Endogenous CO2 ice mixture on the surface of Europa and no detection of plume activity.

Jupiter's moon Europa has a subsurface ocean beneath an icy crust. Conditions within the ocean are unknown, and it is unclear whether it is connected to the surface. We observed Europa with the James Webb Space Telescope (JWST) to search for active release of material by probing its surface and atmosphere. A search for plumes yielded no detection of water, carbon monoxide, methanol, ethane, or methane fluorescence emissions. Four spectral features of carbon dioxide (CO2) ice were detected; their spectral shapes and distribution across Europa's surface indicate that the CO2 is mixed with other compounds and concentrated in Tara Regio. The 13CO2 absorption is consistent with an isotopic ratio of 12C/13C = 83 ± 19. We interpret these observations as indicating that carbon is sourced from within Europa.

A multifunctional setup to record FTIR and UV-vis spectra of organic molecules and their photoproducts in astronomical ices.

This article describes a new, multi-functional, high-vacuum ice setup that allows to record the in situ and real-time spectra of vacuum UV (VUV)-irradiated non-volatile molecules embedded in a low-temperature (10 K) amorphous solid water environment. Three complementary diagnostic tools-UV-visible (UV-vis) and Fourier Transform Infrared (FTIR) spectroscopy and temperature-programmed desorption quadrupole mass spectrometry-can be used to simultaneously study the physical and chemical behavior of the organic molecules in the ice upon VUV irradiation. The setup is equipped with a temperature-controlled sublimation oven that enables the controlled homogeneous deposition of solid species such as amino acids, nucleobases, and polycyclic aromatic hydrocarbons (PAHs) in ice mixtures prepared from precursor gases and/or liquids. The resulting ice is photo-processed with a microwave discharge hydrogen lamp, generating VUV radiation with a spectral energy distribution representative for the interstellar medium. The characteristics, performance, and future potential of the system are discussed by describing three different applications. First, a new method is introduced, which uses broadband interference transmission fringes recorded during ice deposition, to determine the wavelength-dependent refractive index, nλ, of amorphous solid water. This approach is also applicable to other solids, pure and mixed. Second, the UV-vis and FTIR spectroscopy of an VUV-irradiated triphenylene:water ice mixture is discussed, monitoring the ionization efficiency of PAHs in interstellar ice environments. The third and final example investigates the stability of solid glycine upon VUV irradiation by monitoring the formation of dissociation products in real time.

Hydroxylation of camphor by reduced oxy-cytochrome P450cam: mechanistic implications of EPR and ENDOR studies of catalytic intermediates in native and mutant enzymes.

We have employed gamma-irradiation at cryogenic temperatures (77 K and also approximately 6 K) of the ternary complexes of camphor, dioxygen, and ferro-cytochrome P450cam to inject the "second" electron of the catalytic process. We have used EPR and ENDOR spectroscopies to characterize the primary product of reduction as well as subsequent states created by annealing reduced oxyP450, both the WT enzyme and the D251N and T252A mutants, at progressively higher temperatures. (i) The primary product upon reduction of oxyP450 4 is the end-on, "H-bonded peroxo" intermediate 5A. (ii) This converts even at cryogenic temperatures to the hydroperoxo-ferriheme species, 5B, in a step that is sensitive to these mutations. Yields of 5B are as high as 40%. (iii) In WT and D251N P450s, brief annealing in a narrow temperature range around 200 K causes 5B to convert to a product state, 7A, in which the product 5-exo-hydroxycamphor is coordinated to the ferriheme in a nonequilibrium configuration. Chemical and EPR quantitations indicate the reaction pathway involving 5B yields 5-exo-hydroxycamphor quantitatively. Analogous (but less extensive) results are seen for the alternate substrate, adamantane. (iv) Although the T252A mutation does not interfere with the formation of 5B, the cryoreduced oxyT252A does not yield product, which suggests that 5B is a key intermediate at or near the branch-point that leads either to product formation or to nonproductive "uncoupling" and H(2)O(2) production. The D251N mutation appears to perturb multiple stages in the catalytic cycle. (v) There is no spectroscopic evidence for the buildup of a high-valence oxyferryl/porphyrin pi-cation radical intermediate, 6. However, ENDOR spectroscopy of 7A in H(2)O and D(2)O buffers shows that 7A contains hydroxycamphor, rather than water, bound to Fe(3+), and that the proton removed from the C(5) carbon of substrate during hydroxylation is trapped as the hydroxyl proton. This demonstrates that hydroxylation of substrates by P450cam in fact occurs by the formation and reaction of 6. (vi) Annealing at > or = 220 K converts the initial product state 7A to the equilibrium product state 7, with the transition occurring via a second nonequilibrium product state, 7B, in the D251N mutant; in states 7B and 7 the hydroxycamphor hydroxyl proton no longer is trapped. (vii) The present results are discussed in the context of other efforts to detect intermediates in the P450 catalytic cycle.

Role of the nonheme Fe(II) center in the biosynthesis of the plant hormone ethylene.

The final step of ethylene biosynthesis in plants is catalyzed by the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). In addition to ACC, Fe(II), O2, CO2, and ascorbate are required for in vitro enzyme activity. Direct evidence for the role of the Fe(II) center in the recombinant avocado ACCO has now been obtained through formation of enzyme.(substrate or cofactor).NO complexes. These NO adducts convert the normally EPR-silent ACCO complexes into EPR-active species with structural properties similar to those of the corresponding O2 complexes. It is shown here that the ternary Fe(II)ACCO.ACC.NO complex is readily formed, but no Fe(II)ACCO.ascorbate.NO complex could be observed, suggesting that ascorbate and NO are mutually exclusive in the active site. The binding modes of ACC and the structural analog alanine specifically labeled with 15N or 17O were examined by using Q-band electron nuclear double resonance (ENDOR). The data indicate that these molecules bind directly to the iron through both the alpha-amino and alpha-carboxylate groups. These observations are inconsistent with the currently favored mechanism for ACCO, in which it is proposed that both ascorbate and O2 bind to the iron as a step in O2 activation. We propose a different mechanism in which the iron serves instead to simultaneously bind ACC and O2, thereby fixing their relative orientations and promoting electron transfer between them to initiate catalysis.