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S A Asher

Publications and source records attributed to S A Asher.

29 records · Page 2Linked to original sources
PubMed1989

Resonance Raman evidence for tyrosine involvement in the radical site of galactose oxidase.

Resonance Raman data are reported for the redox-activated form of galactose oxidase from Dactylium dendroides. Excitation within the red (659 nm) and blue (457.9 nm) absorption bands leads to strong resonance enhancement of ligated tyrosine vibrational modes at 550, 1170, 1247, 1484, and 1595 cm-1. The ring mode frequencies are unusually low, indicating a decreased bond order in the ring. The spectra clearly differ in both frequencies and relative intensities from those characteristic of known aromatic pi-radicals. Enhancement of tyrosine ring modes on excitation within absorption bands previously associated with the presence of the radical in the active site suggests that the ligated tyrosine residue is present in the radical site and may stabilize this radical species through formation of a charge transfer complex. A dramatically different Raman spectrum is observed for the N3- adduct of galactose oxidase, exhibiting a single strong 1483 cm-1 feature. The intense visible-near IR absorption bands for galactose oxidase may derive from transitions within a charge transfer complex between an aromatic free radical and a tyrosine-copper complex.

Azides↗
PubMed1984

Raman spectroscopy of a coal liquid shows that fluorescence interference is minimized with ultraviolet excitation.

The first ultraviolet resonance Raman measurements of a coal liquid are reported. The spectra detail the presence of numerous polycyclic aromatic hydrocarbons with ring systems similar to those of naphthalene, fluorene, phenanthrene, pyrene, and triphenylene . The ultraviolet resonance Raman measurements of this highly complex sample show no significant interference from fluorescence. The lack of fluorescence interference and the high selectivity indicate that ultraviolet resonance Raman spectroscopy is a powerful new technique for characterizing highly complex samples and mixtures.

Coal↗
PubMed1981

Resonance Raman and absorption spectroscopic detection of distal histidine--fluoride interactions in human methemoglobin fluoride and sperm whale metmyoglobin fluoride: measurements of distal histidine ionization constants.

The pH dependence of the resonance Raman and absorption spectra of human methemoglobin fluoride (HbIIIF) and sperm whale metmyoglobin fluoride (MbIIIF) has been examined. Both the Raman and absorption spectra of HbIIIF and MbIIIF indicate the existence at alkaline pH of an equilibrium between the hydroxide and fluoride complexes. The absorption maxima of HbIIIF and MbIIIF solutions shift to longer wavelengths as the pH is decreased from neutrality. The Raman data show a corresponding shift of the 461- and 468-cm-1 Fe-F vibrational stretching peaks at pH 7.0 [Asher, S. A., & Schuster, T. M. (1979) Biochemistry 18, 5377] to 399 and 407 cm-1 at acid pH in MbIIIF and HbIIIF, respectively. These shifts are interpreted to result from protonation of the distal histidine and the formation of a hydrogen bond to the fluoride ligand. Measurements of the pH dependence of the absorption and resonance Raman spectra give distal histidine ionization constants (apparent) corresponding to pK = 5.1 (+/- 0.1) for HbIIIF and pK = 5.5 (+/- 0.1) for MbIIIF. An examination of the distal histidine pK values and the frequency of the hydrogen-bonded Fe--F stretching vibration at pH 5.0 of HbIIIF with and without inositol hexaphosphate indicates little difference in the distal histidine--heme distance between the so-called R and T quaternary forms of HbIIIF. These results indicate that the changes in the electronic spectrum of HbIIIF that occur upon switching from the R to the T form do not result from alterations in (1) the iron--fluoride bond distance, (2) the iron out-of-heme plane distance, or (3) the distal histidine--fluoride distance.

Animals↗
PubMed1981

Differences in iron-fluoride bonding between the isolated subunits of human methemoglobin fluoride and sperm whale metmyoglobin fluoride as measured by resonance Raman spectroscopy.

The heme geometries of the isolated alpha and beta subunits of human methemoglobin fluoride (HbIIIF) and sperm whale metmyoglobin fluoride (MbIIIF) have been examined by exciting their Raman spectra within their ca. 6000-A charge-transfer absorption bands. The Fe-F stretching vibration at 471 cm-1 in the beta subunits shifts to 466 cm-1 in the alpha subunits and to 461 cm-1 in MbIIIF. The Fe-F bond is estimated to elongate by 0.02 A in the alpha subunits and 0.03 AZ in MbIIIF compared with that in the beta subunits. This bond elongation is interpreted to result from an increased iron displacement toward the proximal histidine side of the heme in the series MbIIIF greater than alpha greater than beta. A comparison of the isolated subunit spectra with that of tetrameric HbIIIF indicates little change occurs in isolated subunit heme geometry upon association into tetrameric HbIIIF. A correlation is found between the gamma max of the 600-A charge-transfer absorption band and the Fe-F bond length. Elongation of the Fe-F bond is associated with a shift of the absorption spectral maximum to a longer wavelength. However, the absorption spectral shift induced by the inositsol hexaphosphate induced R leads to T conversion does not result from a change in the Fe-F stretching frequency (+/- 0.5 cm-1). In contrast, frequency shifts are observed for heme macrocyclic vibrational modes. The data are interpreted to indicate that the effect of the R leads to T conversion in HbIIIF is to perturb heme macrocycle conformation without altering the heme out-of-plane iron distance or the Fe-F bond length.

Animals↗
PubMed1981

Hydration of single crystals of dipalmitoylphosphatidylcholine.

Optical birefringence and x-ray measurements are reported for single crystals (0.5 mm X 2 mm X 5 micrometers) of dipalmitoylphosphatidylcholine as a function of temperature and relative humidity. Large single crystals grown in CHCl3/acetone at a low water concentration undergo an irreversible phase transformation at 20 degrees C and 70% relative humidity or at 60 degrees C and 65% relative humidity. After this transformation occurs, the crystals can be made to undergo a reversible transformation into and out of the L alpha phase. The x-ray-determined mosaic spread in the hydrated crystal is less than approximately 0.04 degrees both above and below the main transition.

Birefringence↗
PubMed1979

Resonance Raman examination of axial ligand bonding and spin-state equilibria in metmyoglobin hydroxide and other heme derivatives.

Resonance Raman spectra and excitation profiles have been obtained within the 5700-6300-A absorption band of purified sperm whale metmyoglobin hydroxide (MbIIIOH) solutions. A large enhancement occurs for a Raman peak at 490 cm-1 which is shown by isotopic substitution of 18O for 16O to be almost purely an Fe-O stretch. The Fe-O vibration in MbIIIOH occurs 5 cm-1 to lower energy than the corresponding vibration at 495 cm-1 in human methemoglobin hydroxide (HbIIIOH) [Asher, S., Vickery, L., Schuster, T., & Sauer, K. (1977) Biochemistry 16, 5849], reflecting differences in ligand bonding between Mb(III) and Hb(III). A larger frequency difference (10 cm-1) exists between MbIIIF and HbIIIF for the Fe-F stretch. We do not observe separate Fe-O or Fe-F stretches from the alpha and beta chains of either HbIIIOH or HbIIIF. Excitation profile measurements for MbIIOH indicate that the 5700-6300-A absorption band is composed of two separate absorption bands which result from a high- and a low-spin form of MbIIIOH. The spin-state-sensitive Raman band at 1608 cm-1 reflects the high-spin species and has an excitation profile maximum at about 6000 A while the low-spin Raman band occurs at 1644 cm-1 and shows an excitation profile maximum at 5800 A. The Fe-O stretch at 490 cm-1 has an excitation profile maximum at about 6000 A. The differences in frequency and Raman cross section between the Fe-X vibrations in MbIIIX and HbIIIX (X = OH-, F-) can be related to increases in the out-of-plane iron distance for the high-spin species of MbIIIX. The shift in the 1644-cm-1 MbIIIOH low-spin state Raman band indicative of the heme core size to 1636 cm-1 in HbIIIOH indicates a larger heme core size in HbIIIOH. Raman frequency shifts are used to estimate differences in bond strain energies between MbIIIX and HbIIIX (X = OH-, F-). Previous resonance Raman excitation profile data can be interpreted in terms of separate contributions from different spin-state species.

Animals↗
PubMed1979

Alignment and defect structures in oriented phosphatidylcholine multilayers.

The alignment of dilauryl-, dimyristoyl-, and dipalmitoylphosphatidylcholine at various water concentrations into large oriented monodomain multilayers by annealing at elevated temperatures (Powers and Clark, 1975, Proc. Natl. Acad. Sci. U.S.A. 72:840; Powers and Pershan. 1977. Biophys. J. 20:137) is accompanied by the formation and subsequent dissolution of various defect structures. Some of these defects appear similar to those observed in thermotropic and other lyotropic liquid crystals, reflecting the lamellar structure of these materials. The formation and evolution of defects during the alignment of the lipids into the defect-free, monodomain, multilamellar geometry is studied using polarized microscopy. A combination of polarized and dark-field microscopy facilitated characterization of the defects; specific structural models are proposed. A new alignment technique involving compression and dilation of the lipid, which effects sample alignment at temperatures that are lower than those required by the Powers technique, is described. Lower temperature alignment avoids thermal decomposition that will sometimes occur if the lipid is maintained at elevated temperatures for prolonged periods. With this technique, samples (80 micrometer thick) of dilaurylphosphatidylcholine with 20% water by weight were aligned at room temperature.

Chemical Phenomena↗