Molecular Envelope Research Articles

The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA

The S-type asymptotic giant branch (AGB) star $\pi^{1}$ Gruis has a known companion at a separation of $\approx$400 AU. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive. The new ALMA $^{12}$CO and $^{13}$CO $J$=3-2 data, together with previously published $^{12}$CO $J$=2-1 data from the Submillimeter Array (SMA), and the $^{12}$CO $J$=5-4 and $J$=9-8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s$^{-1}$ along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s$^{-1}$; and a fast, bipolar outflow with velocity proportionally increasing from 14 km s$^{-1}$ at the base up to 100 km s$^{-1}$ at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7$\times$10$^{-7}$ M$_{\odot}$ yr$^{-1}$. The total mass and linear momentum of the fast outflow are estimated at 7.3$\times$10$^{-4}$ M$_{\odot}$ and 9.6$\times$10$^{37}$ g cm s$^{-1}$, respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a $^{12}$CO/$^{13}$CO abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed

Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core

The Juno spacecraft has measured Jupiter's low‐order, even gravitational moments, J2–J8, to an unprecedented precision, providing important constraints on the density profile and core mass of the planet. Here we report on a selection of interior models based on ab initio computer simulations of hydrogen‐helium mixtures. We demonstrate that a dilute core, expanded to a significant fraction of the planet's radius, is helpful in reconciling the calculated Jn with Juno's observations. Although model predictions are strongly affected by the chosen equation of state, the prediction of an enrichment of Z in the deep, metallic envelope over that in the shallow, molecular envelope holds. We estimate Jupiter's core to contain a 7–25 Earth mass of heavy elements. We discuss the current difficulties in reconciling measured Jn with the equations of state and with theory for formation and evolution of the planet.

Molecular envelope and atomic model of an anti-terminated glyQS T-box regulator in complex with tRNAGly.

A T-box regulator or riboswitch actively monitors the levels of charged/uncharged tRNA and participates in amino acid homeostasis by regulating genes involved in their utilization or biosynthesis. It has an aptamer domain for cognate tRNA recognition and an expression platform to sense the charge state and modulate gene expression. These two conserved domains are connected by a variable linker that harbors additional secondary structural elements, such as Stem III. The structural basis for specific tRNA binding is known, but the structural basis for charge sensing and the role of other elements remains elusive. To gain new structural insights on the T-box mechanism, a molecular envelope was calculated from small angle X-ray scattering data for the Bacillus subtilis glyQS T-box riboswitch in complex with an uncharged tRNAGly. A structural model of an anti-terminated glyQS T-box in complex with its cognate tRNAGly was derived based on the molecular envelope. It shows the location and relative orientation of various secondary structural elements. The model was validated by comparing the envelopes of the wild-type complex and two variants. The structural model suggests that in addition to a possible regulatory role, Stem III could aid in preferential stabilization of the T-box anti-terminated state allowing read-through of regulated genes.

The ALMA early science view of FUor/EXor objects – III. The slow and wide outflow of V883 Ori

We present Atacama Large Millimeter/ sub-millimeter Array (ALMA) observations of V883 Ori, an FU Ori object. We describe the molecular outflow and envelope of the system based on the $^{12}$CO and $^{13}$CO emissions, which together trace a bipolar molecular outflow. The C$^{18}$O emission traces the rotational motion of the circumstellar disk. From the $^{12}$CO blue-shifted emission, we estimate a wide opening angle of $\sim$ 150$^{^{\circ}}$ for the outflow cavities. Also, we find that the outflow is very slow (characteristic velocity of only 0.65 km~s$^{-1}$), which is unique for an FU Ori object. We calculate the kinematic properties of the outflow in the standard manner using the $^{12}$CO and $^{13}$CO emissions. In addition, we present a P Cygni profile observed in the high-resolution optical spectrum, evidence of a wind driven by the accretion and being the cause for the particular morphology of the outflows. We discuss the implications of our findings and the rise of these slow outflows during and/or after the formation of a rotationally supported disk.

The phase problem for one-dimensional crystals.

The phase problem for diffraction amplitudes measured from a one-dimensional crystal is examined. In the absence of any a priori information, the solution to this problem is shown to be unique up to a parameterized, low-dimensional set of solutions. Minimal additional a priori information is expected to render the solution unique. The effects of additional information such as positivity, molecular envelope and helical symmetry on uniqueness are characterized. The results are pertinent to structural studies of polymeric and rod-like biomolecular assemblies that form one-dimensional, rather than three-dimensional, crystals. This shows the potential for ab initio phasing of diffraction data from single such assemblies measured using new X-ray free-electron laser sources. Such an approach would circumvent the complicated inversion of cylindrically averaged diffraction that is necessary in traditional X-ray fibre diffraction analysis.

Investigating Ca2+-Dependent Regulation of Sodium Channels via Thermodynamic and Structural Analysis of Nav1.4 and Nav1.5 Carboxy Tail Interactions with Calmodulin

Voltage-gated sodium channels (Nav) are essential for cardiac and skeletal muscle function. Channelopathic mutations in cardiac sodium channel (Nav1.5) and skeletal muscle sodium channel (Nav1.4), particularly in their cytoplasmic carboxy tails (CTNav), give rise to numerous arrhythmias and myotonias. In Nav regulation, CTNav partners with the Ca2+-sensing protein calmodulin (CaM). We aim to understand how CaM regulates channel function. Ca2+-free CaM (apoCaM) is a regulatory modulator of Nav, and apoCaM bound to the CTNav increases the channel's open probability. CaM also participates in the Ca2+-dependent inactivation (CDI) of Nav1.4. In contrast, Nav1.5 does not show CDI and no role of Ca2+-CaM is known in Nav1.5. To understand the Ca2+-control of CaM regulation of these sodium channels we have collected binding data of CTNavs with CaM, in the presence and absence of Ca2+. Binding data of CTNavs with CaM mutants with Ca2+-binding knocked out in either of CaM's functional domains (lobes) have also been collected to understand the distinct roles of CaM's lobes. Collectively, these binding data have allowed us to predict CTNav and CaM populations as a function of Ca2+ concentration. To gain information on the structural changes induced by Ca2+ on the CTNav-CaM complexes, we conducted small angle scattering (SAXS) experiments on CTNav1.4-CaM and CTNav1.5-CaM in the presence and absence of Ca2+. The distance distribution function, P(r), shows changes in both complexes upon addition of Ca2+. Computational modeling of flexible forms of the complexes reveals conformations of CTNav-CaM, in the presence and absence of Ca2+, that are compatible with our SAXS data. The molecular envelope of the CTNav1.5-apoCaM matches well with the apoCaM-CTNav1.5 crystal structure previously determined by our lab.

Can the removal of molecular cloud envelopes by external feedback affect the efficiency of star formation?

We investigate how star formation efficiency can be significantly decreased by the removal of a molecular cloud's envelope by feedback from an external source. Feedback from star formation has difficulties halting the process in dense gas but can easily remove the less dense and warmer envelopes where star formation does not occur. However, the envelopes can play an important role keeping their host clouds bound by deepening the gravitational potential and providing a constraining pressure boundary. We use numerical simulations to show that removal of the cloud envelopes results in all cases in a fall in the star formation efficiency (SFE). At 1.38 free-fall times our 4 pc cloud simulation experienced a drop in the SFE from 16 to six percent, while our 5 pc cloud fell from 27 to 16 per cent. At the same time, our 3 pc cloud (the least bound) fell from an SFE of 5.67 per cent to zero when the envelope was lost. The star formation efficiency per free-fall time varied from zero to $\approx$ 0.25 according to $\alpha$, defined to be the ratio of the kinetic plus thermal to gravitational energy, and irrespective of the absolute star forming mass available. Furthermore the fall in SFE associated with the loss of the envelope is found to even occur at later times. We conclude that the SFE will always fall should a star forming cloud lose its envelope due to stellar feedback, with less bound clouds suffering the greatest decrease.

A laboratory model for deep-seated jets on the gas giants

A laboratory study of turbulent flows reproduces the properties of jets in the atmospheres of gas giants, providing a better understanding of how these jets could extend deep into the planetary atmosphere. The strong east–west jet flows on the gas giants, Jupiter1 and Saturn2, have persisted for hundreds of years. Yet, experimental studies cannot reach the planetary regime and similarly strong and quasi-steady jets have been reproduced in numerical models only under simplifying assumptions and limitations. Two models have been proposed: a shallow model where jets are confined to the weather layer and a deep model where the jets extend into the planetary molecular envelope. Here we show that turbulent laboratory flows naturally generate multiple, alternating jets in a rapidly rotating cylindrical container. The observed properties of gas giants’ jets are only now reproduced in a laboratory experiment emulating the deep model. Our findings demonstrate that long-lived jets can persist at high latitudes even under conditions including viscous dissipation and friction and bear relevance to the shallow versus deep models debate in the context of the ongoing Juno mission3.

The ALMA early science view of FUor/EXor objects – II. The very wide outflow driven by HBC 494

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) Cycle-2 observations of the HBC 494 molecular outflow and envelope. HBC 494 is an FU Ori-like object embedded in the Orion A cloud and is associated with the reflection nebulae Re50 and Re50N. We use $^{12}$CO, $^{13}$CO and C$^{18}$O spectral line data to independently describe the outflow and envelope structures associated with HBC 494. The moment-1 map of the $^{12}$CO emission shows the widest outflow cavities in a Class I object known to date (opening angle $\sim$ 150$^{^{\circ}}$). The morphology of the wide outflow is likely to be due to the interaction between winds originating in the inner disc and the surrounding envelope. The low-velocity blue- and red-shifted $^{13}$CO and C$^{18}$O emission trace the rotation and infall motion of the circumstellar envelope. Using molecular line data and adopting standard methods for correcting optical depth effects, we estimate its kinematic properties, including an outflow mass on the order of 10$^{-1}$ M$_{\odot}$. Considering the large estimated outflow mass for HBC 494, our results support recent theoretical work suggesting that wind-driven processes might dominate the evolution of protoplanetary discs via energetic outflows.

The millimeter IRAM-30 m line survey toward IK Tau.

We aim to investigate the physical and chemical properties of the molecular envelope of the oxygen-rich AGB star IK Tau. We carried out a millimeter wavelength line survey between ~79 and 356 GHz with the IRAM-30 m telescope. We analysed the molecular lines detected in IK Tau using the population diagram technique to derive rotational temperatures and column densities. We conducted a radiative transfer analysis of the SO2 lines, which also helped us to verify the validity of the approximated method of the population diagram for the rest of the molecules. For the first time in this source we detected rotational lines in the ground vibrational state of HCO+, NS, NO, and H2CO, as well as several isotopologues of molecules previously identified, namely, C18O, Si17O, Si18O, 29SiS, 30SiS, Si34S, H13CN, 13CS, C34S, H234S, 34SO, and 34SO2. We also detected several rotational lines in vibrationally excited states of SiS and SiO isotopologues, as well as rotational lines of H2O in the vibrationally excited state ν2=2. We have also increased the number of rotational lines detected of molecules that were previously identified toward IK Tau, including vibrationally excited states, enabling a detailed study of the molecular abundances and excitation temperatures. In particular, we highlight the detection of NS and H2CO with fractional abundances of f(NS)~10-8 and f(H2CO)~[10-7-10-8 ]. Most of the molecules display rotational temperatures between 15 and 40 K. NaCl and SiS isotopologues display rotational temperatures higher than the average (~65 K). In the case of SO2 a warm component with Trot~290 K is also detected. With a total of ~350 lines detected of 34 different molecular species (including different isotopologues), IK Tau displays a rich chemistry for an oxygen-rich circumstellar envelope. The detection of carbon bearing molecules like H2CO, as well as the discrepancies found between our derived abundances and the predictions from chemical models for some molecules, highlight the need for a revision of standard chemical models. We were able to identify at least two different emission components in terms of rotational temperatures. The warm component, which is mainly traced out by SO2, is probably arising from the inner regions of the envelope (at ≲8R∗) where SO2 has a fractional abundance of f(SO2)~10-6. This result should be considered for future investigation of the main formation channels of this, and other, parent species in the inner winds of O-rich AGB stars, which at present are not well reproduced by current chemistry models.

EVIDENCE FROM SOFIA IMAGING OF POLYCYCLIC AROMATIC HYDROCARBON FORMATION ALONG A RECENT OUTFLOW IN NGC 7027

ABSTRACT We report spatially resolved (FWHM ∼ 3.″8–4.″6) mid-IR imaging observations of the planetary nebula (PN) NGC 7027 taken with the 2.5 m telescope on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). Images of NGC 7027 were acquired at 6.3, 6.6, 11.1, 19.7, 24.2, 33.6, and 37.1 using the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST). The observations reveal emission from polycyclic aromatic hydrocarbons (PAHs) and warm dust ( K) from the illuminated inner edge of the molecular envelope surrounding the ionized gas and central star. The DustEM code was used to fit the spectral energy distribution of fluxes obtained by FORCAST and the archival infrared spectrum of NGC 7027 acquired by the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO). Best-fit dust models provide a total dust mass of , where carbonaceous large (a = 1.5 μm) and very small ( ) grains, and PAHs ( ) compose 96.5, 2.2, and 1.3% of the dust by mass, respectively. The 37 μm optical depth map shows minima in the dust column density at regions in the envelope that are coincident with a previously identified collimated outflow from the central star. The optical depth minima are also spatially coincident with enhancements in the 6.2 μm PAH feature, which is derived from the 6.3 and 6.6 μm maps. We interpret the spatial anti-correlation of the dust optical depth and PAH 6.2 μm feature strength and their alignment with the outflow from the central star as evidence of dust processing and rapid PAH formation via grain–grain collisions in the post-shock environment of the dense ( ) photo-dissociation region and molecular envelope.

Functional and structural characterization of a novel putative cysteine protease cell wall-modifying multi-domain enzyme selected from a microbial metagenome.

A current metagenomics focus is to interpret and transform collected genomic data into biological information. By combining structural, functional and genomic data we have assessed a novel bacterial protein selected from a carbohydrate-related activity screen in a microbial metagenomic library from Capra hircus (domestic goat) gut. This uncharacterized protein was predicted as a bacterial cell wall-modifying enzyme (CWME) and shown to contain four domains: an N-terminal, a cysteine protease, a peptidoglycan-binding and an SH3 bacterial domain. We successfully cloned, expressed and purified this putative cysteine protease (PCP), which presented autoproteolytic activity and inhibition by protease inhibitors. We observed cell wall hydrolytic activity and ampicillin binding capacity, a characteristic of most bacterial CWME. Fluorimetric binding analysis yielded a Kb of 1.8 × 105 M−1 for ampicillin. Small-angle X-ray scattering (SAXS) showed a maximum particle dimension of 95 Å with a real-space Rg of 28.35 Å. The elongated molecular envelope corroborates the dynamic light scattering (DLS) estimated size. Furthermore, homology modeling and SAXS allowed the construction of a model that explains the stability and secondary structural changes observed by circular dichroism (CD). In short, we report a novel cell wall-modifying autoproteolytic PCP with insight into its biochemical, biophysical and structural features.

Rice Cellulose SynthaseA8 Plant-Conserved Region Is a Coiled-Coil at the Catalytic Core Entrance.

The crystallographic structure of a rice (Oryza sativa) cellulose synthase, OsCesA8, plant-conserved region (P-CR), one of two unique domains in the catalytic domain of plant CesAs, was solved to 2.4 Å resolution. Two antiparallel α-helices form a coiled-coil domain linked by a large extended connector loop containing a conserved trio of aromatic residues. The P-CR structure was fit into a molecular envelope for the P-CR domain derived from small-angle X-ray scattering data. The P-CR structure and molecular envelope, combined with a homology-based chain trace of the CesA8 catalytic core, were modeled into a previously determined CesA8 small-angle X-ray scattering molecular envelope to produce a detailed topological model of the CesA8 catalytic domain. The predicted position for the P-CR domain from the molecular docking models places the P-CR connector loop into a hydrophobic pocket of the catalytic core, with the coiled-coil aligned near the entrance of the substrate UDP-glucose into the active site. In this configuration, the P-CR coiled-coil alone is unlikely to regulate substrate access to the active site, but it could interact with other domains of CesA, accessory proteins, or other CesA catalytic domains to control substrate delivery.

BAYESIAN EVOLUTION MODELS FOR JUPITER WITH HELIUM RAIN AND DOUBLE-DIFFUSIVE CONVECTION

ABSTRACT Hydrogen and helium demix when sufficiently cool, and this bears on the evolution of all giant planets at large separations at or below roughly a Jupiter mass. We model the thermal evolution of Jupiter, including its evolving helium distribution following results of ab initio simulations for helium immiscibility in metallic hydrogen. After 4 Gyr of homogeneous evolution, differentiation establishes a thin helium gradient below 1 Mbar that dynamically stabilizes the fluid to convection. The region undergoes overstable double-diffusive convection (ODDC), whose weak heat transport maintains a superadiabatic temperature gradient. With a generic parameterization for the ODDC efficiency, the models can reconcile Jupiter’s intrinsix flux, atmospheric helium content, and radius at the age of the solar system if the Lorenzen et al. H–He phase diagram is translated to lower temperatures. We cast the evolutionary models in an MCMC framework to explore tens of thousands of evolutionary sequences, retrieving probability distributions for the total heavy-element mass, the superadiabaticity of the temperature gradient due to ODDC, and the phase diagram perturbation. The adopted SCvH-I equation of state (EOS) favors inefficient ODDC such that a thermal boundary layer is formed, allowing the molecular envelope to cool rapidly while the deeper interior actually heats up over time. If the overall cooling time is modulated with an additional free parameter to imitate the effect of a colder or warmer EOS, the models favor those that are colder than SCvH-I. In this case the superadiabaticity is modest and warming and cooling deep interiors are equally likely.

Molecular Architecture of Contactin-associated Protein-like 2 (CNTNAP2) and Its Interaction with Contactin 2 (CNTN2)

Contactin-associated protein-like 2 (CNTNAP2) is a large multidomain neuronal adhesion molecule implicated in a number of neurological disorders, including epilepsy, schizophrenia, autism spectrum disorder, intellectual disability, and language delay. We reveal here by electron microscopy that the architecture of CNTNAP2 is composed of a large, medium, and small lobe that flex with respect to each other. Using epitope labeling and fragments, we assign the F58C, L1, and L2 domains to the large lobe, the FBG and L3 domains to the middle lobe, and the L4 domain to the small lobe of the CNTNAP2 molecular envelope. Our data reveal that CNTNAP2 has a very different architecture compared with neurexin 1α, a fellow member of the neurexin superfamily and a prototype, suggesting that CNTNAP2 uses a different strategy to integrate into the synaptic protein network. We show that the ectodomains of CNTNAP2 and contactin 2 (CNTN2) bind directly and specifically, with low nanomolar affinity. We show further that mutations in CNTNAP2 implicated in autism spectrum disorder are not segregated but are distributed over the whole ectodomain. The molecular shape and dimensions of CNTNAP2 place constraints on how CNTNAP2 integrates in the cleft of axo-glial and neuronal contact sites and how it functions as an organizing and adhesive molecule.

The Biophysical Characterisation and SAXS Analysis of Human NLRP1 Uncover a New Level of Complexity of NLR Proteins.

NOD-like receptors represent an important class of germline-encoded pattern recognition receptors that play key roles in the regulation of inflammatory signalling pathways. They function as danger sensors and initiate inflammatory responses and the production of cytokines. Since NLR malfunction results in chronic inflammation and auto-immune diseases, there is a great interest in understanding how they work on a molecular level. To date, a lot of insight into the biological functions of NLRs is available but biophysical and structural studies have been hampered by the difficulty to produce soluble and stable recombinant NLR proteins. NLRP1 is an inflammasome forming NLR that is believed to be activated by binding to MDP and induces activation of caspase 1. Here, we report the identification of a soluble fragment of NLRP1 that contains the NACHT oligomerization domain and the putative MDP-sensing LRR domain. We describe the biophysical and biochemical characterization of this construct and a SEC-SAXS analysis that allowed the calculation of a low resolution molecular envelope. Our data indicate that the protein is constitutively bound to ATP with a negligible ability to hydrolyse the triphosphate nucleotide and that it adopts a monomeric extended conformation that is reminiscent of the structure adopted by NLRC4 in the inflammasome complex. Furthermore, we show that the presence of MDP is not sufficient to promote self-oligomerization of the NACHT-LRR fragment suggesting that MDP may either bind to regions outside the NACHT-LRR module or that it may not be the natural ligand of NLRP1. Taken together, our data suggest that the NLRP1 mechanism of action differs from that recently reported for other NLRs.

Biochemical Characterization and Low-Resolution SAXS Molecular Envelope of GH1 β-Glycosidase from Saccharophagus degradans.

The marine bacteria Saccharophagus degradans (also known as Microbulbifer degradans), are rod-shaped and gram-negative motile γ-proteobacteria, capable of both degrading a variety of complex polysaccharides and fermenting monosaccharides into ethanol. In order to obtain insights into structure-function relationships of the enzymes, involved in these biochemical processes, we characterized a S. degradans β-glycosidase from glycoside hydrolase family 1 (SdBgl1B). SdBgl1B has the optimum pH of 6.0 and a melting temperature T m of approximately 50°C. The enzyme has high specificity toward short D-glucose saccharides with β-linkages with the following preferences β-1,3>β-1,4≫β-1,6. The enzyme kinetic parameters, obtained using artificial substrates p-β-NPGlu and p-β-NPFuc and also the disaccharides cellobiose, gentiobiose and laminaribiose, revealed SdBgl1B preference for p-β-NPGlu and laminaribiose, which indicates its affinity for glucose and also preference for β-1,3 linkages. To better understand structural basis of the enzyme activity its 3D model was built and analysed. The 3D model fits well into the experimentally retrieved low-resolution SAXS-based envelope of the enzyme, confirming monomeric state of SdBgl1B in solution.

A FULLY SELF-CONSISTENT MULTI-LAYERED MODEL OF JUPITER

ABSTRACT We construct a three-dimensional, fully self-consistent, multi-layered, non-spheroidal model of Jupiter consisting of an inner core, a metallic electrically conducting dynamo region, and an outer molecular electrically insulating envelope. We assume that the Jovian zonal winds are on cylinders parallel to the rotation axis but, due to the effect of magnetic braking, are confined within the outer molecular envelope. We also assume that the location of the molecular-metallic interface is characterized by its equatorial radius , where R e is the equatorial radius of Jupiter at the 1 bar pressure level and H is treated as a parameter of the model. We solve the relevant mathematical problem via a perturbation approach. The leading-order problem determines the density, size, and shape of the inner core, the irregular shape of the 1 bar pressure level, and the internal structure of Jupiter that accounts for the full effect of rotational distortion, but without the influence of the zonal winds; the next-order problem determines the variation of the gravitational field solely caused by the effect of the zonal winds on the rotationally distorted non-spheroidal Jupiter. The leading-order solution produces the known mass, the known equatorial and polar radii, and the known zonal gravitational coefficient J 2 of Jupiter within their error bars; it also yields the coefficients J 4 and J 6 within about 5% accuracy, the core equatorial radius and the core density corresponding to 3.73 Earth masses; the next-order solution yields the wind-induced variation of the zonal gravitational coefficients of Jupiter.

L1599B: CLOUD ENVELOPE AND C+ EMISSION IN A REGION OF MODERATELY ENHANCED RADIATION FIELD

ABSTRACT We study the effects of an asymmetric radiation field on the properties of a molecular cloud envelope. We employ observations of carbon monoxide (12CO and 13CO), atomic carbon, ionized carbon, and atomic hydrogen to analyze the chemical and physical properties of the core and envelope of L1599B, a molecular cloud forming a portion of the ring at ≃27 pc from the star Λ Ori. The O8 star provides an asymmetric radiation field that produces a moderate enhancement of the external radiation field. Observations of the [C ii] fine structure line with the GREAT instrument on SOFIA indicate a significant enhanced emission on the side of the cloud facing the star, while the [C i], 12CO and 13CO J = 1–0 and 2–1, and 12CO J = 3–2 data from the Purple Mountain Observatory and APEX telescopes suggest a relatively typical cloud interior. The atomic, ionic, and molecular line centroid velocities track each other very closely, and indicate that the cloud may be undergoing differential radial motion. The H i data from the Arecibo GALFA survey and the SOFIA/GREAT [C ii] data do not suggest any systematic motion of the halo gas, relative to the dense central portion of the cloud traced by 12CO and 13CO.

Abstract 27: Structure of Lipid-free and Lipid-bound Apolipoprotein A-I Determined by Stable Isotope-assisted Cross-linking and Small Angle X-ray Scattering

Apolipoprotein (apo)A-I, the major protein constituent of high density lipoproteins (HDL), acts as a conformationally dynamic scaffold on the surface of the particle. Evidence suggests apoA-I conformation dictates HDL functionality and capacity to interface with other HDL proteins; however, the details of apoA-I structure and HDL formation are not completely understood. X-ray crystal structures of truncated forms of apoA-I and its cousin, apoA-IV, show reciprocal domain swapping structures in the lipid-free state. Recently, we showed the truncated mutant of apoA-I is also highly dynamic in solution with a flexible N-terminus. The purpose of this study was to a) derive a structural model of wild-type (WT) apoA-I using the new in-solution truncation model as a template and b) determine the relationship between monomeric, and oligomeric forms of lipid-free WT apoA-I. Using a novel isotope labeling strategy and chemical cross-linking, we defined spatial relationships within and between different molecules of WT apoA-I. The cross-linking patterns revealed a high degree of structural similarity between WT apoA-I oligomers. We found that monomeric and dimeric WT apoA-I structures were similar, but not identical, to those predicted by the crystal and solution structures of truncated apoA-I. Using cross-linking data, we derived two interconverting structures each for monomeric WT apoA-I, both containing dynamic N- and C-terminal regions. Similar structures were generated for WT dimeric and trimeric species. Small angle X-ray scattering was used to derive molecular envelopes of the molecules and refine the arrangements to propose the most detailed molecular models of lipid-free WT apoA-I to date. The models were used in conjunction with the reported double-belt and trefoil structures reconstituted HDL to reveal evidence of a parsimonious relationship between lipid-free and lipid-bound WT apoA-I. We propose a structural framework of WT apoA-I self-association and lipidation, centric to the C-terminal region of the molecule, that forms a basis for understanding HDL particle formation at the cell surface.