Intramolecular general acid catalysis in the aminolysis of beta-lactam antibiotics

The rate of aminolysis of benzylpenicillin and cephaloridine by hydroxylamine, unlike other amines, shows only a first order dependence on amine concentration. The rate enhancement compared with that predicted from a Bronsted plot for other primary amines with benzylpenicillin is greater than 10(6). This is much more than an alpha-effect and is compatible with rate-limiting formation of the tetrahedral intermediate due to a rapid intramolecular general acid catalysed breakdown of the intermediate. For cephaloridine, the rate enhancement is greater than 10(4) which demonstrates that beta-lactam C-N bond fission and expulsion of the leaving group at C3' are not concerted.     

Variables affecting simulated use determination of residual ethylene oxide in medical devices

In autumn 1993, AAMI/ST/WG 63, Sterilization Residuals Working Group undertook the task of studying factors involved in determining the amount of residual ethylene oxide in medical devices after sterilization and developing a protocol for controlling the relevant variables. The protocol was evaluated by conducting a round robin study consisting of 8 participating laboratories from around the country. Results of this round robin study demonstrated the range over which results may vary despite controls placed on the time and temperature at which determinations were conducted. The data from the study suggest that small, random variations in technique during short sample extraction times can lead to variability in the results. Variables such as initial water temperature, oven temperature, weighing of sample, and length of extraction should be carefully controlled. Inherent variations in the material composition of similar devices are possible contributing factors. The efforts of this working group and the subsequent evaluation and discussion of its findings are presented.     

How big is the universe today?

If one defines the size of the present universe in terms of a hypersurface of simultaneity generated by the spatial geodesies orthogonal to our world line today, then it is finite in all expanding Big Bang Friedmann models.     

Tables of Waiting Times for <Emphasis Type="Italic">M/M/n</Emphasis>, <Emphasis Type="Italic">M/D/n</Emphasis> and <Emphasis Type="Italic">D/M/n</Emphasis> and their Use to Give Approximate Waiting Times in More General Queues

An approximation formula for average waiting time in multiserver queues is considered using tables for the queues M/M/n, M/D/n and D/M/n. The approximation is compared with that proposed by Sakasegawa. Both approximations predominantly overestimate the waiting time, the first being more accurate, but the Sakasegawa approximation is simpler to apply.     

Coulometric determination of nickel and cobalt

A method has been developed for the successive determinations of nickel and cobalt by controlled potential coulomctric analysis with a. mercury cathode, using an aqueous pyridine electrolyte, Quantitles of the metals from about 10 to l00 mg per 100 ml are determinable with an average error of ±0.5 mg. Because this controlled potential electrolysis technique produces a clean separation of nickel and cobalt it can be used for preparing nickel-free cobalt compounds and cobalt-free nickel compounds.     

Crystal structure of β-CaNi5D0.77 by rietveld analysis of neutron powder-diffraction data

β-CaNi₅D_0.77 is orthorhombic (a = 8.6033(13) b = 5.0810(9) c = 7.8557(15)Å, Pmcm, Z = 4, D_x = 6.48 g cm^?3, V = 343.4ų, FW = 1339.8 amu. Rietveld analysis of a neutron powder-diffraction pattern shows that D atoms occupy the centers of [Ni₄Ca₂] square dipyramids (“octahedra”) linked through the Ca atoms at opposite apices and forming chains. About 80% of the deuterium atoms occupy fairly regular sites (DNi = 1.53, 1.80, 1.86, 1.86 Å; DCa = 2.42, 2.45 Å). The remaining 20% occupy very distorted sites (DNi = 1.21, 1.38, 2.07, 2.07 Å; DCa = 2.54, 2.60 Å). The average DD distance in a chain is 4.3 Å.     

Electrospray ionization ion trap mass spectrometry for structural characterization of oligosaccharides derivatized with 2-aminobenzamide

The use of electrospray ionization (ESI) quadrupole ion trap mass spectrometry and reversed-phase high-performance liquid chromatography (HPLC) for the characterization of 2-aminobenzamide (2AB)-labeled oligosaccharides and N-linked protein oligosaccharide mixtures is described. The major signals were obtained under these conditions from the [M+Na]+ ions for all 2AB-derivatized oligosaccharides. Under collision-induced dissociation, sodiated molecular species generated in the ESI mode yield simple and predictable mass spectra. Tandem mass spectrometry (MS/MS) experiments with orders higher than two offer a number of ways to enhance MS/MS spectra and to derive information not present in MS and MS2 spectra. Information on composition, sequence, branching and, to some extent, interglycosidic linkages can be deduced from fragments resulting from the cleavage of glycosidic bonds and from weak cross-ring cleavage products. Reversed-phase HPLC and derivatization by reductive amination using 2-aminobenzamide were finally applied to characterize a glycan pool enzymatically released from glycoproteins.     

Charge competition and the linear dynamic range of detection in electrospray ionization mass spectrometry

An experimental investigation and theoretical analysis are reported on charge competition in electrospray ionization (ESI) and its effects on the linear dynamic range of ESI mass spectrometric (MS) measurements. The experiments confirmed the expected increase of MS sensitivities as the ESI flow rate decreases. However, different compounds show somewhat different mass spectral peak intensities even at the lowest flow rates, at the same concentration and electrospray operating conditions. MS response for each compound solution shows good linearity at lower concentrations and levels off at high concentration, consistent with analyte "saturation" in the ESI process. The extent of charge competition leading to saturation in the ESI process is consistent with the relative magnitude of excess charge in the electrospray compared to the total number of analyte molecules in the solution. This ESI capacity model allows one to predict the sample concentration limits for charge competition and the on-set of ionization suppression effects, as well as the linear dynamic range for ESI-MS. The implications for quantitative MS analysis and possibilities for effectively extending the dynamic range of ESI measurements are discussed.     

Reaction path analysis of the "tunable" photoisomerization selectivity of free and locked retinal chromophores

Multiconfigurational second-order perturbation theory computations and reaction path mapping for the retinal protonated Schiff base models all-trans-nona-2,4,6,8-tetraeniminium and 2-cis-nona-2,4,6,8-tetraeniminium cation demonstrate that, in isolated conditions, retinal chromophores exhibit at least three competing excited-state double bond isomerization paths. These paths are associated with the photoisomerization of the double bonds in positions 9, 11, and 13, respectively, and are controlled by barriers that favor the position 11. The computations provide a basis for the understanding of the observed excited-state lifetime in both naturally occurring and synthetic chromophores in solution and, tentatively, in the protein environment. In particular, we provide a rationalization of the excited-state lifetimes observed for a group of locked retinal chromophores which suggests that photoisomerization in bacteriorhodopsin is the result of simultaneous specific "catalysis" (all-trans --> 13-cis path) accompanied by specific "inhibition" (all-trans --> 11-cis path). The nature of the S(1) --> S(0) decay channel associated with the three paths has also been investigated at the CASSCF level of theory. It is shown that the energy surfaces in the vicinity of the conical intersection for the photoisomerization about the central double bond of retinal (position 11) and the two corresponding lateral double bonds (positions 9 and 13) are structurally different.