future study was surveyed up to two times from modal ages 19 through 22 (in 2008-2019). Respondents self-reported negative consequences related to their own past 12-month marijuana use. Bivariate and multivariable models examined subgroup differences in consequence prevalence. RESULTS: Approximately 60% of those using frequently (20+ use occasions in the past 30 days) and 35% of those using non-frequently reported negative consequences. Among all young adult marijuana users, 31.1% reported emotional/physical consequences, 12.9% performance/financial consequences, and 12.3% relational consequences. Use frequency was positively associated with consequence likelihood, excluding regret and unsafe driving. Among college students, frequent use was more strongly associated with any and performance/financial consequences. Controlling for use frequency, men reported more performance/financial consequences; relational consequences were higher among Hispanic (vs. White) respondents, and those living with parents, employed full-time, and not attending 4-year colleges. CONCLUSION: Young adults using marijuana reported a wide range of negative use consequences; likelihood of most consequences increased with higher use frequency. Perceived consequences varied by college attendance, living situation, employment, sex, and race/ethnicity. Efforts to reduce negative marijuana consequences may be strengthened by recognizing and addressing the different types of negative consequences users perceive.

Social Role, Behavior, and Belief Changes Associated With Driving After Using Marijuana Among U.S. Young Adults, and Comparisons With Driving After 5+ Drinking.

OBJECTIVE: This study examined past-2-week driving after marijuana use (DMU) and driving after having five or more drinks (D5D) during young adulthood, specifically focusing on associations between within-person change in social roles (living situation, marriage, parenthood, education, employment) and mediators (perceived risk, evenings out, and religiosity) from modal ages 19 to 30. METHOD: Multilevel analyses were conducted using survey data collected from 2013 to 2019 from 1,873 adults (1,060 women; total number of data collection waves = 7,037) participating in the longitudinal Monitoring the Future study. RESULTS: Change across waves from not being married to married was associated with lower DMU likelihood at any particular wave both directly and via mediation through wave-level change in evenings out. Change in employment (not employed to employed full time) was associated with higher D5D likelihood at any particular wave both directly and via mediation through change in evenings out. Wave-level change in other social roles was indirectly associated with DMU/D5D likelihood via wave-level change in evenings out. CONCLUSIONS: Change in all social roles examined was associated with change in evenings out, which appears to be a primary, proximal predictor of young adult DMU/D5D. Improved understanding of how socialization change is associated with driving after substance use may strengthen efforts to reduce the harms associated with such driving behaviors.

S3 State Models of Nature's Water Oxidizing Complex: Analysis of Bonding and Magnetic Exchange Pathways, Assessment of Experimental Electron Paramagnetic Resonance Data, and Implications for the Water Oxidation Mechanism.

Broken symmetry density functional theory (BS-DFT) calculations on large models of Nature's water oxidizing complex (WOC) are used to investigate the electronic structure and associated magnetic interactions of this key intermediate state. The electronic origins of the ferromagnetic and antiferromagnetic couplings between neighboring Mn ions are investigated and illustrated by using corresponding orbital transformations. Protonation of the O4 and/or O6 atoms leads to large variation in the distribution of spin around the complex with associated changes in its magnetic resonance properties. Models for Sr2+ exchange and methanol addition indicate minor perturbations reflected in slightly altered spin projection coefficients for the Mn1 and Mn2 ions. These are shown to account for the observed changes observed experimentally via electron paramagnetic resonance methods and suggest a reinterpretation of the experimental findings. By comparison with experimental determinations, we show that the spin projections and resulting calculated 55Mn hyperfine couplings support the open cubane form of an oxo (O5)-hydroxo (O6) cluster in all cases with no need to invoke a closed cubane intermediate. The implications of these findings for the water oxidation mechanism are discussed.