Theoretical analysis of the kinetic isotope effect on carboxylation in RubisCO.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) fixes atmospheric carbon dioxide into bioavailable sugar molecules. It is also well known that a kinetic isotope effect (KIE; CO2 carbon atoms) accompanies the carboxylation process. To describe the reaction and the KIE α, two different types of molecular dynamics (MD) simulations (ab initio MD and classical MD) have been performed with an Own N-layered Integrated molecular Orbitals and molecular Mechanics (ONIOM)-hybrid model. A channel structure for CO2 transport has been observed during the MD simulation in RubisCO, and assuming the reaction path from the inlet to the product through the coordinate complex with Mg2+ , simulations have been performed on several molecular configuration models fixing several distances between CO2 and ribulose-1,5-bisphosphate along the channel. Free energy analysis and diffusion coefficient analysis have been evaluated for different phases of the process. It is confirmed that the isotopic fractionation effect for CO2 containing either 13 C or 12 C would appear through the transiting path in the channel structure identified in RubisCO. The estimated isotope fractionation constant was quite close to the experimental value.

Exoplanet Biosignatures: Future Directions.

We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets-Biosignatures-Life detection-Bayesian analysis. Astrobiology 18, 779-824.

A Strategy for Origins of Life Research.

Contents 1. Introduction 1.1. A workshop and this document 1.2. Framing origins of life science 1.2.1. What do we mean by the origins of life (OoL)? 1.2.2. Defining life 1.2.3. How should we characterize approaches to OoL science? 1.2.4. One path to life or many? 2. A Strategy for Origins of Life Research 2.1. Outcomes-key questions and investigations 2.1.1. Domain 1: Theory 2.1.2. Domain 2: Practice 2.1.3. Domain 3: Process 2.1.4. Domain 4: Future studies 2.2. EON Roadmap 2.3. Relationship to NASA Astrobiology Roadmap and Strategy documents and the European AstRoMap Appendix I Appendix II Supplementary Materials References.

Decoding Redox Evolution Before Oxygenic Photosynthesis Based on the Sulfur-Mass Independent Fractionation (S-MIF) Record.

The isotopic anomaly of stable sulfur isotope could be a useful new tracer for decoding atmospheric chemistry of early Earth from ancient rock samples. Here, we summarize current status of the experimental works and discuss what the isotopic anomaly tells about early Earth's atmosphere.

Nonadiabatic calculations of ultraviolet absorption cross section of sulfur monoxide: isotopic effects on the photodissociation reaction.

Ultraviolet absorption cross sections of the main and substituted sulfur monoxide (SO) isotopologues were calculated using R-Matrix expansion technique. Energies, transition dipole moments, and nonadiabatic coupling matrix elements were calculated at MRCI/AV6Z level. The calculated absorption cross section of (32)S(16)O was compared with experimental spectrum; the spectral feature and the absolute value of photoabsorption cross sections are in good agreement. Our calculation predicts a long lived photoexcited SO* species which causes large non-mass dependent isotopic effects depending on the excitation energy in the ultraviolet region.

SO2 photoexcitation mechanism links mass-independent sulfur isotopic fractionation in cryospheric sulfate to climate impacting volcanism.

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO2, that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of (32)SO2, (33)SO2, (34)SO2, and (36)SO2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ(33)S/δ(34)S and Δ(36)S/Δ(33)S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.

Isotope effect in the carbonyl sulfide reaction with O(3P).

The sulfur kinetic isotope effect (KIE) in the reaction of carbonyl sulfide (OCS) with O((3)P) was studied in relative rate experiments at 298 ± 2 K and 955 ± 10 mbar. The reaction was carried out in a photochemical reactor using long path FTIR detection, and data were analyzed using a nonlinear least-squares spectral fitting procedure with line parameters from the HITRAN database. The ratio of the rate of the reaction of OC(34)S relative to OC(32)S was found to be 0.9783 ± 0.0062 ((34)ε = (-21.7 ± 6.2)‰). The KIE was also calculated using quantum chemistry and classical transition state theory; at 300 K, the isotopic fractionation was found to be (34)ε = -14.8‰. The OCS sink reaction with O((3)P) cannot explain the large fractionation in (34)S, over +73‰, indicated by remote sensing data. In addition, (34)ε in OCS photolysis and OH oxidation are not larger than 10‰, indicating that, on the basis of isotopic analysis, OCS is an acceptable source of background stratospheric sulfate aerosol.

Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox.

Distributions of sulfur isotopes in geological samples would provide a record of atmospheric composition if the mechanism producing the isotope effects could be described quantitatively. We determined the UV absorption spectra of 32SO2, 33SO2, and 34SO2 and use them to interpret the geological record. The calculated isotopic fractionation factors for SO2 photolysis give mass independent distributions that are highly sensitive to the atmospheric concentrations of O2, O3, CO2, H2O, CS2, NH3, N2O, H2S, OCS, and SO2 itself. Various UV-shielding scenarios are considered and we conclude that the negative Delta33S observed in the Archean sulfate deposits can only be explained by OCS shielding. Of relevant Archean gases, OCS has the unique ability to prevent SO2 photolysis by sunlight at lambda >202 nm. Scenarios run using a photochemical box model show that ppm levels of OCS will accumulate in a CO-rich, reducing Archean atmosphere. The radiative forcing, due to this level of OCS, is able to resolve the faint young sun paradox. Further, the decline of atmospheric OCS may have caused the late Archean glaciation.

Carbonyl sulfide isotopologues: ultraviolet absorption cross sections and stratospheric photolysis.

Ultraviolet absorption cross sections of the main and substituted carbonyl sulfide isotopologues were calculated using wavepacket dynamics. The calculated absorption cross section of (16)O(12)C(32)S is in very good agreement with the accepted experimental spectrum between 190 and 250 nm. Relative to (16)O(12)C(32)S, isotopic substitution shows a significant enhancement of the cross section for (16)O(13)C(32)S, a significant reduction for (18)O(12)C(32)S and (17)O(12)C(32)S and almost no change for the sulfur isotopologues (16)O(12)C(33)S, (16)O(12)C(34)S, and (16)O(12)C(36)S. The analysis of the initial wavepackets shows that these changes can be explained in terms of the change in the norm of the initial wavepacket. Implications for our understanding of the stratospheric sulfur cycle are discussed.