Changes In Raman Spectrum Research Articles

A Raman Spectroscopic Study of Uranyl Species Adsorbed onto Colloidal Particles

Raman spectra of uranyl ion−loaded silver and gold colloidal particles have been measured. The Raman active symmetric stretching frequency of uranyl ions shifts to a smaller wavenumber by the adsorption onto silver and gold sols. The shift is so large that it discounts the case of weak bonding between uranyl species and both sols and suggests extensive σ donation from the surface of the sols to the equatorial plane of the adsorbed uranyl species. For the silver sols, the Raman spectrum changes significantly upon varying the pH of the colloidal suspensions and corresponds to different adsorbed species. For the gold sols, the Raman spectrum does not change upon varying the pH and thus corresponds to only one adsorbed species. The notion that adsorption of uranyl ions involves the release of ligands was confirmed for gold sols but discounted for silver sols.

Sequence-specific Changes in the Metal Site of Ferric Bleomycin Induced by the Binding of DNA

The binding of the iron complex of the antineoplastic glycopeptide bleomycin A2 (Fe-BLM) to calf thymus DNA and the self-complementary oligonucleotides d(CGCGCG) and d(ATATAT) has been studied using optical, EPR, and resonance Raman spectroscopies. An increase in the intensity of the bands at 365 and 384 nm is observed in the optical spectrum of Fe(III)-BLM when the drug binds to either oligonucleotide. However, in the presence of phosphate, this increase is observed only with d(CGCGCG) and not with d(ATATAT). In addition, the gmax feature in the EPR spectrum of low spin Fe(III)-BLM is narrowed in a way suggesting a reduction of possible conformers that the drug can achieve when it is bound to d(CGCGCG) or to calf thymus DNA but not when bound to d(ATATAT). When Fe(III)-BLM is bound to d(CGCGCG), changes in the resonance Raman spectrum of the metal drug complex suggest conformational changes in three of the ligands to iron: the beta-hydroxyhistidyl amide, the pyrimidine, and the axial hydroxide. In addition, the Fe-OH band undergoes narrowing, again consistent, with the reduction of conformers of the drug. No such resonance Raman changes are observed upon binding to d(ATATAT). The changes in the pyrimidine modes upon binding d(CGCGCG) to the drug are consistent with a recently proposed model (Wu, W., Vanderwall, D. E., Turner, C. J., Kozarich, J. W., and Stubbe, J. (1996) J. Am. Chem. Soc. 118, 1281-1294) of DNA recognition by activated bleomycin, HOO-Fe(III)-BLM, in which the pyrimidine moiety of the drug is important for the preferential cleavage of 5'-GpPy-3' sequences.

NMR and Raman Studies of Na<sub>2</sub>O-P<sub>2</sub>O<sub>5</sub>-SiO<sub>2</sub> Glasses

The 29Si and 31P nuclear magnetic resonance (NMR) chemical shifts and the Qn distribution in ternary sodium phosphosilicate glasses were examined. In the NaPO3-SiO2 glasses, only the Si(Q4) unit from SiO2 and the P(Q2) unit from NaPO3 are observed. When the P2O5 content increases at fixed mole fractions of SiO2 equal to 0.1 and 0.05, the 6-coordinated silicon atoms appear. The presence of 6-coordinated silicon atoms is expected in glasses where the [P2O5]/[Na2O] composition ratio is greater than unity. Their appearances can be understood in terms of the relative acidic strength of SiO2 and P2O5. The profile of Raman spectra changes appreciably due to the formation of 6-coordinated silicon atoms.

Ordered water molecules as key allosteric mediators in a cooperative dimeric hemoglobin.

One of the most remarkable structural aspects of Scapharca dimeric hemoglobin is the disruption of a very well-ordered water cluster at the subunit interface upon ligand binding. We have explored the role of these crystallographically observed water molecules by site-directed mutagenesis and osmotic stress techniques. The isosteric mutation of Thr-72-->Val in the interface increases oxygen affinity more than 40-fold with a surprising enhancement of cooperativity. The only significant structural effect of this mutation is to destabilize two ordered water molecules in the deoxy interface. Wild-type Scapharca hemoglobin is strongly sensitive to osmotic conditions. Upon addition of glycerol, striking changes in Raman spectrum of the deoxy form are observed that indicate a transition toward the liganded form. Increased osmotic pressure, which lowers the oxygen affinity in human hemoglobin, raises the oxygen affinity of Scapharca hemoglobin regardless of whether the solute is glycerol, glucose, or sucrose. Analysis of these results provides an estimate of six water molecules lost upon oxygen binding to the dimer, in good agreement with eight predicted from crystal structures. These experiments suggest that the observed cluster of interfacial water molecules plays a crucial role in communication between subunits.

Raman spectroscopic study of structural changes in calcium aluminate (CaAl 2O 4) glass at high pressure and high temperature

The Raman spectrum of calcium aluminate (CaAl 20 4) glass has been obtained as a function of pressure at ambient temperature to ∼ 15 GPa, and as a function of temperature at room pressure to 1650 K. On increasing pressure at 300 K, the Raman spectrum changes drastically between 8 and 10 GPa. The intensity of the overall spectrum weakens, the intensity of the 580-cm −1 band decreases markedly, and a new broad band centered near 750 cm −1 begins to dominate the spectrum. We interpret these spectral changes as due to the formation of highly coordinated A1 species through the involvement of bridging oxygens (i.e. formation of [3]O species). Upon decompression, the pressure-induced spectral changes are mostly reversible. However, the decompressed samples do not relax completely to their normal ambient state, and the spectra reflect important changes in the inter-tetrahedral AlOA1 linkages. These changes are consistent with some high-coordinate species being retained in the decompressed sample. On increasing temperature through the glass transformation range ( T g = 1178 K) at room pressure, our Raman spectra show evidence for important configurational changes, after vibrational effects are removed. The major spectral change consists of growth of a broad band between 600 and 1100 cm −1 appearing immediately above T g. We suggest that these configurational changes concern the removal of O 2− ions from the fully polymerized aluminate framework, with the resulting formation at high temperature of [3]O species (“triclusters”), coordinated by three tetrahedral aluminate groups.

Biochemical and Spectroscopic Characterization of the B800-850 Light-Harvesting Complex from Rhodobacter sulfidophilus and Its B800-830 Spectral Form

We demonstrate that the B800-830 spectral form of the B800-850 peripheral light-harvesting complex of Rhodobacter sulphidophilus, which is formed at low ionic strengths in the presence of the zwitterionic detergent LDAO, results from a local modification of the bacteriochlorophyll binding site and not the dissociation of the complex. This perturbation does not result in significant changes to the interactions between the pigments as studied by circular dichroism or fluorescence spectroscopy; however, modifications in the pigment binding sites are inferred from changes in the preresonance Raman spectrum. Specifically, an alteration of the hydrogen bonding of the 2-acetyl group of at least one of the bacteriochlorophyll groups that make up the 850 nm absorbing pair is observed. This implies an alteration in the conformation of the C-terminal domain of the alpha-polypeptide, in which are located the two tyrosyl residues that are believed to act as H-bond donors to these groups, induced by the protein-bound detergent in the absence of bound cations. We suggest that the ability of this complex to form an 800-830 complex is linked to the presence of an aspartyl residue immediately upstream of the tyrosyl residues. This study therefore provides a further illustration of the importance of hydrogen bonds to the 2-acetyl group of the bacteriochlorophyll in the determination of its spectral properties; furthermore, we provide a description of a conformational change that is able to modulate chromophore binding in these complexes.

Thermal stability of biatomic Si sheets in Ge

Certain short-period Si/Ge superlattices containing biatomic sheets of Si have been proposed as candidates for light-emitting Si-based structures. The thermal stability of biatomic sheets of Si in such structures is thus of interest and has been investigated here using the way in which their distinctive double-peaked lines in the Raman spectrum changes with annealing. The Raman data obtained were correlated with the results of an electron microscopical examination of the annealed samples which further demonstrated that, at 700 degrees C, rather than the layers becoming mixed through a simple interdiffusion process they instead become periodically of varying thickness in a way which would lower their strain energy. It is postulated that the changes in layer thickness arise as a result of interface diffusion. The results provide a useful starting point for understanding how direct-gap Si-based structures can be processed.

Infrared-excited Raman scattering and photoluminescence of deep intragap states in semiconducting YBa2Cu3O6+x.

The semiconducting parent compounds of the high-${\mathit{T}}_{\mathit{c}}$ superconducting system ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$, at low oxygen content (0x0.25) and different oxygen isotopes, have been investigated by means of infrared (IR) excited (1.16 eV) Raman scattering and photoluminescence (PL). The IR-excited Raman spectrum of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ is dominated by a group of three bands in the spectral range of the apex oxygen O(4) ${\mathit{A}}_{\mathit{g}}$ mode (475 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the $^{16}\mathrm{O}$ compound). The Raman spectrum changes dramatically upon oxygen doping with the appearance of a strong resonant band at 507 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ accompanied by several overtones. By studying site-selective $^{18}\mathrm{substituted}$ samples this band is assigned to ${\mathit{A}}_{\mathit{g}}$ vibrations of apex O(4) atoms adjacent to the O(1) oxygen atoms occupying a chain site upon doping. The energy shift of this phonon and its associated overtones is caused by a strong lattice relaxation around the O(4)-Cu(1)-O(4) complex due to electronic charge-transfer processes involving O(4) sites. This behavior implies a strong coupling between O(4) vibration and charge-transfer excitations. PL, with a maximum at about 1.3 eV, has been observed in samples with very low oxygen concentrations. Peak energy and intensity are temperature and doping dependent. Both Raman and PL data are interpreted assuming the existence of a narrow band, associated with the O(4)-Cu(1)-O(4) complex (in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$) which is located, for small x, within the ${\mathrm{CuO}}_{2}$ plane charge-transfer gap.

Deposition of high quality amorphous silicon films with strong hydrogen diluted silane as reactant gas source

By using strongly hydrogen diluted silane as a reactant gas source [C(=SiH4/(SiH4+H2))<5%] in a conventional diode PECVD system, we have deposited high quality a-Si:H films which exhibit almost no Steable-Wronski (S-W) effect. The [H] radical in rf plasma erodes the growing surface and eliminates the weak Si-Si bonds, thus reducing the density of metastable defects of a-Si:H films and causing the amorphous network to be more perfect. Our results show that as C value decreases from 5.4% to 0.8%, the peak location of TO mode in Raman spectra changes from 480 to 500 cm-1, the average distortion of bond angles Δθ, which is calculated from the width of half full height of TO peaks, reduces from 9.0° to 3.8°. The hydrogen content CH of the samples which show almost no S-W effect is less than 10 at%.

Resonance Raman study of halorhodopsin photocycle kinetics, chromophore structure, and chloride-pumping mechanism.

Kinetic resonance Raman spectra of the HR520, HR640, and HR578 species in the halorhodopsin photocycle are obtained using time delays ranging from 5 microseconds to 10 ms in 0.3 M NO3-, 0.3 M Cl-, and 3 M Cl-. The Raman intensities are converted to absolute concentrations by using a conservation of molecules constraint. The simplest kinetic scheme that satisfactorily models the data is HR578-->HR520 in equilibrium with HR640-->HR578. The rate constant for the HR640-->HR578 transition increases with Cl- concentration, suggesting that Cl- is taken up between HR640 and HR578. The ratio of the forward to the reverse rate constants connecting HR520 and HR640 increases as the inverse of the Cl- concentration, suggesting that Cl- is released during the HR520-->HR640 step. The configuration about the C13 = C14 bond of the retinal chromophore in HR640 is examined by regenerating the protein with [12,14-2H2]retinal. The C12-2H + C14-2H rocking vibration for HR640 is observed at 943 cm-1, demonstrating that the chromophore is 13-cis. The changes in the resonance Raman spectrum of HR640 in response to 2H2O suspension indicates that the Schiff base linkage to the protein is protonated. None of the HR640 fingerprint vibrations shift significantly in 2H2O, suggesting that the Schiff base adopts a C = N anti configuration; this assignment is supported by the frequency of the C15-2H rocking mode (1002 cm-1). The 13-cis structure for the chromophore in HR640 requires that thermal isomerization back to all-trans occurs in the HR640-->HR578 transition. These structural and kinetic results are incorporated into a two-state C-T model for Cl- pumping.

Interrelation of the structure, vibrational spectra and critical temperature of (123)-superconductors

Raman spectrum changes resulting from different variations of the crystal lattice YBa 2Cu 3O δ (such as the substitutions Y → rare-earth element R, Ba → Sr, δ = 6 → δ = 7) have been studied. The concentration associated shift of vibrational frequencies of the oxygen atoms located in different layers of the lattice is shown to obey a general law: certain vibrations soften whereas the others harden. This vibrational behaviour is explained by the wave-like reconstruction of the crystal lattice (123). For the whole superconductor family RBa 2Cu 3O 7 and for YBa 2Cu 3O 7 at hydrostatis pressure the increase in T c with increasing frequency of the highest frequency A g-vibration is established. The results show the essential role of phonons in the high- T c superconductivity mechanism.

Substrate polarization by residues in Δ5‐3‐ketosteroid isomerase probed by site‐directed mutagenesis and UV resonance Raman spectroscopy

delta 5-3-Ketosteroid isomerase (KSI: EC 5.3.3.1) of Pseudomonas testosteroni catalyzes the isomerization of delta 5-3-ketosteroids to delta 4-3-ketosteroids by the stereospecific transfer of the steroid 4 beta-proton to the 6 beta-position, using Tyr-14 as a general acid and Asp-38 as a base. Ultraviolet resonance Raman (UVRR) spectra have been obtained for the catalytically active double mutant Y55F + Y88F, which retains Tyr-14 as the only tyrosine residue (referred to as the Y14(0) mutant), and the Y14F mutant, which has 50,000-fold lower activity. The UVRR results establish that binding of the product analog and competitive inhibitors 19-nortestosterone or 4-fluoro-19-nortestosterone to the Y14(0) mutant does not result in the formation of deprotonated Tyr-14. The UVRR spectra of the steroid inhibitors show large decreases in the vinyl and carbonyl stretching frequencies on binding to the Y14(0) enzyme but not on binding to the Y14F enzyme. These changes cannot be mimicked by protonation of the steroids. For 19-nortestosterone, the vinyl and carbonyl stretching frequencies shift down (with respect to the values in aqueous solution) by 18 and 27 cm-1, respectively, on binding to Y14(0) KSI. It is proposed that the changes in the steroid resonance Raman spectrum arise from polarization of the enone moiety via the close proximity of the charged Asp-38 side chain to the vinyl group and the directional hydrogen bond between Tyr-14 and the 3-carbonyl oxygen of the steroid enone. The 230-nm-excited UVRR spectra do not, however, show changes that are characteristic of strong hydrogen bonding from the tyrosine hydrogen. It is proposed that this hydrogen bonding is compensated by a second hydrogen bond to the Tyr-14 oxygen from another protein residue. UVRR spectra of the Y14(0) enzyme obtained using 200 nm excitation show enhancement of the amide II and S Raman bands. The secondary structure of KSI was estimated from the amide II and S intensities and was found to be low in alpha-helical structure. The alpha-helix content was estimated to be in the range of 0-25% (i.e., 10 +/- 15%).

Crystallization of nitrobenzene multilayers on Ni(111) studied by Raman scattering

We have observed an anomalous coverage dependence of the Raman scattering intensity of the NO 2 symmetric stretching mode (1346 cm −1) of nitrobenzene on a clean, well-defined single crystal surface of Ni(111). When the coverage is below 3 monolayers the Raman intensity is very weak, and it rises suddenly between 3.1 and 3.7 monolayers as the coverage increases. Above 4 monolayers it increases linearly with coverage. During the sudden rise of the intensity, the Raman spectrum changes from that of a single broad peak around 1346 cm −1 to two peaks at 1341.6 and 1350.3 cm −1. This sudden intensity rise and the spectral split are due to the formation of a polycrystalline structure in the layers already formed, although the first and the second nitrobenzene layers do not crystallize. These facts indicate that the molecules adsorbed above the second layer are relatively free to move so that they can easily fall into a crystalline structure, but that the first two layers have a moderately strong interaction with the Ni(111) surface which prevents their crystallization. Above 4 monolayers the nitrobenzene molecules form a polycrystalline structure immediately upon adsorption. A model for this crystallization process is proposed.

Phonon Raman scattering and structure of Co substituted YBa2Cu3O6+δ

Raman and x-ray diffraction measurements have been performed on Co substituted YBa2Cu3−xCoxO6+δ. The x-ray diffraction data show that the structure transforms from orthorhombic to tetragonal at x∼0.13. The features in the Raman spectra change smoothly throughout the investigated concentration range without any observable abrupt changes at the phase transition. The observed changes in the Raman spectra for increasing Co concentration are reminiscent of those observed for pure YBCO samples of increasing oxygen deficiency. In particular, the frequency of the O(4) stretching vibration is observed to be similarly correlated to Tc for both Co substituted and oxygen deficient YBCO. It is suggested that the O(4) stretching vibration is sensitive to small changes of the charge balance between the CuO chains and the CuO2 planes.

Axial coordination of ferric Aplysia myoglobin

Resonance Raman spectra of ferric Aplysia myoglobin in the ligand-free and the azide-bound forms have been studied over a wide pH range to determine the coordination states of the heme iron atom. In the hydroxide form at high pH (approximately 9) the iron is six-coordinate and is in a high/low spin equilibrium. As the pH is lowered below the acid/alkaline transition (pKa = 7.5), the heme becomes five-coordinate. When the pH is lowered even further no other changes in the resonance Raman spectrum are detected; thus, the heme remains five-coordinate down to pH 4, the lowest value studied. For ferric azide-bound Aplysia myoglobin, the iron is six-coordinate in a high/low spin equilibrium at all pH values (4.8-9). These data indicate (i) that the unusual reactivity toward azide previously observed at neutral pH is indeed related to the absence of a coordinated water molecule, and (ii) that causes other than the heme coordination are responsible for the spectral differences and the ligand-binding kinetics differences observed below pH 6.

Resonance Raman analysis of astaxanthin–protein complexes

AbstractA survey of resonance Raman spectra of carotenoproteins, including astaxanthin, is presented. The data are discussed and compared with absorption measurements in order to establish the mode of binding. Three mechanisms seem to be dominant in explaining the changes which occur in the resonance Raman and absorption spectra of astaxanthin. The polarization of the π‐electron system is responsible for the large red shift in the absorption λmax and the decrease in the CC stretching wavenumber. A distortion of the backbone plane involving double bonds can produce an increase of the CC stretching wavenumber and a red shift of the absorption band. Effects of the medium on excited states can produce red or blue shifts of the absorption band without perturbation of the ground electronic state properties. The interference of these mechanisms can explain the observed changes in the resonance Raman spectrum of astaxanthin bound to proteins and the great diversity of the results.

Component Assignments of the Raman Spectrum from Highly Elongated Silica Glass Filbers

Raman spectra from glass fibers subjected to high tensile stresses (up to 4 GPa) were decomposed into their Gaussian components, which were then assigned to normal vibrational modes of either SiO4 or Si2O molecules using an “isolated molecule" model. Such molecule-related parameters as force constants, Raman coupling constants and the dispersion of the intertetrahedral bond angle were estimated from the frequencies, heights and widths of the components. It is concluded from the analysis that the Raman spectrum change associated with the application of tensile stress arises principally, not from a Si–O bond stretching or Si–O–Si bond angle broadening, but from a change in the tetrahedral angles of the SiO4 molecules constituting silica glass networks.

TIME‐RESOLVED RESONANCE RAMAN SPECTRA OF THE PHOTOCYCLE INTERMEDIATES OF ACID AND DEIONIZED BACTERIORHODOPSIN

Abstract— The photocycle of bacteriorhodopsin (bR) and its perturbed forms are investigated by a time‐resolved resonance Raman study. These experiments were performed in the C=C stretching and in the fingerprint spectral regions for the acid blue, acid purple and deionized forms of bR.The main observations are as follows: (1) isomerization of the retinal, from all‐trans to 13‐cis, occurs in native bR and in all of the acid and deionized perturbed bR species; (2) formation of the early intermediates (the K610 and L550 analogues) also occur in native bR and in all of the perturbed species; and (3) deprotonation of the protonated Schiff base (PSB), to give the M412 type intermediate, occurs in native bR, but is inhibited in all of the perturbed bR species on the time‐scale of the native bR photocycle.The results show that isomerization alone is not a prerequisite for the PSB deprotonation process. The observed photocycle, initiated with retinal isomerization, is found to occur from all‐trans to 13‐cis in all of the perturbed forms of bR. In addition, the results imply that removal of the cations, of an increase in the hydrogen ion concentration, prevent only the PSB deprotonation process and not the formation of earlier cycle intermediates. Some attention is focused on the two blue forms of bR (acid and deionized) due to the fact that their ground‐state absorption maximum, unphotolyzed Raman spectra, and Raman spectra changes during the photocycle are all very similar. The similarities between the acid blue and deionized blue forms in the fingerprint region support previous suggestions that both blue species have nearly the same retinal active site.

Erratum: Changes in the Raman spectrum of LiIO3induced by uniaxial stress

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Excitations in Fe1-xCoxCl2: a randomly mixed antiferromagnet with competing spin anisotropies

Magnons, phonons and electronic excitations in the mixed crystal Fe1-xCoxCl2 have been studied by Raman scattering spectroscopy. Magnon-like excitations have been observed for T<TN in crystals with 0<or=x<0.03 and 0.9<X<or=1. The temperature dependences of the one-magnon peak position, linewidth and intensity have also been studied. For FeCl2 an interacting magnon theory provides an excellent description of the peak position and linewidth results for TN<or approximately=0.7 TN. The FeCl2 and CoCl2 magnon frequencies and linewidths are relatively insensitive to addition of impurities, but their intensities are drastically diminished such that the magnon Raman scattering is unobservable for 0.03<x<0.9. Ferromagnetic resonance experiments on iron-rich samples produced similar results. The reason for this loss of intensity is unknown but it may indicate a reduction in the density of stats at k=0. The magnetic ordering is also reflected in changes in the electronic Raman spectrum of Co2+ and Fe2+ ions. The authors' results for the concentration and temperature dependence of this scattering suggest that the magnetic ions may experience random fields in the mixed phase. The temperature and concentration dependences of the two Raman-active k=0 phonons have been analysed over the whole concentration range. Both phonons exhibit the expected linear concentration dependence of their frequencies provided allowance is made for Fe2+-electron-Eg-phonon and Co2+-electron-A1g-phonon coupling.