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1. Thermodynamics and Reaction Mechanisms for Decomposition of a Simple Protonated Tripeptide, H ⁺ GGA: From H ⁺ GGG to H ⁺ GAG to H ⁺ GGA

   https://doi.org/10.1021/jasms.1c00345  

   Mookherjee, Abhigya; Armentrout, P. B. (February 2022, Journal of the American Society for Mass Spectrometry)
2. Branching Ratio for O + H ₃ ⁺ Forming OH ⁺ + H ₂ and H ₂ O ⁺ + H

   https://doi.org/10.3847/1538-4357/ac41ce  

   Hillenbrand, Pierre-Michel; Ruette, Nathalie de; Urbain, Xavier; Savin, Daniel W. (March 2022, The Astrophysical Journal)

   Abstract The gas-phase reaction of O + H 3 + has two exothermic product channels: OH + + H 2 and H 2 O + + H. In the present study, we analyze experimental data from a merged-beams measurement to derive thermal rate coefficients resolved by product channel for the temperature range from 10 to 1000 K. Published astrochemical models either ignore the second product channel or apply a temperature-independent branching ratio of 70% versus 30% for the formation of OH + + H 2 versus H 2 O + + H, respectively, which originates from a single experimental data point measured at 295 K. Our results are consistent with this data point, but show a branching ratio that varies with temperature reaching 58% versus 42% at 10 K. We provide recommended rate coefficients for the two product channels for two cases, one where the initial fine-structure population of the O( 3 P J ) reactant is in its J = 2 ground state and the other one where it is in thermal equilibrium. 

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3. ² H/ ¹ H variation in microbial lipids is controlled by NADPH metabolism

   https://doi.org/10.1073/pnas.1818372116  

   Wijker, Reto S.; Sessions, Alex L.; Fuhrer, Tobias; Phan, Michelle (June 2019, Proceedings of the National Academy of Sciences)

   null (Ed.)

   The hydrogen-isotopic compositions ( 2 H/ 1 H ratios) of lipids in microbial heterotrophs are known to vary enormously, by at least 40% (400‰) relative. This is particularly surprising, given that most C-bound H in their lipids appear to derive from the growth medium water, rather than from organic substrates, implying that the isotopic fractionation between lipids and water is itself highly variable. Changes in the lipid/water fractionation are also strongly correlated with the type of energy metabolism operating in the host. Because lipids are well preserved in the geologic record, there is thus significant potential for using lipid 2 H/ 1 H ratios to decipher the metabolism of uncultured microorganisms in both modern and ancient ecosystems. But despite over a decade of research, the precise mechanisms underlying this isotopic variability remain unclear. Differences in the kinetic isotope effects (KIEs) accompanying NADP + reduction by dehydrogenases and transhydrogenases have been hypothesized as a plausible mechanism. However, this relationship has been difficult to prove because multiple oxidoreductases affect the NADPH pool simultaneously. Here, we cultured five diverse aerobic heterotrophs, plus five Escherichia coli mutants, and used metabolic flux analysis to show that 2 H/ 1 H fractionations are highly correlated with fluxes through NADP + -reducing and NADPH-balancing reactions. Mass-balance calculations indicate that the full range of 2 H/ 1 H variability in the investigated organisms can be quantitatively explained by varying fluxes, i.e., with constant KIEs for each involved oxidoreductase across all species. This proves that lipid 2 H/ 1 H ratios of heterotrophic microbes are quantitatively related to central metabolism and provides a foundation for interpreting 2 H/ 1 H ratios of environmental lipids and sedimentary hydrocarbons. 

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4. Mechanisms and dynamics of the NH ⁺ ₂ + H ⁺ and NH ⁺ + H ⁺ + H fragmentation channels upon single-photon double ionization of NH ₃

   https://doi.org/10.1088/1361-6455/abc3aa  

   Larsen, Kirk A; Rescigno, Thomas N; Streeter, Zachary L; Iskandar, Wael; Heck, Saijoscha; Gatton, Averell; Champenois, Elio G; Severt, Travis; Strom, Richard; Jochim, Bethany; et al (November 2020, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We present state-selective measurements on the N H 2 + + H + and NH + + H + + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH 3 , where the two photoelectrons and two cations are measured in coincidence using 3D momentum imaging. Three dication electronic states are identified to contribute to the N H 2 + + H + dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold N H 2 + fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited N H 2 + fragment with roughly 1 eV of internal energy. The NH + + H + + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the N H 2 + + H + channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states. 

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5. Probing O–H Bonding through Proton Detected ¹ H– ¹⁷ O Double Resonance Solid-State NMR Spectroscopy

   https://doi.org/10.1021/jacs.8b10878  

   Carnahan, Scott L.; Lampkin, Bryan J.; Naik, Pranjali; Hanrahan, Michael P.; Slowing, Igor I.; VanVeller, Brett; Wu, Gang; Rossini, Aaron J. (November 2018, Journal of the American Chemical Society)