Paramagnetic liposomes: inner versus outer membrane relaxivity of DPPC liposomes incorporating lipophilic gadolinium complexes

Proton relaxometric properties of unilamellar DPPC liposomes embedding an amphiphilic paramagnetic chelate (Gd-DTPA-BC(14)A) in both layers of the phospholipid membrane or only in the external one are compared. The results show that the membrane's water permeability is able to quench the effect of the paramagnetic complexes located in the internal layer of DPPC liposomes, leading thus to an apparent lower global relaxivity.     

Absolute line intensities for formic acid and dissociation constant of the dimer

Absolute line intensities in the nu(6) and nu(8) interacting bands of trans-HCOOH, observed near 1105.4 and 1033.5 cm(-1), respectively, and the dissociation constant of the formic acid dimer (HCOOH)(2) have been measured using Fourier transform spectroscopy at a resolution of 0.002 cm(-1). Eleven spectra of formic acid, at 296.0(5) K and pressures ranging from 14.28(25) to 314.0(24) Pa, have been recorded between 600 and 1900 cm(-1) with an absorption path length of 19.7(2) cm. 437 integrated absorption coefficients have been measured for 72 lines in the nu(6) band. Analysis of the pressure dependence yielded the dissociation constant of the formic acid dimer, K(p)=361(45) Pa, and the absolute intensity of the 72 lines of HCOOH. The accuracy of these results was carefully estimated. The absolute intensities of four lines of the weak nu(8) band were also measured. Using an appropriate theory, the integrated intensity of the nu(6) and nu(8) bands was determined to be 3.47 x 10(-17) and 4.68 x 10(-19) cm(-1)(molecule cm(-2)) respectively, at 296 K. Both the dissociation constant and integrated intensities were compared to earlier measurements.     

Evaluation of polymeric methacrylate-based monoliths in capillary electrochromatography for their potential to separate pharmaceutical compounds

Polymeric methacrylate-based monoliths are evaluated in capillary electrochromatography (CEC) and pressurized capillary electrochromatography (p-CEC) for their potential in pharmaceutical analysis. Using a given polymerization mixture as a basis for the monolith synthesis, different mobile phase pH at constant organic modifier concentrations are tested in both CEC and p-CEC. The test set consists of basic, acidic, amphoteric, and neutral compounds, which are mainly pharmaceuticals. Because of the mainly hydrophobic character of the stationary phase, the interactions are largest when the compounds appear in an uncharged state, but some ion-exchange phenomena with negatively charged compounds can also be observed. In CEC, acidic substances are most retained at low pH. For amphoteric and neutral compounds, no preference regarding analyzing pH can be derived from these experiments. For basics, a high pH is chosen, but a reduced solvent strength is needed to enhance the retention of these compounds. The retention mechanism in p-CEC can also be assigned to both hydrophobic and ionic interactions. For acidic, amphoteric, and neutral compounds, acceptable retention is seen. For the basic compounds, the retention with a mobile phase containing 50% organic modifier is low, as in CEC. However, when the organic modifier content in the mobile phase is decreased, retention increases and the selectivity of the stationary phase is more pronounced. This mode of operation presents a possibility for separating some test mixtures, thus some potential for pharmaceutical analysis is seen. More efforts are needed to obtain higher efficiencies and better peak shapes, which might be solved by a further optimization of both the stationary phase synthesis and the mobile phase composition.     

Ruggedness and robustness testing

Due to the strict regulatory requirements, especially in pharmaceutical analysis, analysis results with an acceptable quality should be reported. Thus, a proper validation of the measurement method is required. In this context, ruggedness and robustness testing becomes increasingly more important. In this review, the definitions of ruggedness and robustness are given, followed by a short explanation of the different approaches applied to examine the ruggedness or the robustness of an analytical method. Then, case studies, describing ruggedness or robustness tests of high-performance liquid chromatographic (HPLC), capillary electrophoretic (CE), gas chromatographic (GC), supercritical fluid chromatographic (SFC), and ultra-performance liquid chromatographic (UPLC) assay methods, are critically reviewed and discussed. Mainly publications of the last 10 years are considered.     

Magnetization Transfer via Residual Dipolar Couplings: Application to Proton–Proton Correlations in Partially Aligned Proteins

A novel three-dimensional NMR experiment is reported that allows the observation of correlations between amide and other protons via residual dipolar couplings in partially oriented proteins. The experiment is designed to permit quantitative measurement of the magnitude of proton–proton residual dipolar couplings in larger molecules and at higher degree of alignments. The observed couplings contain data valuable for protein resonance assignment, local protein structure refinement, and determination of low-resolution protein folds.     

Precise mass measurement of a double-stranded 500 base-pair (309 kDa) polymerase chain reaction product by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) has been used to determine the mass of a double-stranded 500 base-pair (bp) polymerase chain reaction (PCR) product with an average theoretical mass of the blunt-ended (i.e. unadenylated) species of 308 859.35 Da. The PCR product was generated from the linearized bacteriophage Lambda genome which is a double-stranded template. Utilization of ethanol precipitation in tandem with a rapid microdialysis step to purify and desalt the PCR product was crucial to obtain a precise mass measurement. The PCR product (0.8 pmol/μL) was electrosprayed from a solution containing 75% acetonitrile, 25 mM piperidine, and 25 mM imidazole and was infused at a rate of 200 nL/min. The average molecular mass and the corresponding precision were determined using the charge-states ranging from 172 to 235 net negative charges. The experimental mass and corresponding precision (reported as the 95% confidence interval of the mean) was 309 406 +/- 27 Da (87 ppm). The mass accuracy was compromised due to the fact that the PCR generates multiple products when using Taq polymerase due to the non-template directed 3'-adenylation. This results in a mixture of three PCR products with nearly identical mass (i.e. blunt-ended, mono-adenylated and di-adenylated) with unknown relative abundances that were not resolved in the spectrum. Thus, the experimental mass will be a weighted average of the three species which, under our experimental conditions, reflects a nearly equal concentration of the mono- and di-adenylated species. This report demonstrates that precise mass measurements of PCR products up to 309 kDa (500 bp) can be routinely obtained by ESI-FTICR requiring low femtomole amounts. Copyright 1999 John Wiley & Sons, Ltd.     

Sharp bounds for decompositions of graphs into complete r-partite subgraphs

If G is a graph on n vertices and r ≥ 2, we let m_r(G) denote the minimum number of complete multipartite subgraphs, with r or fewer parts, needed to partition the edge set, E(G). In determining m_r(G), we may assume that no two vertices of G have the same neighbor set. For such reducedgraphs G, we prove that m_r(G) ≥ log₂ (n + r − 1)/r. Furthermore, for each k ≥ 0 and r ≥ 2, there is a unique reduced graph G = G(r, k) with m_r(G) = k for which equality holds. We conclude with a short proof of the known eigenvalue bound m_r(G) ≥ max{n₊ (G, n₋(G)/(r − 1)}, and show that equality holds if G = G(r, k). © 1996 John Wiley & Sons, Inc.     

High-resolution Fourier transform study of the ν3 fundamental band of trans-formic acid

Using high-resolution Fourier transform spectra of trans-HCOOH recorded at 5.6 μm, we carried out an extensive analysis of the strong ν₃ fundamental band (carbonyl stretching mode) at 1776.83 cm^?1, starting from results of a previous analysis [Weber WH, Maker PD, Johns JWC, Weinberger E. J Mol Spectrosc 1987; 121: 243–60]. As pointed out in the literature, the ν₃ band is significantly perturbed by resonances due to numerous dark bands. We were able to assign series belonging to the ν₅+ν₇, ν₅+ν₉, ν₆+ν₇ and ν₆+ν₉ dark bands, located at 1843.48, 1792.63, 1737.96 and 1726.40 cm^?1, respectively. The model used to calculate energy levels accounts partly for the observed resonances, and enabled us to reproduce most of the observed line positions, within their experimental uncertainties. We also determined absolute line intensities with an accuracy estimated to 15%. Finally, we generated, for the first time, a list of line parameters for the 5.6 μm region of trans-formic acid.     

Calix[4]arenes as Molecular Platforms for Magnetic Resonance Imaging (MRI) Contrast Agents

The novel amphipilic conjugate of a calix[4]arene with four Gd–1,4,7,10‐ tetra(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane (DOTA) chelates has potential as a magnetic resonance imaging contrast agent, both in its monomeric and in its micellar form. The system, illustrated here with its nuclear magnetic relaxation profile, shows good relaxivities, thanks to its high rigidity.

     

Dynamic switching between binding sites in the complexation of macrocyclic ‘push-pull’ chromophores to lanthanides

The introduction of a 1,3-diacetylpyridine moiety, as an additional binding niche in a macrocyclic receptor ligand containing a conjugated, push-pull malonate functionality, leads to, in addition to the 1:1 binding stoichiometry, the equilibrium formation of multiple complexes, specifically 1:2, 2:1, and 3:1 ligand:metal complexes with lanthanide trications. Various binding modes elicit distinctive responses in the visible region, which enhance the potential recognition of lanthanide ions. This paper also highlights the usefulness of a full factor analysis in the elucidation of complex binding phenomena.