News
ANALYTICAL CURRENTS
Using a Iight-scattering detector for SFC In packed-column SFC, modifiers and additives are often necessary to efficiently elute polar compoundsfromthe column. Unfortunately, these modifiers can interfere with the use offlameionization and UV detectors. Alternatively, Larry T. Taylor and J. Thompson B. Strode, III, of the Virginia Polytechnic Institute and State University modified an evaporative lightscattering detector designed for LC and interfaced it with a supercritical fluid chromatograph. They evaluated the performance of the detector by monitoring the response of progesterone, testosterone, and 17-ahydroxyprogesterone with varying nitrogen makeup gas flow rate, C0 2 modifier type, modifier concentration, detector
Friction yields nanolatch After three centuries of studying friction, adhesion, and wear at the macroscopic level, researchers can now explore tribological properties at the nanometer scale by using atomic force microscopy (AFM). Theoretically, results should be interpretable in terms of fundamental principles.
Computer model of the atomic structure of the Mo03/MoS2 interface. (Adapted with permission from the American Association for the Advancement of Science.)
detector. A total gas flow rate of 1000 mL/ min gave acceptable peak widths and signal response. Detection limits of < 10 ng were obtained for all three steroids using 2%, 10%, and 20% (v/v) methanol-modified C0 2 on a Deltabond cyano column. The authors were also able to separate Detection of ginkgolides in a Ginkgo biloba leaf using UV and evaporative light-scattering detection. (Adapted with permission and detect the five ginkgolides in an exfrom Preston Publications.) tract from a Ginkgo biloba leaf and suggest that SFC with evaporative lightorifice size, and detector temperature. scattering detection is a promising techThe researchers found that increased nique for assaying food stuffs, polymers, nitrogen gasflowresults in decreased and pharmaceuticals. (J. Chromatogr. analyte response but in improved peak shape similar to that obtained with a UV Sci. 1996, 34, 261-71.)
However, AFM investigations into tribology are hampered by the lack of a welldefined description of the probe tipsample sliding interface. Paul E. Sheehan and Charles M. Lieber of Harvard University have circumvented this problem by investigating the friction of Mo03 nanocrystals sliding across single-crystal MoS2, a widely used solid lubricant. The interface structure and contact area are atomically defined, and the Mo03 nanocrystals can be moved controllably with the AFM tip. When they ran the experiments, the authors found that the friction was extremely anisotropic, with Mo03 sliding along only one of the three equivalent MoS2 surface directions. An atomic model of the interface was developed to explain these observations. The researchers exploited this friction anisotropy to fabricate a nanometer-scale mechanical latch and groove lock using two Mo03 crystals that have different preferred sliding directions. The latch and groove slide in and out reversibly, whereas breaking the latch required a large lateral force of 41 nN. Such reversible latches could be the basis for mechanical logic gates. (Science, 1996,272, 1158-61.)
Studying the catalytic properties of an enzyme The mechanism by which complex redox metalloenzymes catalyze enzyme transport and coupled reactions is important for understanding biological energy transduction. The various kinetic and spectroscopic techniques used to study electron transport tend to produce disparate pieces of information rather than a unified picture of the process. Fraser A. Armstrong and colleagues at the Oxford Inorganic Chemistry Laboratory (U.K.) and the University of CaliforniaSan Francisco have used electrocatalytic voltammetry to study the catalytic behavior of succinate dehydrogenase (SDH), the membrane-extrinsic component of Complex II. SDH adsorbs onto a pyrolytic graphite edge electrode and catalyzes interconversion of succinate and fumarate depending on the applied electrochemical potential. Under reversible conditions (near the reduction potential of the fumarate/succinate couple) and at the physiological temperature of 38 °C, SDH
Analytical Chemistry News & Features, August 1, 1996 461 A
News used to reveal characteristics not apparent from conventional studies of enzymes that have strong electronic coupling with the electrode and are capable of displaying high rates of substrate turnover. The degree to which an enzyme is biased to catalyze more effectively in one direction can be quantified, and the ability Resolution of SDH from Complex II and adsorption of SDH onto to scan across a conthe electrode surface. tinuous potential range allows detection of small changes in electron-transis biased to catalyze fumarate reduction port activity that occur as sites switch (reversal of the tricarboxylic acid cycle) at pH values below 7.7. The authors con- between redox states. (J. Am. Chem. Soc. 1996,118, 5031-38.) cluded that electrochemistry can be
A transient metabolite in the field
munity during the sample processing. Analysis by GC/MS was completed within 12 h. The researchers found the intermediate in all the samples collected within the contaminant plume; the intermediate was absent in the negative controls. (Environ. Sci. Technol. 1996,30, 2099-103.)
Observation of an intermediary metabolite provides strong evidence that biodegradation of an environmental contaminant is taking place. If that metabolite is unstable, identifying it is complicated, because prior to analysis, microorganisms respond quickly to changes arising from removal from the contaminated field site. As a result, studies relying on the identification of an unstable intermediary metabolite to document in situ realtime biodegradation are rare. Mark S. Wilson and Eugene L. Madsen of Cornell University have reported the field identification of l,2-dihydroxy-l,2-dihydronapthalene (1,2-DHDN), a transient intermediate in the biodegradation of napthalene, which is a component of several pollutant mixtures, including coal tars and crude oil. Using solid-phase extraction and GC/MS, the authors determined 1,2DHDN in waters flowing though a contaminated site. To minimize sample artifacts, a field laboratory was set up and collection, extraction, and derivatization steps were completed within Mass spectra of authentic 1,2-DHDN (top) 30 min. In addition, several steps were compared with the metabolite formed in an taken, such as adding the respiratory enriched culture inoculated with a mixture of inhibitor sodium azide, to avoid chang- site-derived microorganisms (middle) and ing the native aquatic microbial comcollected in waters on site (bottom). 462 A
Analytical Chemistry News & Features, August 1, 1996
Interatomic distance measurements by NMR Knowledge of interatomic distances is important in correlating the observed function of materials with their structure. Although traditional methods of determining interatomic distances, such as crystallography and solution-state NMR, can provide information on atomic location with accuracies of 0.5-2.0 A, these methods are time-consuming and are not always representative of in vivo conformation. K. T Mueller of The Pennsylvania State University and T. P. Jarvie and G. T Went of CuraGen Corp. have used rotational-echo double-resonance (REDOR), anew analysis method for solid-state NMR, to study biologically relevant molecules. They performed the REDOR experiment on a mixture of two small cyclic peptides bound to mBHA resin via the terminal Gly. The two peptides were identical except for their labeling; one peptide was labeled with 15N,1-13C Gly and the other with 15N,2-13C Gly. To obtain a spectrum of the dipolar coupling present in the peptide system, a REDOR transform was applied to the dephasing data. The results agreed well with X-ray crystallographic data from pure Gly. The authors believe that the REDOR transform has the potential to aid in the drug discovery process via improved local structural studies of regions of peptides that bind proteins. (J. Am. Chem. Soc. 1996,118,5330-31.)
Two effective MALDI matrices Matrix-assisted laser desorption/ionization MS (MALDI-MS) allows the analysis of large biomolecules. Commonly used matrices include a-cyano-4-hydroxycinnamic acid (ce-CHCA), sinapinic acid (SA), and 2,5-dihydroxybenzoic acid (DHB). David M. Lubman and co-workers at The University of Michigan evaluated three structurally related compounds, 4-hydroxy-3-me-
