News
Filtered probes improve tissue Raman spectra
A variety of rabbit tissues, including adipose, muscle, esophagus, aorta, brain, and skin, were analyzed with the IFPs. For most tissues, the two beveled probes performed comparably. For skin and aorta, the probe with the smaller bevel angle appeared to perform better, which contradicted the point response function data. The authors attribute this to the level 5 probe sampling a larger tissue volume than the level 10 probe. (Appl. Spectrosc. 1999, 93, 619-27)
Raman spectroscopy can be used to analyze intact tissues. Although Raman has more molecular specificity than other spectroscopies, such as fluorescence, it is also a much weaker effect. Brian C. Wilson and co-workers at the University of Toronto (Canada) and Visionex describe internally filtered probes (IFPs), which are designed to suppress fluorescence and the Raman signal generated by the fiber itself. The IFPs consist of a central fiber that delivers light from the laser and seven collection fibers. Filters were incorporated onto the core of the fibers—a bandpassfilterin the delivery fiber and longpaasfiltersin the collectionfibers.Beam-steering optics were incorporated in the fibers by beveling them and coating the surface with a reflective layer. The beam steering improved the collection efficiency and provided a way to control the spatial sampling volume. Three probes with different bevel angles tested and identified as level 0 (no beam-steering optics) 5 and 10 probes (The numerical values of the levels were arbitrary) The spectral quality achieved by each of the filtered probes was better than Illustrations of (a) the general architecture of the its unfiltered counterpart. The bifurcated fiber-optic probes and (b) the tip of an filters drasttcally reduced but internally filtered probe. The cross sections in (a) did not completely eliminate show the orientation of the individual fiber-optic the contribution from silica to cables within the probe. (Adapted with permission. the Raman spectrum. Copyright 1999 Society for Applied Spectroscopy.)
VCD of oligosaccharides Vibrational circular dichroism (VCD), which measures the differential absorption of left and right circularly polarized IR light by chiral molecules, can provide detailed stereochemical information about molecules. The VCD signal, although weak, can generate more information than electronic circular dichroism, because a limited number of electronic transitions are usually accessible. However, limited by the small 588 A
signals, VCD of carbohydrates has been restricted to monosaccharides. Prasad L. Polavarapu and Pranati K. Bose of Vanderbilt University demonstrate that VCD can be used to probe the glycosidic linkages of oligosaccharides as well. All the analyzed oligosaccharides contained two or more D-glucose units connected by O-glycosidic linkages—D-maltose, a-D-cyccodextrin, a,a-D-trehalose, D-gentobiose, and D-cellobiose. The glucose units are connected through oc(l->4), a(l->l),
Analytical Chemistry News & &eatures, September 1, 1999
P(l->4), and |3(1—>6) linkages. Fairly strong VCD signals are seen for the oligosaccharides with a linkages (a,a-trehalose, cc-cyclodextrin, and maltose) but not for those with (3 llnkages (cellobiose and gentobiose)) According to the authors, the results suggest that VCD in the 1200-900 cm-1 1egion can be used to distinguish between a- and (3-glycosidic linkages. In addition, VCD can help distinguish the types of a linkages because their VCD patterns differ. (J. .m. Chem. Soc. 1999,121, 6094-95)
New metalloprotein tool Although magnetic circular dichroism (MCD) is commonly used to study electronic transitions of metalloproteins and their derivatives, comparable magnetic linear dichroism (MLD) studies of such systems have not previously been reported. Jim Peterson and co-workers at Carnegie Mellon University and the University of Pittsburgh School of Medicine show, for the first time, tiiat two ferrous hemoprotein derivatives— ferrocytochrome c and deoxymyoglobin— do indeed exhibit detectable MLD signals in the near-UV to visible region. MLD spectra were recorded on a CD spectrometer combined with a superconducting magnet and an optical cryostat. The instrument, which was originally constructed for MCD experiments, was modified to operate in MLD mode. MLD uses linearly rather than circularly polarized light and a different orientation of the applied magnetic field than MCD. Ferrocytochrome c and deoxymyoglobin were selected as model compounds because they are strong chromophores that exhibit well-characterized electronic transitions. In contrast to ferrocytochrome c, deoxymyoglobin spectra were found to be temperaturedependent, which is consistent with a chromophore having a paramagnetic ground state. In addition, the deoxymyoglobin spectra were significantly more intense than those exhibited by low-spin ferric hemes. Other paramagnetic metalloprotein derivatives are expected also to yield MLD spectra, opening the door for a host of potential applications. Interestingly, the MLD spectra differ qualitatively from MCD spectra in that they apparently do not reveal transitions that are predominantly d-d. Therefore comparison of the two kinds of spectra may ciid in assigning electronic transitions of metalloproteins (T. Am Chem SoS 1999 121 5972-80)
