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ANALYTICAL CURRENTS
Intelligent chemical sensors Polymer hydrogels expand or contract when exposed to changes in environmental conditions such as temperature, pH, and solvent. Such volume changes can indicate chemical change if functional groups that interact with specific analytes are attached to the polymer chains. Sanford A Asher and John H. Holtz of the University of Pittsburgh are investigating polymerized crys-
Absorption spectra showing dependence of diffraction wavelength on glucose concentration for PCCA glucose sensor. (Adapted with permission. Copyright 1997 Macmiilan Magazines.)
Studying noncovalent complexes w i t h MALDI For many proteins, the physiological structure is actually an ensemble of two or more subunits, known as the quaternary structure. Knowledge of the noncovalent interactions that hold the proteins together and that bind enzymes to their substrates should help elucidate protein reaction mechanisms. Matrix-assisted laser desorption ionization (MALDI) MS has been used only sparingly to investigate noncovalent complexes, because common matrices yield spectra of the subunits only. Franz Hillenkamp and co-workers at the University of Munster (Germany) have taken MALDI MS closer to the study of noncovalent complexes. They investigated various matrix-organic sol-
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talline colloidal arrays (PCCAs) within hydrogels that respond to changes in chemical speciation. Because the lattice is trapped within the hydrogel, changes in the hydrogel volume affect the lattice spacing and the diffracted wavelength. An intelligent PCCA (IPCCA) that is sensitive to Pb2+, Ba2+, and K+ was developed by copolymerization of a crown ether, which selectively binds the cations, into a PCCA. The volume of IPCCA and diffraction wavelength increased with increasing Pb2+ concentrations. The swelling was reversible, and the diffraction returned to its original wavelength upon removal of the Pb2+ from the complex. Another intelligent sensor was developed by attaching glucose oxidase to a PCCA In the presence of oxygen, glucose solutions cause die IPCCA to swell and the diffraction to shift to longer wavelengths. Upon removal of glucose from the IPCCA it again reverts to its original diffraction wavelength. Other biomolecular recognition elements could potentially be incorporated into IPCCA—raising the possibility of developing sensors that respond to low concentrations of viruses or other pathogens. (Nature 1997 389 829-32)
vent combinations that allow the study of intact protein complexes. Several hydroxyacetophenone matrices were successfully used to obtain spectra of intact streptavidin, which is a tetramer with identical subunits. Interestingly, the tetramer was observed only for the first laser shot at any given matrix spot The researchers were also able to obtain spectra of the tetramers of yeast alcohol dehydrogenase and beef liver catalase, both of which are stable over a narrower pH range than streptavidin. The authors observed no general pattern or mechanism that accounts for the spectra of the intact tetramers obtained on just die first laser shot. They suggest that the most likely model is precipitation of the quaternary complex on the surface of the matrix crystal. (J. Am. Soc. Mass Spectrom. 1997,8,1046-52)
Open wide Nobody looks forward to visiting the dentist, particularly when the dentist probes for cavities. Near-IR Raman spectroscopy may provide a more precise method for distinguishing decayed and healthy tooth material than do tactile methods based on increased tooth porosity. Wieland Hill of the Institut fur Spektrochemie und Angewandte Spektroskopie and Verena Petrou of Universitat Erlangen-Nurnberg (both in Germany) have investigated the changes in the near-IR spectra of human molars due to caries infection. The researchers used 1-mm slices of carious human molars to study sound dentin (tooth material) and enamel and carious tissue—including ordinary, chronic, and rampant caries in dentin and initial and fissure caries in enamel. The types of caries are classified by their rate of progression, with ordinary being the slowest and rampant the fastest. They also studied the Raman spectra of three white and two transparent composite dimethacrylate resins used for filling cavities. The Raman spectrum of tooth enamel is dominated by hydroxyapatite; the spectrum of dentin is dominated by hydroxyapatite and collagen, the primary organic component of teeth. The spectra of carious dentin are also dominated by hydroxyapatite and collagen, but they show an intense broadband luminescence background. The authors attribute the luminescence to bacterially infected material or brownish substances generated by incomplete microbial metabolism. (Appl. Spectrosc. 1199751,1265-68)
A slice of tooth containing several different kinds of defects. (Adapted with permission. Copyright 1997 Society for Applied Spectroscopy.)
Analytical Chemistry News & Features, December 1, 1997 709 A
