Shifts in the relative intensities of oligomer ions are found to accompany changes in the cone potential in the electrospray ion source, which introduce uncertainties into average molecular weight determinations for polymer distributions. Similar shifts with changes in cone potential have long been recognized in the multiple-charge distributions of proteins and other biomolecules. In the case of multiple-charge distributions of a single, or small number of, species there are no major consequences for calculation of molecular weight; however, mass distributions and the averages thereof, are of major concern with synthetic polymers and understanding the shifts in relative intensities becomes critically important. We report here an evaluation of the effects of cone potentials on the molecular weight distributions of synthetic polymers, which we compare with the effects on charge-state distributions of peptides. The effects of cone potential have been modeled mathematically, from which we conclude that cone potentials exert a focusing effect dependent on the mass-to-charge ratios of ions. It is largely this focusing effect that determines the dependence of oligomer ion intensities upon cone potential in the ESI mass spectra of polymers. The influence of cone potential on molecular weight determinations of polymers of varying polydispersities (P(o)) is compared and discussed. For polymers with low polydispersities (e.g., narrow molecular weight poly(ethyleneglycol) standards with P(o) < 1.5), the variation in molecular weight determinations tends to be small (typically <5%), whereas with synthetic polymers with polydispersities greater than 2, variations in cone potential can influence molecular weight determinations significantly (by 100% or even more).
Pyruvate dehydrogenase (PDH) deficiency has been described in many patients with primary lactic acidosis. However, there are very few cases in which a structural defect in the complex has been clearly demonstrated. Measurement of the activity of the PDH complex in cultured human cells has proved unreliable, and a combination of structural and functional studies are required to make a definitive diagnosis. For this reason, an immunochemical strategy has been developed to complement direct enzyme assay in the detection and further characterization of PDH deficiency. We illustrate the usefulness of this approach by describing defects in the alpha-subunit of the pyruvate decarboxylase component of the PDH complex in two patients with primary lactic acidosis. In one patient, there is no immunologically cross-reacting material corresponding to this subunit. In the second patient, there appears to be an intrinsic structural defect in the subunit which restricts dephosphorylation (and hence activation) of the inactive phosphorylated complex.
Seismology records the presence of various heterogeneities throughout the lower mantle e.g. 1,2 , however, the origins of these signals, whether thermal or chemical, remain uncertain and therefore much of the information they hold about the nature of the deep Earth is obscured. Accurate interpretation of observed velocities requires knowledge of the seismic properties of all of Earth's possible mineral components. Calcium silicate perovskite (hereafter "calcium perovskite") is believed to be the third most abundant mineral throughout the lower mantle. Here we measure the crystal structure, compressional and shear wave velocity of calcium perovskite samples , and provide direct constraints for calcium perovskite's adiabatic bulk and shear moduli. We observe that titanium incorporation into calcium perovskite stabilises the tetragonal structure to higher temperatures, and that the shear modulus of calcium perovskite is significantly lower than is predicted by computations 3-5 or thermodynamic datasets 6. When combined with literature data and extrapolated our results suggest subducted oceanic crust will be visible as low velocity anomalies throughout the lower mantle. In particular we show that large low-velocity provinces (LLVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal-cubic phase transition in Ti-bearing calcium perovskite. The lower mantle is vast, extending from the seismic discontinuity observed at ~ 660 km depth to the core-mantle boundary (CMB) at ~ 2890 km. Tomographic images demonstrate that despite a smooth variation of vp, vs and ρ in 1D velocity models the lower mantle is heterogeneous and regularly refertilised by subducting slabs 7,8. Sluggish diffusive re-equilibration and incomplete mechanical mixing e.g. 9 means that large-scale patterns of mantle convection may be directly observed via tomographic velocity anomalies and/or the distribution of seismic scatterers. Identifying the causes of heterogeneities requires accurate mineralogical models of Earth's mantle to facilitate comparisons between geophysical observations and predicted seismic velocities. However, a major uncertainty in many models e.g. 10,11 has been the influence of calcium silicate perovskite (capv, here corresponding to Ca[SixTi[1-x]]O3) on velocity, despite the widespread expectation that it is the lower mantle's third most abundant phase comprising 5-10 and 24-29 vol.% of peridotitic 12 and basaltic 13 assemblages respectively. Uncertainties stem from a sparsity of reliable measurements of capv's physical properties, which are technically challenging because CaSiO3 is unrecoverable 14 , undergoing spontaneous amorphisation at room temperature during decompression. The widely used thermodynamic model of Stixrude et al. 6 predicts that capv is significantly faster than PREM 15 , and therefore slow velocity anomalies are difficult to explain using recycled crustal material. Whilst
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