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The RNA world hypothesis proposes that nucleic acids were once responsible for both information storage and chemical catalysis, before the advent of coded protein synthesis. However, it is difficult to imagine how nucleic acid polymers first appeared, as the abiotic chemical formation of long nucleic acid polymers from mononucleotides or short oligonucleotides remains elusive, and barriers to achieving this goal are substantial. One specific obstacle to abiotic nucleic acid polymerization is strand cyclization. Chemically activated short oligonucleotides cyclize efficiently, which severely impairs polymer growth. We show that intercalation, which stabilizes and rigidities nucleic acid duplexes, almost totally eliminates strand cyclization, allowing for chemical ligation of tetranucleotides into duplex polymers of up to 100 base pairs in length. In contrast, when these reactions are performed in the absence of intercalators, almost exclusively cyclic tetra- and octanucleotides are produced. Intercalator-free polymerization is not observed, even at tetranucleotide concentrations >10;000-fold greater than those at which intercalators enable polymerization. We also demonstrate that intercalation-mediated polymerization is most favored if the size of the intercalator matches that of the base pair; intercalators that bind to Watson--Crick base pairs promote the polymerization of oligonucleotides that form these base pairs. Additionally, we demonstrate that intercalation-mediated polymerization is possible with an alternative, non-Watson--Crick-paired duplex that selectively binds a complementary intercalator. These results support the hypothesis that intercalators (acting as 'molecular midwives') could have facilitated the polymerization of the first nucleic acids and possibly helped select the first base pairs, even if only trace amounts of suitable oligomers were available. base pair selection | origin of life | RNA world | polymerization | molecular evolution www.pnas.org/cgi/doi/10.1073/pnas.0914172107

The studies have shown that the nonintercalating bimetallic complex [[[(bpy).sub.2]Ru(tpphz)Ru[(bpy).sub.2]].sup.4+], where bpy is 2,2E-bipyridine, tpphz is tetrapyrido[[3,2-a:2E,3E-c:3EE,2EE-h:2EEE,3EEE-j]phenazine, has behaved like a DNA light-switch complex. This is the first example of a nonintercalating light-switch metal complex, which has shown that light-switch behavior cannot be used exclusively as confirmation of intercalation.

The thermoelectric properties of quaternary type-I clathrate compounds, [Ba.sub.8][Ga.sub.16-x][In.sub.x][Ge.sub.30] (x=0-9) are examined as a function of In content and temperature. The solid solubility of indium in [Ba.sub.8][Ga.sub.16][Ge.sub.30] is determined between x=6 and 9 when expressed in the form of [Ba.sub.8][Ga.sub.16-x][In.sub.x][Ge.sub.30].

The primary mechanism by which the kidneys mediate net acid excretion is through ammonia metabolism. In the current study, we examined whether chronic metabolic acidosis, which increases ammonia metabolism, alters the cell-specific and/or the subcellular expression of the ammonia transporter family member, Rhcg, in the outer medullary collecting duct in the inner stripe (OMCDi). Chronic metabolic acidosis was induced in normal SD rats by HCl ingestion for 7 days; controls were pair-fed. The subcellular distribution of Rhcg was determined using immunogold electron microscopy and morphometric analyses. In intercalated cells, acidosis increased total Rhcg, apical plasma membrane Rhcg, and the proportion of total cellular Rhcg in the apical plasma membrane. Intracellular Rhcg decreased significantly, and basolateral Rhcg was unchanged. Because apical plasma membrane length increased in parallel with apical Rhcg immunolabel, apical plasma membrane Rhcg density was unchanged. In principal cells, acidosis increased total Rhcg, apical plasma membrane Rhcg, and the proportion of total cellular Rhcg in the apical plasma membrane while decreasing the intracellular proportion. In contrast to the intercalated cell, chronic metabolic acidosis did not significantly alter apical boundary length; accordingly, apical plasma membrane Rhcg density increased. In addition, basolateral Rhcg immunolabel increased in response to chronic metabolic acidosis. These results indicate that in the rat OMCDi 1) chronic metabolic acidosis increases apical plasma membrane Rhcg in both the intercalated cell and principal cell where it may contribute to enhanced apical ammonia secretion; 2) increased apical plasma membrane Rhcg results from both increased total protein and changes in the subcellular distribution of Rhcg; 3) the mechanism of Rhcg subcellular redistribution differs in intercalated and principal cells; and 4) Rhcg may contribute to regulated basolateral ammonia transport in the principal cell. intercalated cell; principal cell; immunogold; morphometry

Using the patch-clamp technique, we investigated [Cl.sup.-] channels on the basolateral membrane of the connecting tubule (CNT) and cortical collecting duct (CCD). We found a ~10-pS channel in CNT cell-attached patches. Substitution of sodium gluconate for NaCl in the pipette shifted the reversal potential by +25 mV, whereas N-methyl-Dg-luconate chloride had no effect, indicating anion selectivity. On inside-out patches, we determined a selectivity sequence of [Cl.sup.-] > [Br.sup-] ~ N[Osup.-.sub.3] > [F.sup.-], which is compatible with that of CIC-K2, a [Cl.sup.-] channel in the distal nephron. In addition, the number of open channels (N[P.sub.o]) measured in cell-attached patches was significantly increased when [Ca.sup.2+] concentration or pH in the pipette was increased, which is another characteristic of CIC-K. These findings suggest that the basis for this channel is CIC-K2. A similar [Cl.sup.-] channel was found in CCD patches. Because CNT and CCD are heterogeneous tissues, we studied the cellular distribution of the [Cl.sup.-] channel using recording conditions (KCl-rich solution in the pipette) that allowed us to detect simultaneously [Cl.sup.-] channels and inwardly rectifying [K.sup.+] channels. We detected [Cl.sup.-] channels alone in 45% and 42% and [K.sup.+] channels alone in 51% and 58% of CNT and CCD patches, respectively. [Cl.sup.-] and [K.sup.+] channels were recorded simultaneously from two patches (4% of patches) in the CNT and from none of the patches in the CCD. This indicates that [Cl.sup.-] and [K.sup.+] channels are located in different cell types, which we suggest may be the intercalated cells and principal cells, respectively. ClC-K; renal tubule; intercalated cell

A core transect across the southwestern Greenland Sea reveals coeval events of extremely negative planktic and benthic [[delta].sup.13]C excursions between 40 and 87 ka. The most pronounced event, event 1, began at peak Dansgaard-Oeschger stadial 22 (85 ka) with a duration of 18 k.y. During this episode, incursions of Atlantic Intermediate Water caused a bottom-water warming of up to 8 [degrees]C. The amplitude, timing, and geographic pattern of the [[delta].sup.13]C events suggest that this bottom-water warming triggered clathrate instability along the East Greenland slope and a methane-induced depletion of [[delta].sup.13][C.sub.DIC] (DIC--dissolved inorganic carbon). Since [[delta].sup.13]C event 1 matches a major peak in atmospheric C[H.sub.4] concentration, this clathrate destabilization may have contributed to the rise in atmospheric C[H.sub.4] and thus to climate warming over marine isotope stage 5.1. Keywords: Clathrates, global warming, Denmark Strait Overflow, Heinrich events.

Abstract: Layered double hydroxides of aluminum and zinc (Zn/Al-LDH's) were synthesized directly by hydrolysis at room temperature as reported elsewhere. After characterization, the material was reacted with m-aminobenzoat anion (m-N[H.sub.2][C.sub.6][H.sub.4]CO[O.sup.-]) [...]

Aragonitic clathrites are methane-derived precipitates that are found at sites of massive near-seafloor gas hydrate (clathrate) accumulations at the summit of southern Hydrate Ridge, Cascadia margin. These platy carbonate precipitates form inside or in proximity to gas hydrate, which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation. The clathrites record the preferential incorporation of [sup.18]O into the hydrate structure and hence the enrichment of [sup.16]O in the surrounding brine. We measured [[delta].sup.18]O values as high as 2.27[per thousand] relative to Peedee belemnite that correspond to a fluid composition of -1.18[per thousand] relative to standard mean ocean water. The same trend can be observed in Ca isotopes. Ongoing clathrite precipitation causes enrichment of the [44.sup.Ca] in the fluid and hence in the carbonates. Carbon isotopes confirm a methane source for the carbonates. Our triple stable isotope approach that uses the three main components of carbonates (Ca, C, O) provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites. This approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation, thus recording the evolution of the geochemical environment of gas hydrate systems. Keywords: clathrite, clathrate, [[delta].sup.13]C,[[delta].sup.18]O, [[delta].sup.44/40]Ca, gas hydrate brine, aragonite, Ca isotope fractionation.

Using recorded data of pressure and temperature, we developed a nondestructive method to estimate the precipitation rate of carbon dioxide (C[O.sub.2]) hydrate by considering the C[O.sub.2] solution density equation in association with the mass conservation equation. We applied this method to the investigation of the dynamic process of clathrate hydrate precipitation from C[O.sub.2] solution using a high-pressure dissolution system consisting of a high-pressure vessel and optical detecting instruments. The role of stirring was examined. The temperatures studied in the experiments were from 275 to 288 K over a pressure range of 4-8 MPa. Experimental results showed that our method can quantitatively monitor this dynamic process. The volume ratio of precipitated hydrate to that of the pressure-vessel approached 0.09 when a steady state was reached, which took about 100 seconds; more than 60% of the total clathrate hydrate precipitated within 10 seconds at the cooling rates of this experiment. For precipitation, stirring enhanced the cooling and led to a large hydrate precipitation rate from 0.004 to 0.008 in volume ratio per second. For nucleation, however, the residual structure of the solution decreased the amount of hysteresis in the formation of hydrate nuclei during supercooling, of which induction times were reduced by 4-9 minutes.

Raman spectroscopy and differential scanning calorimetry have been used to determine the kinetics and mode of action of a range of inhibitors on tetrahydrofuran (THF) hydrate formation. The results from these kinetic measurements were compared to those obtained from gas hydrate studies. The thermodynamic inhibitory nature of methanol was reflected in the findings as expected. However, some nucleation retardation was also observed at all concentrations studied, with higher concentrations of methanol resulting in ice formation. Addition of poly-N-vinylpyrrolidone (PVP) to THF/water mixtures caused nucleation times to increase and crystal growth to decelerate. No significant synergic effect was observed for THF hydrate inhibition using a combination of PVP and one of two glycol ethers, which were found to dramatically increase nucleation periods in natural gas hydrate formation. Only a slight synergic tendency was observed for the combination from the DSC results, while the Raman data gave no indication of a synergic effect for the combination. The results suggest that the nature of the guest species may be an important factor in the mechanism of hydrate inhibition.

Scanning electron microscopy (SEM) was used to investigate grain texture and pore structure development within various compositions of pure sI and sII gas hydrates synthesized in the laboratory, as well as in natural samples retrieved from marine (Gulf of Mexico) and permafrost (NW Canada) settings. Several samples of methane hydrate were also quenched after various extents of partial reaction for assessment of mid-synthesis textural progression. All laboratory-synthesized hydrates were grown under relatively high-temperature and high-pressure conditions from rounded ice grains with geometrically simple pore shapes, yet all resulting samples displayed extensive recrystallization with complex pore geometry. Growth fronts of mesoporous methane hydrate advancing into dense ice reactant were prevalent in those samples quenched after limited reaction below and at the ice point. As temperatures transgress the ice point, grain surfaces continue to develop a discrete 'rind' of hydrate, typically 5 to 30 [micro]m thick. The cores then commonly melt, with rind microfracturing allowing migration of the melt to adjacent grain boundaries where it also forms hydrate. As the reaction continues under progressively warmer conditions, the hydrate product anneals to form dense and relatively pore-free regions of hydrate grains, in which grain size is typically several tens of micrometers. The prevalence of hollow, spheroidal shells of hydrate, coupled with extensive redistribution of reactant and product phases throughout reaction, implies that a diffusion-controlled shrinking-core model is an inappropriate description of sustained hydrate growth from melting ice. Completion of reaction at peak synthesis conditions then produces exceptional faceting and euhedral crystal growth along exposed pore walls. Further recrystallization or regrowth can then accompany even short-term exposure of synthetic hydrates to natural ocean-floor conditions, such that the final textures may closely mimic those observed in natural samples of marine origin. Of particular note, both the mesoporous and highly faceted textures seen at different stages during synthetic hydrate growth were notably absent from all examined hydrates recovered from a natural marine-environment setting.

Neutron scattering has been used to study the dynamics of trimethylene oxide (TMO) in a structure II clathrate hydrate. Two regimes of guest dynamics have been identified in the range 10 K to 100 K. Below 50 K, the hydrogen atoms on TMO execute jumps between nonequivalent sites with a jump distance of 2.1 [Angstrom], consistent with a model of rotations of 90 [degrees] about the [C.sub.2v], molecular axis between sites with unequal occupancy probabilities, and corresponding to an energy difference between sites of ~7 meV. Above 50 K, a second dynamical regime appears in which rotational motions of H occur about both the [C.sub.2v] axis and a second axis perpendicular to the plane of the molecule. An increase in the activation energy barrier to the motion that appears to accompany the onset of multi-axis motion could be a result of the additional high-energy rotations taking place within more restricted cross-sections of the cage.

The stability of the recently characterized type II hydrogen clathrate [Mao, W. L., Mao, H.-K., Goncharov, A. F., Struzhkin, V. V., Guo, Q., et al. (2002) Science 297, 2247-2249] with respect to hydrogen occupancy is examined with a statistical mechanical model in conjunction with first-principles quantum chemistry calculations. It is found that the stability of the clathrate is mainly caused by dispersive interactions between [H.sub.2] molecules and the water forming the cage walls. Theoretical analysis shows that both individual hydrogen molecules and n[H.sub.2] guest clusters undergo essentially free rotations inside the clathrate cages. Calculations at the experimental conditions -2,000 bar (1 bar = 100 kPa) and 250 K confirm multiple occupancy of the clathrate cages with average occupations of 2.00 and 3.96 [H.sub.2] molecules per D-[5.sup.12] (small) and H-[5.sup.12][6.sup.4] (large) cage, respectively. The [H.sub.2]-[H.sub.2]O interactions also are responsible for the experimentally observed softening of the H--H stretching modes. The clathrate is found to be thermodynamically stable at 25 bar and 150 K.

Raman spectra of the internal vibrations of tetrahydrofuran molecules in two gas hydrate phases formed in the tetrahydrofuran-water system, as well as the spectra of solid and liquid tetrahydrofuran, under high-pressure conditions is studied. A conclusion is drawn on the basis of spectral data that the high-pressure hydrate is, most probably, of clathrate nature.

Trimethylene oxide (TMO) forms structure I or structure II hydrates depending on stoichiometry and affords the opportunity to compare and contrast the temperature dependence of the crystal structure parameters for the two major hydrate structure types occluding a common guest module. Changes with respect to temperature were observed in structure I and not in structure II.

The (super 23)Na and (super 29)Si NMR spectra of the sodium-silicon clathrate materials Na(sub 8)Si(sub 46) and Na(sub x)Si(sub 136) are studied in detail. NMR, XPS and XANES results showed an interaction between Na atom and the Si framework of the clathrates.