Geophysical imaging of ophiolite structure in the United Arab Emirates.

The Oman-United Arab Emirates ophiolite has been used extensively to document the geological processes that form oceanic crust. The geometry of the ophiolite, its extension into the Gulf of Oman, and the nature of the crust that underlies it are, however, unknown. Here, we show the ophiolite forms a high velocity, high density, >15 km thick east-dipping body that during emplacement flexed down a previously rifted continental margin thereby contributing to subsidence of flanking sedimentary basins. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40-45°, east-dipping, normal fault. The fault is interpreted as the southwestern margin of an incipient suture zone that separates the Arabian plate from in situ Gulf of Oman oceanic crust and mantle presently subducting northwards beneath the Eurasian plate along the Makran trench.

Post-eruptive flooding of Santorini caldera and implications for tsunami generation.

Caldera-forming eruptions of island volcanoes generate tsunamis by the interaction of different eruptive phenomena with the sea. Such tsunamis are a major hazard, but forward models of their impacts are limited by poor understanding of source mechanisms. The caldera-forming eruption of Santorini in the Late Bronze Age is known to have been tsunamigenic, and caldera collapse has been proposed as a mechanism. Here, we present bathymetric and seismic evidence showing that the caldera was not open to the sea during the main phase of the eruption, but was flooded once the eruption had finished. Inflow of water and associated landsliding cut a deep, 2.0-2.5 km3, submarine channel, thus filling the caldera in less than a couple of days. If, as at most such volcanoes, caldera collapse occurred syn-eruptively, then it cannot have generated tsunamis. Entry of pyroclastic flows into the sea, combined with slumping of submarine pyroclastic accumulations, were the main mechanisms of tsunami production.

Earthquake rupture stalled by a subducting fracture zone.

We showed that the rupture produced by the great Peru earthquake (moment magnitude 8.4) on 23 June 2001 propagated for approximately 70 kilometers before encountering a 6000-square-kilometer area of fault that acted as a barrier. The rupture continued around this barrier, which remained unbroken for approximately 30 seconds and then began to break when the main rupture front was approximately 200 kilometers from the epicenter. The barrier had relatively low rupture speed, slip, and aftershock density as compared to its surroundings, and the time of the main energy release in the earthquake coincided with the barrier's rupture. We associate this barrier with a fracture zone feature on the subducting oceanic plate.

The long-term strength of Europe and its implications for plate-forming processes.

Field-based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons. This partitioning of deformation suggests that the cratons are stronger than surrounding younger Phanerozoic provinces. However, although Archaean and Phanerozoic lithosphere differ in their thickness and composition, their relative strength is a matter of much debate. One proxy of strength is the effective elastic thickness of the lithosphere, Te. Unfortunately, spatial variations in Te are not well understood, as different methods yield different results. The differences are most apparent in cratons, where the 'Bouguer coherence' method yields large Te values (> 60 km) whereas the 'free-air admittance' method yields low values (< 25 km). Here we present estimates of the variability of Te in Europe using both methods. We show that when they are consistently formulated, both methods yield comparable Te values that correlate with geology, and that the strength of old lithosphere (> or = 1.5 Gyr old) is much larger (mean Te > 60 km) than that of younger lithosphere (mean Te < 30 km). We propose that this strength difference reflects changes in lithospheric plate structure (thickness, geothermal gradient and composition) that result from mantle temperature and volatile content decrease through Earth's history.

The theory and application of transition state pK(a) values: the reaction of papain with a series of trimethylene disulphide reactivity probes.

For many years methods to describe the structure of the transition state for a reaction have been sought. Most commonly these structures have been inferred from kinetic isotope effects. We report here for the first time the application of transition state pK(a) values to describe the relationship between molecular recognition and the transition state for the catalytic mechanism of papain. The background to the theory is presented and applied to the reactions of papain with a series of trimethylene disulphide reactivity probes. The common feature of these reactions is a loss in reactivity on ionization of the imidazolium cation for those probes containing molecular recognition features and an increase in reactivity on ionization of the electrostatic switch residue. The use of transition state pK(a) values enhances this information by providing details regarding the protonic distribution within the transition state. This has led to the reconsideration of the effect of the electrostatic switch ionization and the role of the hydrogen bond formed between the catalytic-site imidazolium cation and the leaving group of the reaction in the catalytic mechanism of papain.

A classical enzyme active center motif lacks catalytic competence until modulated electrostatically.

The cysteine proteinase superfamily is a source of natural structural variants of value in the investigation of mechanism. It has long been considered axiomatic that catalytic competence of these enzymes mirrors the generation of the ubiquitous catalytic site imidazolium-thiolate ion pair. We here report definitive evidence from kinetic studies supported by electrostatic potential calculations, however, that at least for some of these enzymes the ion pair state which provides the nucleophilic and acid-base chemistry is essentially fully developed at low pH where the enzymes are inactive. Catalytic competence requires an additional protonic dissociation with a common pKa value close to 4 possibly from the Glu50 cluster to control ion pair geometry. The pH dependence of the second-order rate constant (k) for the reactions of the catalytic site thiol groups with 4,4'-dipyrimidyl disulfide is shown to provide the pKa values for the formation and deprotonation of the (Cys)-S-/(His)-Im+H ion pair state. Analogous study of the reactions with 2,2'-dipyridyl disulfide reveals other kinetically influential ionizations, and all of these pKa values are compared with those observed in the pH dependence of kcat/Km for the catalyzed hydrolysis of N-acetylphenylalanylglycine 4-nitroanilide. The discrepancy between the pKa value for ion pair formation and the common pKa value close to 4 related to generation of catalytic activity is particularly marked for ficin (pKa 2.49 +/- 0.02) and caricain (pKa 2.88 +/- 0.02) but exists also for papain (pKa 3.32 +/- 0.01).

Characterization of the hydrolytic activity of a polyclonal catalytic antibody preparation by pH-dependence and chemical modification studies: evidence for the involvement of Tyr and Arg side chains as hydrogen-bond donors.

The hydrolyses of 4-nitrophenyl 4'-(3-aza-2-oxoheptyl)phenyl carbonate and of a new, more soluble, substrate, 4-nitrophenyl 4'-(3-aza-7-hydroxy-2-oxoheptyl)phenyl carbonate, each catalysed by a polyclonal antibody preparation elicited in a sheep by use of an analogous phosphate immunogen, were shown to adhere closely to the Michaelis-Menten equation, in accordance with the growing awareness that polyclonal catalytic antibodies may be much less heterogeneous than had been supposed. The particular value of studies on polyclonal catalytic antibodies is discussed briefly. Both the kcat and kcat/K(m) values were shown to increase with increase in pH across a pKa of approx. 9. Group-selective chemical modification studies established that the side chains of tyrosine and arginine residues are essential for catalytic activity, and provided no evidence for the involvement of side chains of lysine, histidine or cysteine residues. The combination of evidence from the kinetic and chemical modification studies and from studies on the pH-dependence of binding suggests that catalysis involves assistance to the reaction of the substrate with hydroxide ions by hydrogen-bond donation at the reaction centre by tyrosine and arginine side chains. This combination of hydrogen-bond donors appears to be a feature common to a number of other hydrolytic catalytic antibodies. High-pKa acidic side chains may be essential for the effectiveness of catalytic antibodies that utilize hydroxide ions.

The effect of constant ambient temperature and ration on the performance of sexed broilers.

Two trials involving 480 Cobb color-sexed broiler chicks were conducted to determine the effect of various constant ambient temperatures on the performance of broilers. Temperatures in Trial 1 were 18 and 29 C and in Trial 2 were 24 and 35 C. The interacting effect of dietary energy (3.142 or 3.252 kcal ME/g of diet) and protein (16, 19, or 22%) on performance criteria was also examined within each trial. There was no indication of selective consumption of any of the ratios at any temperature. Differences in feed consumption observed in either trial were totally contributed by temperature effect. Within a trial, and irrespective of temperature treatment, the rate of growth and feed consumption of the females were less than that of the males. Males and females responded equally to the ambient temperature; there was no significant sex X temperature interaction in Trials 1 or 2.

Cholesterol metabolism in the chicken.

Four hundred and eighty, sexed (White Leghorn) chicks were divided so that each sex was fed one of eight rations to determine the effect of sex, dietary cholesterol, and stearic, oleic, and/or linoleic acid on cholesterol deposition in blood, liver and aortic tissues. After 5 months on the dietary regimen, 3 cockerels from each of the rations were orally administered 100 mC. sodium-1-C14-acetate and 200 mC. cholesterol-1,2-H3 to determine whether body tissues contained cholesterol from endogenous or exogenous origin. After 5 months on the experimental rations it was observed that male birds had higher free cholesterol levels than did females. The addition of cholesterol (2%) and fatty acids (6%) to ration increased free cholesterol levels in liver and aorta. Cholesterol stored in livers and aortas occurs principally as free cholesterol while blood cholesterol was in esterified form. Cholesterol levels in both liver and aorta are primarily of endogenous origin.