Electrochemical titration of the S = 32 and S = 12 states of the iron protein of nitrogenase

AbstractThe iron protein of nitrogenase delivers electrons and ATP to the iron-molybdenum protein, which in turn reduces dinitrogen to ammonia. The iron protein contains a single four-iron, four-sulfur prosthetic group, detectable by ESR spectroscopy m its reduced form. Until recently, spin quantitations suggested that only a portion of the reduced iron protein was being detected by ESR, but recent work in several laboratories has shown that there is a spin = 32 signal near g = 5 in addition to the well characterized spin = 12 signal near g = 1.94. In this paper we characterize the redox properties of both states in the presence and absence of ATP and ADP, and find that the new spin state has identical redox properties to those previously determined for the spin = 12 state

Electrochemistry of ubiquinones Menaquinones and plastoquinones in aprotic solvents

AbstractFirst and second half-wave reduction potentials of a series of 1,4-benzo- and 1,4-naphtho-quinones related to the naturally occurring ubiquinones, plastoquinones and menaquinones are correlated with substituent effects. Notably, E12 of 2,3-dimethoxy-1,4-benzoquinone is positive of the values for the 2,5- and 2,6-dimethoxy isomers, and of the value for methoxy-1,4-benzoquinone. This phenomenon is attributed to steric inhibition of resonance when two methoxy groups occupy adjacent positions, and the significance of this orientation in the ubiquinone series is highlighted

Mo\u3csup\u3eV\u3c/sup\u3e Electron Paramagnetic Resonance of Sulfite Oxidase Revisited: The Low-pH Chloride Signal

Valuable information on the active sites of molybdenum enzymes has been provided by MoV electron paramagnetic resonance (EPR) spectroscopy. In recent years, multiple resonance techniques have been extensively used to examine details of the active-site structure, but basic continuous-wave (CW) EPR has not been re-evaluated in several decades. Here, we present a re-examination of the CW EPR spectroscopy of the sulfite oxidase low-pH chloride species and provide evidence for direct coordination of molybdenum by chloride

Zwicky Transient Facility constraints on the optical emission from the nearby repeating FRB 180916.J0158+65

The discovery rate of fast radio bursts (FRBs) is increasing dramatically thanks to new radio facilities. Meanwhile, wide-field instruments such as the 47 deg$^2$ Zwicky Transient Facility (ZTF) survey the optical sky to study transient and variable sources. We present serendipitous ZTF observations of the CHIME repeating source FRB 180916.J0158+65, that was localized to a spiral galaxy 149 Mpc away and is the first FRB suggesting periodic modulation in its activity. While 147 ZTF exposures corresponded to expected high-activity periods of this FRB, no single ZTF exposure was at the same time as a CHIME detection. No $>3\sigma$ optical source was found at the FRB location in 683 ZTF exposures, totalling 5.69 hours of integration time. We combined ZTF upper limits and expected repetitions from FRB 180916.J0158+65 in a statistical framework using a Weibull distribution, agnostic of periodic modulation priors. The analysis yielded a constraint on the ratio between the optical and radio fluences of $\eta \lesssim 200$, corresponding to an optical energy $E_{\rm opt} \lesssim 3 \times 10^{46}$ erg for a fiducial 10 Jy ms FRB (90% confidence). A deeper (but less statistically robust) constraint of $\eta \lesssim 3$ can be placed assuming a rate of $r(>5$ Jy ms)= hr$^{-1}$ and $1.2\pm 1.1$ FRB occurring during exposures taken in high-activity windows. The constraint can be improved with shorter per-image exposures and longer integration time, or observing FRBs at higher Galactic latitudes. This work demonstrated how current surveys can statistically constrain multi-wavelength counterparts to FRBs even without deliberately scheduled simultaneous radio observation.Comment: Accepted for publication in ApJL, 9 pages, 4 figures, 1 tabl

A new class of large-amplitude radial-mode hot subdwarf pulsators

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g − r ≤ −0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200–475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T eff, and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T eff and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25–0.35 M ⊙ helium-core pre-WDs.Published versio

Marine Oil Snow, a Microbial Perspective

Under certain conditions, dispersed crude oil in the sea combines with organisms, organic matter, and minerals to form marine oil snow (MOS), thereby contributing to the sinking of oil to the seafloor. Marine microbes are the main players in MOS formation, particularly via the production of extracellular polymeric substances. Distinct groups of microbes also consume the majority of the hydrocarbons during descent, leading to enrichment of the less bioavailable hydrocarbons and asphaltenes in the residue. Here we discuss the dynamics of microbial communities in MOS together with their impacts on MOS evolution. We explore the effects of dispersant application on MOS formation, and consider ways in which laboratory experiments investigating MOS formation can be more representative of the situation in the marine environment, which in turn will improve our understanding of the contribution of MOS to the fate of spilled oil

Cataclysmic Variables in the First Year of the Zwicky Transient Facility

Using selection criteria based on amplitude, time, and color, we have identified 329 objects as known or candidate cataclysmic variables (CVs) during the first year of testing and operation of the Zwicky Transient Facility. Of these, 90 are previously confirmed CVs, 218 are strong candidates based on the shape and color of their light curves obtained during 3–562 days of observation, and the remaining 21 are possible CVs but with too few data points to be listed as good candidates. Almost half of the strong candidates are within 10 deg of the galactic plane, in contrast to most other large surveys that have avoided crowded fields. The available Gaia parallaxes are consistent with sampling the low mass transfer CVs, as predicted by population models. Our follow-up spectra have confirmed Balmer/helium emission lines in 27 objects, with four showing high-excitation He ii emission, including candidates for an AM CVn, a polar, and an intermediate polar. Our results demonstrate that a complete survey of the Galactic plane is needed to accomplish an accurate determination of the number of CVs existing in the Milky Way