Laboratory surface and solid state astrochemistry of biomolecular precursors on grain mimics

The existence of water (H2O) and acetonitrile (CH3CN) in various extraterrestrial environments attracts considerable attention as both molecules are crucial to understanding the genesis of life. This thesis involves investigations of the physical and chemical interactions of CH3CN and H2O ices condensed on an amorphous silica surface using ultrahigh vacuum-based surface science methods (reflection-absorption infrared spectroscopy, temperature programmed desorption, proton- and electron-irradiation experiments) in an effort to understand how such molecules will behave when they are processed by radiation and heat in space environments. The thermal behaviour of CH3CN on H2O and on the silica surface is governed by a balance of intermolecular forces and shows evidence for wetting of the silica surface (formation of a distinct first adsorption layer before multilayer growth) and of dewetting or island formation on the H2O surface (multilayer growth for all CH3CN exposures). This illustrates the importance of the substrate in determining the solid state growth morphology. Physical and chemical processes induced by 250 to 500 eV electrons have been investigated in the CH3CN and H2O systems. Such interactions promote only desorption from both CH3CN and H2O ice surfaces. 200 keV protons irradiation experiments of CH3CN ices, conducted by collaborators in Italy, in contrast showed evidence for chemical reactions in the solid CH3CN. Quantitative measurements of desorption and reaction cross-sections are used to reconcile this stark contrast. The impact of such processes in relation to H2O on astrophysics is investigated through simple numerical simulations using results derived from this work

A study of Single-Cell RNA Sequencing Analysis Reveals Sequential Cell Fate Transition during Human Spermatogenesis. Wang et al

(1A) Immunofluorescence of FGFR3 (red) co-stained with DDX4, UTF1, KIT, or STRA8 (green) in adult human testicular paraffin sections from one donor with OA. Triangles indicate SSCs; yellow arrowheads indicate Diff.ing SPG; arrows indicate Diff.ed SPG. SSC, spermatogonial stem cells; Diff.ing SPG, differentiating spermatogonia; Diff.ed SPG, differentiated spermatogonia. Scale bar, 10 μm. (1B) Immunofluorescence of BMPR1B (red) co-stained with DDX4, UTF1, KIT, or STRA8 (green) in adult human testicular paraffin sections from one donor with OA. Triangles indicate SSCs; yellow arrowheads indicate Diff.ing SPG; arrows indicate Diff.ed SPG. SSC, spermatogonial stem cells; Diff.ing SPG, differentiating spermatogonia; Diff.ed SPG, differentiated spermatogonia. Scale bar, 10 μm. (2A) Immunofluorescence of DMC1 and γH2AX (red) co-stained with SYCP3 (green) in adult human testicular paraffin sections from one donor with OA. Triangles indicate leptotene (L) spermatocytes; yellow arrowheads indicate zygotene (Z) spermatocytes; arrows indicate pachytene (P) spermatocytes. Scale bar, 10 μm. (2B) Boxplot showing the relative expression levels (average TPM) of sex chromosome genes versus autosomes (Chr1, Chr3 to Chr5, Chr7 to Chr10, Chr12 to Chr19, Chr21 to Chr22) in each cluster. (3A) Immunofluorescence of TNP1 (green) co-stained with γH2AX (red) and PNA (pink) in adult human testicular paraffin sections from one donor with OA. Triangles indicate γH2AX+ spermatids. Scale bar, 10 μm. (3B) Immunofluorescence of TNP1, PRM1, H4K16AC, KAT8 and CHD5 (green) co-stained with DDX4 (red) and PNA (pink) in adult human testicular paraffin sections from one donor with OA respectively. Triangles indicate S1; yellow arrowheads indicate S2; arrows indicate S3. S, spermatids. Scale bar, 10 μm. (4A) Enriched GO terms and P values of 2,821 human-mouse homologous DEGs in four categories of adult human testicular cells. SPG, spermatogonia; SPC, spermatocytes; S, spermatids. (4B) Enriched GO terms and P values (P value < 0.05) of 1,710 widely expressed human-mouse homologous genes in testicular cells. (4C) Enriched GO terms and P values (P value < 0.05) of 998 widely expressed human-mouse non-homologous genes in testicular cells. (4D) Enriched GO terms and P values of 1,158 non-housekeeping (also non-homologous) DEGs in four categories of adult human testicular cells. SPG, spermatogonia; SPC, spermatocytes; S, spermatids

Laboratory studies of electron and ion irradiation of solid acetonitrile (CH3CN)

The structure and bonding of solid acetonitrile (CH 3 CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250–400 eV electrons are quantified using transmission infrared spectroscopy, reflection–absorption infrared spectroscopy and temperature-programmed desorption, with the assistance of basic computational chemistry and nuclear materials calculations. The thermal desorption profiles are found to depend strongly on the balance between CH 3 CN–surface and CH 3 CN–CH 3 CN interactions, passing from a sub-monolayer regime (binding energy: 35–50 kJ mol −1 ) to a multilayer regime (binding energy: 38.2±1.0 kJ mol −1 ) via a fractional order desorption regime characteristic of islanding as the coverage increases. Calculations using the SRIM code reveal that the effects of the ion irradiation are dominated by electronic stopping of incident protons, and the subsequent generation of secondary electrons. Therefore, ion irradiation and electron irradiation experiments can be quantitatively compared. During ion irradiation of thicker CH 3 CN films, a cross section for secondary electron-promoted chemical destruction of CH 3 CN of 4 (±1)×10 −18  cm 2 was measured, while electron-promoted desorption was not detected. A significantly higher cross section for electron-promoted desorption of 0.82–3.2×10 −15  cm 2 was measured during electron irradiation of thinner CH 3 CN films, while no chemical products were detected. The differences between the experimental results can be rationalized by recognizing that chemical reaction is a bulk effect in the CH 3 CN film, whereas desorption is a surface sensitive process. In thicker films, electron-promoted desorption is expected to occur a rate that is independent of the film thickness; i.e. show zeroth-order kinetics with respect to the surface concentration. </jats:p

Surface Science Investigations of Icy Mantle Growth on Interstellar Dust Grains in Cooling Environments

Experimental measurements on the thermal and nonthermal behavior of water and other simple molecules, including organic compounds such as methanol and benzene, on model interstellar dust grain and solid water ice surfaces using science techniques and methodologies are reviewed. A simple qualitative model of the early stages of mantle growth arising from a synthesis of the results of such investigations from our own laboratory and others is presented