Ion Mobility-Mass Spectrometry Study of Folded Ubiquitin Conformers Induced By Treatment with <Emphasis Type="Italic">cis</Emphasis>-[Pd(en)(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>]<Superscript>2+</Superscript>

Ion mobility-mass spectrometry is used to study the new conformers of bovine ubiquitin (Ub) and the palladium(II) binding sites after the incubation with cis-[Pd(en)(H₂O)₂]²⁺ where en = ethylenediamine. Palladium(II) complexes are potentially useful proteomic reagents because they selectively bind to the side groups of methionine and histidine and hydrolytically cleave the peptide bond. Incubating 1.0 mM solution of Ub with 10.0 molar excess of cis-[Pd(en)(H₂O)₂]²⁺ results with one to four Pd²⁺ or Pd(en)²⁺ being attached to intact Ub and two conformer families at each of the 4+ to 11+ charge states. The 4+ and 5+ species exhibit a compact form, which is also observed in untreated Ub, and a new highly folded conformer. The 6+ to 10+ exhibit an elongated form, also observed in Ub, and a new partially folded conformer. The new conformers are shown to be more stable if they contain at least one Pd²⁺, rather than all Pd(en)²⁺. IM-MS/MS of [UbPd₂en+5H]⁹⁺ shows that both the partially folded and elongated conformers first lose the en ligand, followed by dissociating into product ions that indicate that Met1, Glu51/Asp52, His68, and Glu16 are binding sites for Pd²⁺. These results suggest that Pd²⁺ is simultaneously binding to multiple side groups across different regions of Ub. This type of sequestering of Pd²⁺ probably reduces the efficiency of Pd²⁺ ions to selectively cleave Ub because it prevents Pd²⁺ anchoring to only Met or His and to an adjacent backbone amide nitrogen and forming the “activated complex” necessary for specific peptide bond cleavage.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

Electrospray ionization multiple-stage tandem mass spectrometric analysis of diglycosyldiacylglycerol glycolipids from the bacteria Bacillus pumilus

Electrospray ionization (ESI) combined with multiple-stage tandem mass spectrometry (MS(n)) was used to directly analyze the glycolipid mixture from bacteria Bacillus pumilus without preliminary separation. Full scan ESI-MS revealed the composition of picomole quantities of glycerolglycolipid species containing C(14)-C(19) fatty acids, some of which were monounsaturated. Two main components were identified from their molecular masses and fragmentation pathways. The fragmentation pathway of the known compound compared with the investigated compound verified the proposed structure as 1(3)-acyl-2-pentadecanoyl-3(1)-O-[beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl]-sn-glycerols. A comparison of the multiple tandem mass spectra of the different alkali-metal cation adducts indicates that the intensity of fragments and the dissociation pathways are dependent on the alkali-metal type. The basic structures of glycerolglycolipids were reflected clearly from the fragmentation patterns of the sodium cations. The intense fragments of the sugar residue from the precursor ions were obtained from the lithiated adduct ions. ESI-MS(n) spectra of [M + K](+) ions did not provide as much fragmentation as [M + Na](+) and [M + Li](+) adducts, but their spectra allow the position of glycerol acylation to be determined. On the basis of MS(2) spectra of [M + K](+) ions, it was established that all components have a C(15:0) fatty acid at the sn-2 position of the glycerol backbone and C(14)-C(19) acids at the sn-1 position of the glycerol backbone. Copyright 1999 John Wiley & Sons, Ltd.     

Analysis of a shielded TE011 mode composite dielectric resonator for stable frequency reference

Analysis of a TE₀₁₁ mode composite sapphire-rutile dielectric resonator has been carried out to study the temperature variation of resonance frequency, close to the Cs atomic clock hyperfine frequency of 9.192 GHz. The complementary behavior of dielectric permittivity with temperature of the composite has been exploited to obtain the desired turning point in the resonant frequency. The frequency of the composite structure is found to be independent of the shield diameter beyond four times the puck diameter.     

The effect of data scaling on dual prices and sensitivity analysis in linear programs

In many practical situations scaling the data is necessary to solve linear programs. This note explores the relationships in translating the sensitivity analysis between the original and the scaled problems.     

Technetium-99m Radiopharmaceuticals for Ideal Myocardial Perfusion Imaging: Lost and Found Opportunities

The favorable nuclear properties in combination with the rich coordination chemistry make technetium-99m the radioisotope of choice for the development of myocardial perfusion tracers. In the early 1980s, [^99mTc]Tc-Sestamibi, [^99mTc]Tc-Tetrofosmin, and [^99mTc]Tc-Teboroxime were approved as commercial radiopharmaceuticals for myocardial perfusion imaging in nuclear cardiology. Despite its peculiar properties, the clinical use of [^99mTc]Tc-Teboroxime was quickly abandoned due to its rapid myocardial washout. Despite their widespread clinical applications, both [^99mTc]Tc-Sestamibi and [^99mTc]Tc-Tetrofosmin do not meet the requirements of an ideal perfusion imaging agent due to their relatively low first-pass extraction fraction and high liver absorption. An ideal radiotracer for myocardial perfusion imaging should have a high myocardial uptake; a high and stable target-to-background ratio with low uptake in the lungs, liver, stomach during the image acquisition period; a high first-pass myocardial extraction fraction and very rapid blood clearance; and a linear relationship between radiotracer myocardial uptake and coronary blood flow. Although it is difficult to reconcile all these properties in a single tracer, scientific research in the field has always channeled its efforts in the development of molecules that are able to meet the characteristics of ideality as much as possible. This short review summarizes the developments in ^99mTc myocardial perfusion tracers, which are able to fulfill hitherto unmet medical needs and serve a large population of patients with heart disease, and underlines their strengths and weaknesses, the lost and found opportunities thanks to the developments of the new ultrafast SPECT technologies.     

Variable-temperature x-ray structural investigation of [Fe[HC(3,5-Me2pz)3]2](BF4)2 (pz= pyrazolyl ring): observation of a thermally induced spin state change from all high spin to an equal high spin-low spin mixture, concomitant with the onset of nonmerohedral twinning

The complex [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) A, b = 10.332(1) A, c = 19.644(2) A, beta = 111.097(2) degrees, and V = 3851.5(6) A(3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS. The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 A, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P1 with a = 10.287(2) A, b = 11.355(3) A, c = 18.949(4) A, alpha = 90.852(4) degrees, beta = 105.245(4) degrees, gamma = 116.304(4) degrees, and V = 1892.3(8) A(3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. Previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.