Residue-specific pKa determination of lysine and arginine side chains by indirect 15N and 13C NMR spectroscopy: application to apo calmodulin

Electrostatic interactions in proteins can be probed experimentally through determination of residue-specific acidity constants. We describe here triple-resonance NMR techniques for direct determination of lysine and arginine side-chain protonation states in proteins. The experiments are based on detection of nonexchangeable protons over the full range of pH and temperature and therefore are well suited for pKa determination of individual amino acid side chains. The experiments follow the side-chain 15Nzeta (lysine) and 15Nepsilon or 13Czeta (arginine) chemical shift, which changes due to sizable changes in the heteronuclear electron distribution upon (de)protonation. Since heteronuclear chemical shifts are overwhelmed by the charge state of the amino acid side chain itself, these methods supersede 1H-based NMR in terms of accuracy, sensitivity, and selectivity. Moreover, the 15Nzeta and 15Nepsilon nuclei may be used to probe changes in the local electrostatic environment. Applications to three proteins are described: apo calmodulin, calbindin D9k, and FKBP12. For apo calmodulin, residue-specific pKa values of lysine side chains were determined to fall between 10.7 and 11.2 as a result of the high net negative charge on the protein surface. Ideal two-state titration behavior observed for all lysines indicates the absence of significant direct charge interactions between the basic residues. These results are compared with earlier studies based on chemical modification.     

Development and validation of a qualitative screening method for the detection of exogenous anabolic steroids in urine by liquid chromatography-tandem mass spectrometry

A screening method for the urinary detection of 34 exogenous anabolic steroids has been developed. The method involves an enzymatic hydrolysis, liquid-liquid extraction and detection by liquid chromatography-tandem mass spectrometry. The use of some adducts such as [M+NH₄]⁺, [M+CH₃COO]⁻ and [M+H+MeOH]⁺ was necessary in order to detect some analytes at the required level (lower than 10 ng/ml). Two transitions were selected for each analyte. Different concentration factors have been studied in order to increase the sensitivity. A concentration factor of 50 was selected for the screening method although the high ion suppression observed under these conditions can hamper its application as a quantitative method. The method was validated and limits of detection were obtained by spiking ten different blank urine samples at five different concentration levels. Up to 29 analytes were detected in all spiked urines at the required level. Limits of detection between 1 and 10 ng/ml were obtained for most analytes which fulfil current requirements. The applicability of the method was shown by analysing positive samples.     

Ionization of anabolic steroids by adduct formation in liquid chromatography electrospray mass spectrometry

The ionization of 46 anabolic steroids has been studied. The absence of basic or acidic moieties in most of these analytes makes their direct ionization as [M + H]+ by atmospheric pressure interfaces difficult. The formation of adducts with different components of the mobile phase has been found to be an efficient way to ionize anabolic steroids by electrospray. Different mobile phases using methanol (MeOH) or acetonitrile as organic solvent and HCOOH, Na+ or NH4+ as additives have been tested to favor the adduct formation. A direct correlation between the chemical structure of the anabolic steroid and the possibility to ionize it in a particular chromatographic condition has been found. According to their ionization, anabolic steroids can be divided into seven different groups depending on both the nature and the relative position of their functional groups. The formation of different adducts such as [M + Na + MeOH]+ or [M + H + CH3 CN - H2O]+ is required in order to ionize some of these groups and the optimal mobile phase composition for each group of anabolic steroids is proposed. Despite the ionization limitations due to their chemical structure, most of tested anabolic steroids could be ionized using the adduct formation approach.     

Development of a qualitative liquid chromatography/tandem mass spectrometric method for the detection of narcotics in urine relevant to doping analysis

A new screening procedure for 18 narcotics in urine for anti-doping purposes has been developed using liquid chromatography/triple quadrupole mass spectrometry (LC/MS). Electrospray ionization (ESI) was used as interface. Infusion experiments were performed for all substances to investigate their mass spectrometric behaviour in terms of selecting product specific ions. These product ions were then used to develop a tandem mass spectrometric method using selected reaction monitoring (SRM). For the LC/MS analysis, chromatography was performed on an octadecylsilane column. The total run time of the chromatographic method was 5.5 min. For the sample preparation prior to LC/MS analysis, the urine samples were liquid-liquid extracted at pH 9.5 after overnight enzymatic hydrolysis. Two extraction solvents were evaluated: dichloromethane/methanol 9/1 (v/v), which is currently used for the extraction of narcotics, and diethyl ether, used for the extraction of steroids. With diethyl ether the detection limits for all compounds ranged between 0.5 and 20 ng/mL and with the mixture containing dichloromethane the detection limits ranged between 0.5 and 10 ng/mL. Taking into account the minimum required performance limits of the World Anti-Doping Agency of 200 ng/mL for narcotics, diethyl ether can also be considered as extraction solvent for narcotics. Finally, the described method was applied to the analysis of urine samples previously found to contain narcotics by our routine gas chromatography/mass spectrometry (GC/MS) method.     

Novel method for measurement of glutathione kinetics in neonates using liquid chromatography coupled to isotope ratio mass spectrometry

A novel analytical method using liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS) was developed for measuring the fractional synthesis rate (FSR) of glutathione (GSH) in neonates after infusion of [1-(13)C]-glycine as a tracer. After transformation of GSH into GSSG, its dimeric form, the intra-erythrocytic concentration and (13)C-isotopic enrichment of GSH were determined using 200 microL of blood. The results showed that, using LC/IRMS, the concentration (range of micromol/mL) was reliably measured using norvaline as internal standard with precision better than 0.1 micromol/mL. In addition, the (13)C-isotopic enrichment measured in the same run gave reliable values with excellent precision (with standard deviation (sd) lower than 0.3 per thousand) and accuracy (measured between 0 and 2 Atom % Excess (APE)). The inter-assay repeatability of delta(13)C of norvaline used as internal standard with in vivo samples was assessed at -26.07 +/- 0.28 per thousand with coefficient of variance (CV) at 1.1%. The FSR calculated either with GSH or GSSG showed similar results with slightly higher values for GSSG (41.6 +/- 4.7 and 46.5 +/- 4.4, respectively). The slightly lower FSR of GSH is probably due to interfering compounds in the biological matrix. Successfully used in a clinical study, this rapid and reliable method opens up a variety of kinetic studies with relatively low administration of tracer infusates, reducing the total cost of the study design. The small volume of blood needed enables studies even in extremely small subjects, such as premature infants, as reported in this study.     

Synthesis and adsorption of shape-persistent macrocycles containing polycyclic aromatic hydrocarbons in the rigid framework

Shape-persistent macrocycles with interiors in the nanometer regime were prepared by the oxidative cyclization of the appropriate bisacetylene precursors under high-dilution conditions. These compounds contain polycyclic aromatic hydrocarbons in the ring backbone and are decorated with extra annular oligoalkyl or silyl side groups. Interestingly, after depositing them on different surfaces and investigating the self-assembled structures by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), various nanostructures were observed. STM showed that these macrocycles are organized in two-dimensional (2D) layers, whereas AFM showed, in addition, the formation of 2D crystallites and one-dimensional fibrils. These results reveal the importance of the extra annular substitution of the macrocycles in creating patterned surfaces and nanoscale objects.     

Longitudinal voice changes: Facts and interpretation

To study intraindividual vocal changes over time, a longitudinal research design is preferred. However, this procedure reveals many restrictions concerning methodology and interpretation, especially in cases of (semi)retrospective studies. This is illustrated by an investigation of the voices of 20 male Dutch-speaking reporters of the Belgian Radio Broadcasting service. These men read the same text they did 30 years previously for a digital registration. After having copied the high-quality archival recordings digitally, the results of the acoustical analysis of both recordings (recent and archival) were compared. This was done to search for the contribution of aging. In this article we report on the results of the speaking fundamental frequency (SFF) and the standard deviation of the SFF (SFFsd) as a measure of intraindividual variability, and the voice onset time (VOT). Results indicate that, in connected speech, SFF decreased, SFFsd became more restricted, and the VOT increased strongly. The interpretation of the results compels us to conclude that it is extremely difficult to define the specific effect of aging alone, since it is only one element among many others, including methodological concerns.     

Hydrogen-bonded oligo(p-phenylenevinylene) functionalized with perylene bisimide: self-assembly and energy transfer

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor-acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.     

Bidentate phosphorus baskets by intramolecular phosphinidene addition

Intramolecular phosphinidene addition to the C==C bond of Mo-complexed, seven-membered phosphorus heterocycles affords three novel [(diphos)Mo(CO)(4)] complexes (18-20). The three bidentate phosphorus baskets differ in the composition of the seven-membered ring: one of the phosphorus atoms is flanked by CH(2), NCH(3), or O. The unsaturated tetrahydrophosphepine precursors are synthesized by either ring-closing metathesis (C and N derivatives) or by a cyclization sequence (O derivative). The crystal structures of the nitrogen- (19) and oxygen-containing (20) baskets have relatively small P-Mo-P angles of 76.240(13) degrees and 77.626(12) degrees , respectively, and complex 20 has slightly shortened Mo--P bond lengths.