Resonance-enhanced Raman Spectra Research Articles

Excited State Resonance Raman of Flavin Mononucleotide: Comparison of Theory and Experiment.

Blue light absorbing flavoproteins play important roles in a variety of photobiological processes. Consequently, there have been numerous investigations of their excited state structure and dynamics, in particular by time-resolved vibrational spectroscopy. The isoalloxazine chromophore of the flavoprotein cofactors has been studied in detail by time-resolved Raman, lending it a benchmark status for mode assignments in excited electronic states of large molecules. However, detailed comparisons of calculated and measured spectra have proven challenging, as there are many more modes calculated than are observed, and the role of resonance enhancement is difficult to characterize in excited electronic states. Here we employ a recently developed approach due to Elles and co-workers ( J. Phys. Chem. A 2018, 122, 8308-8319) for the calculation of resonance-enhanced Raman spectra of excited states and apply it to the lowest singlet and triplet excited states of the isoalloxazine chromophore. There is generally good agreement between calculated and observed enhancements, which allows assignment of vibrational bands of the flavoprotein cofactors to be refined. However, some prominently enhanced bands are found to be absent from the calculations, suggesting the need for further development of the theory.

Cell-phone camera Raman spectrometer.

In this report, we describe the design, construction, and operation of a cell-phone-based Raman and emission spectral detector, which when coupled to a diffraction grating and cell-phone camera system provides means for the detection, recording, and identification of chemicals, drugs, and biological molecules, in situ by means of their Raman and fluorescence spectra. The newly constructed cell-phone spectrometer system was used to record Raman spectra from various chemicals and biological molecules including the resonance enhanced Raman spectra of carrots and bacteria. In addition, we present the quantitative analysis of alcohol-water Raman spectra, performed using our cell-phone spectrometer. The designed and constructed system was also used for constructing Raman images of the samples by utilizing a position scanning stage in conjunction with the system. This compact and portable system is well suited for in situ field applications of Raman and fluorescence spectroscopy and may also be an integrated feature of future cell-phones.

Resonance Raman Spectra for the In Situ Identification of Bacteria Strains and Their Inactivation Mechanism.

The resonance Raman spectra of bacterial carotenoids have been employed to identify bacterial strains and their intensity changes as a function of ultraviolet (UV) radiation dose have been used to differentiate between live and dead bacteria. In addition, the resonance-enhanced Raman spectra enabled us to detect bacteria in water at much lower concentrations (∼108 cells/mL) than normally detected spectroscopically. A handheld spectrometer capable of recording resonance Raman spectra in situ was designed, constructed, and was used to record the spectra. In addition to bacteria, the method presented in this paper may also be used to identify fungi, viruses, and plants, in situ, and detect infections within a very short period of time.

Biological synthesis of gold nanoparticles by the diatom Stephanopyxis turris and in vivo SERS analyses

Cultivation of the sea water diatom Stephanopyxis turris (S. turris) in a gold salt containing medium leads to the formation of gold nanoparticles. These biosynthesized nanoparticles were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). According to their diameter, they are classified into two groups with average diameters of around 30 and 10nm. It is demonstrated that both types of nanoparticles consist exclusively of gold. The location of the gold nanoparticles with respect to the diatom cell was revealed using surface enhanced Raman scattering (SERS). To ensure a reliable observation of SERS spectra in each measured cell, the concentration of the gold solution as well as the incubation time of S. turris in this solution were optimized. 3D Raman imaging of entire S. turris cells was performed to localize SERS spectra of compounds resulting from closely neighbored nanoparticles. Interestingly, intracellular SERS spectra were observed indicating the presence of gold nanoparticles inside S. turris cells. Further investigation shows that the SERS spectra generated by biosynthesized nanoparticles differ significantly from the conventional Raman spectrum of diatoms. While pigments are dominating the resonance enhanced Raman spectrum, SERS spectra exhibit a completely different appearance. Their bands coincide with bands in SERS spectra of various biologically relevant compounds, e.g., hydroxyurea and retinol. The biosynthesized gold nanoparticles therefore have a high potential for prospective in vivo research of algal metabolism and its regulation.

Resonance enhanced Raman scatter in liquid benzene at vapor-phase absorption peaks

The resonance enhanced Raman spectra in the 1B2u mode of the forbidden benzene electronic transition band, ~230-270 nm, has been investigated. Resonance enhanced Raman scattering in both liquid benzene and liquid toluene exhibit the greatest enhancement when the wavelength of excitation is tuned to the vapor-phase absorption peaks; even though the sample volume is in a liquid state. Raman signals for the symmetric breathing mode of the carbon ring are found to be resonantly enhanced by several orders of magnitude (>500X) with deep UV excitation compared to non-resonant visible excitation. Since the benzene absorbs near this resonant wavelength, its effect on the sampled volume cannot be neglected in determining the resonance gain, as we discuss in detail. Large resonant gains correspond with excitation at the 247, 253, and 259 nm absorption peaks in the benzene vapor spectrum. The narrow region of resonance gain is investigated in detail around the absorption peak located at 259 nm using 0.25 nm steps in the excitation wavelength. We observe the resonance gain tracking the vapor phase absorption peaks and valleys within this narrow range. Results are interpreted in terms of the coherence forced by the use of a forbidden transition for resonance excitation.

Adhesion of Mussel Foot Protein-3 to TiO2 Surfaces: the Effect of pH

The underwater adhesion of marine mussels relies on mussel foot proteins (mfps) rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (Dopa). As a side chain, Dopa is capable of strong bidentate interactions with a variety of surfaces, including many minerals and metal oxides. Titanium is among the most widely used medical implant material and quickly forms a TiO2 passivation layer under physiological conditions. Understanding the binding mechanism of Dopa to TiO2 surfaces is therefore of considerable theoretical and practical interest. Using a surface forces apparatus, we explored the force-distance profiles and adhesion energies of mussel foot protein 3 (mfp-3) to TiO2 surfaces at three different pHs (pH 3, 5.5 and 7.5). At pH 3, mfp-3 showed the strongest adhesion force on TiO2, with an adhesion energy of ∼-7.0 mJ/m(2). Increasing the pH gives rise to two opposing effects: (1) increased oxidation of Dopa, thus, decreasing availability for the Dopa-mediated adhesion, and (2) increased bidentate Dopa-Ti coordination, leading to the further stabilization of the Dopa group and, thus, an increase in adhesion force. Both effects were reflected in the resonance-enhanced Raman spectra obtained at the three deposition pHs. The two competing effects give rise to a higher adhesion force of mfp-3 on the TiO2 surface at pH 7.5 than at pH 5.5. Our results suggest that Dopa-containing proteins and synthetic polymers have great potential as coating materials for medical implant materials, particularly if redox activity can be controlled.

Structure of endohedral fullerene Eu@C74

Structure determination of endohedral fullerenes in the absence of X-ray data is difficult and often controversial. Here we show that the structure of endohedral fullerene Eu@C(74) may be determined by density functional theory aided interpretation of its electronic, infrared and Raman spectra. The use of recently developed analytical polarizability gradient methods to simulate resonance-enhanced Raman spectra is crucial for this approach and allows for a nearly complete assignment of the experimental spectra. Eu@C(74) is assigned a pear-shaped C(2v) symmetric structure and shows strong ionic interaction between the encapsulated metal and the fullerene pi system.

Broadband Coherent Anti-Stokes Raman Spectroscopy Characterization of Polymer Thin Films

Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of polymer thin film materials. A simple modification to a 1 kHz broad bandwidth sum frequency generation (SFG) spectrometer permits acquisition of CARS spectra for polymer thin films less than 100 nm thick, a dimension relevant to organic electronic device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance-enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained under these conditions consistently demonstrate enhanced signal-to-noise ratio compared to the spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The dielectic properties of the substrate have a dramatic effect on the detected signal intensity. For ultrathin films, the strongest signals are obtained from fused silica surfaces. Similar to surface-enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS, no surface roughening is necessary.

Molecular Spectroscopy of Uranium(IV) Bis(ketimido) Complexes. Rare Observation of Resonance-Enhanced Raman Scattering from Organoactinide Complexes and Evidence for Broken-Symmetry Excited States

Electronic absorption and resonance-enhanced Raman spectra for ketimido (azavinylidene) complexes of tetravalent uranium, (C(5)Me(5))(2)U[-N=C(Ph)(R)](2) (R = Ph, Me, and CH(2)Ph), have been recorded. The absorption spectra exhibit four broad bands between 13 000 and 24 000 cm(-1). The highest-energy band is assigned to the ketimido-localized p( perpendicular)(N)-->pi(N=C) transition based on comparison to the spectra of (C(5)H(5))(2)Zr[-N=CPh(2)](2) and (C(5)Me(5))(2)Th[-N=CPh(2)](2). Upon excitation into any of these four absorption bands, the (C(5)Me(5))(2)U[-N=C(Ph)(R)](2) complexes exhibit resonance enhancement for several Raman bands attributable to vibrations of the ketimido ligands. Raman bands for both the symmetric and nominally asymmetric N=C stretching bands are resonantly enhanced upon excitation into the p( perpendicular)(N)-->pi(N=C) absorption bands, indicating that the excited state is localized on a single ketimido ligand. Raman excitation profiles for (C(5)Me(5))(2)U[-N=CPh(2)](2) confirm that at least one of the lower-energy electronic absorption bands (E(max) approximately 16300 cm(-1)) is a charge-transfer transition between the U(IV) center and the ketimido ligand(s). The observations of both charge-transfer transitions and resonance enhancement of Raman vibrational bands are exceedingly rare for tetravalent actinide complexes and reflect the strong bonding interactions between the uranium 5f/6d orbitals and those on the ketimido ligands.

Structure and bonding in cerium oxysulfide compounds. III—Electronic, infrared and resonance Raman spectra of and lattice dynamics calculations on mixed‐valence Ce4O4S3

The electronic (350–900 nm), infrared and Raman (700–30 cm-1) spectra of polycrystalline samples of the mixed-valence (Ce3+/Ce4+) oxysulfide compound Ce4O4S3 were investigated. The compound is a low-gap semiconductor with intervalence transitions in the near-infrared (ca. 850 nm) and visible (ca. 540 nm) regions, so that resonance-enhanced Raman spectra were obtained in the 450–650 nm range. From some polarized Raman data and a comparison with the vibrational results for Ce2O2S and Ce2.0O2.5S reported in Parts I and II, tentative infrared and Raman assignments are proposed. These results allow one mainly to localize the totally symmetry Ag modes and to suggest that the higher wavenumber signals in the 580–410 cm-1 range are due to the stretching vibrations of the shortest Ce4+—O bonds. In addition, complete lattice dynamic calculations on the orthorhombic structure (D2h9, Z=2) of Ce4O4S3 were performed by using a valence force field potential function transferred from the force fields previously obtained for Ce2O2S and Ce2.0O2.5S. With these calculations one can satisfactorily reproduce the whole experimental wavenumbers, propose more confident vibrational assignments and confirm that the Ce4+—O bond strengths are definitively stronger than the Ce3+—O counterparts. Such conclusions are corroborated by the establishment of the Raman excitation profiles which maximize near 568 nm. The largest enhancements are obtained for the totally symmetric modes at 509, 412 and 350 cm-1, which contain important potential energy distributions in the Ce4+–O stretching force constants, and at 132 cm-1, which corresponds to a deformation or compressional motion (along b) within the (Ce4O4)6+ units. These vibrations are thus the more effective coupling modes in the intervalence charge-transfer mechanisms. © 1997 John Wiley & Sons, Ltd.

UV-Excited Raman and Resonance Raman Spectra of Synthetic Polymers

High-quality, UV-excited (218–242 nm), fluorescence-free conventional Raman spectra have been generated from UV-transparent polymers such as polytetra-fluoroethylene (Teflon®), polyethylene, polypropylene, and polyoxymethylene (Delrin®). Spectra can be generated with the use of low exciting power (<1 mW), but even at low power, precautions have to be taken to prevent photo-oxidation and thermal degradation of samples. Polyvinylchloride polymers especially have been found to be prone to degradation. Weakly absorbing polymers such as nylon and polybutadiene show special promise for Raman analysis, because they exhibit significant preresonance enhancement of structurally significant modes, and because they still retain information derived from conventional Raman spectra. Strongly preresonance-enhanced ethylenic modes of polybutadiene isomers can be resolved, allowing the easy identification of cis and trans isomers. In contrast, strongly absorbing polymers such as polystyrene, polycarbonate, and polyphenolics show only intense, resonance-enhanced Raman spectra. Unfortunately, Raman spectra of these materials are limited to phenyl ring modes, and all valuable conventional Raman spectra are totally lost due to internal absorption.

Raman spectra of NOCl and Cl2 photodissociation dynamics

The resonance enhanced Raman spectra of NOCl and Cl2 were recorded at a range of laser wavelengths from 514nm to 355nm, in both static and flow cells. A progression of overtone transitions (up to ν′′ = 13) was observed in the Raman spectrum of Cl2 at 355nm. NO overtones were observed for the NOCl dissociation at any of the wavelengths investigated. This suggests that the NOCl photodissociation may not be as fast as was previously thought.

Synthesis, Crystal Structure and Spectroscopy of 3,4-Perylene-dicarboxylic Monoanhydride

Abstract The preparation, via a chemical vapor deposition process, of metallic-green whiskers of 3,4-perylenedicarboxylic monoanhydride (PDCMA), is described. The crystal structure and the electronic, infrared and resonance-enhanced Raman spectra of the PDCMA crystals are reported and compared with the corresponding data for the dianhydride, PTCDA. Interesting differences in the spectroscopy of PDCMA compared with that of PTCDA have been found. This has been correlated with the somewhat increased charge delocalization in PDCMA caused by the packing of the molecules in the crystal. The application of some of these results in characterizing the partly solid state, heat-induced polymerization of PTCDA to the intrinsically conducting polymer, polyperinaphthalene, is also pointed out.

Resonance Raman characterization of the heme prosthetic group in eosinophil peroxidase

The resonance-enhanced Raman spectrum of eosinophil peroxidase (EPO) from horse and human eosinophils is reported. Based upon the spectral energies, distribution and depolarization ratios of the high-frequency skeletal modes and upon the presence of weak bands assignable to vinyl substituent groups, we conclude that the heme prosthetic group is high-spin, 6 coordinate protoporphyrin. The Raman spectrum reveals clear differences from lactoperoxidase (LPO), an enzyme which appears nearly structurally isomorphous by other physical techniques; the data indicate a stronger axial 6th ligand in EPO. Mechanistic implications are discussed in relation to LPO and myeloperoxidase, an enzyme present in neutrophils and monocytes which contains a unique functional active-site chlorin.

Resonance Raman scattering by the excited singlet electronic state (S 1) of naphthalene

Time-resolved resonance-enhanced Raman spectrum of a photochemical transient of naphthalene, associated with broad electronic absorption in the = 425 nm region, is reported. The Raman signals at 1570, 1431, 1415 and 1388 cm −1, which decay at nanosecond times, are ascribed to this species. This transient is positively identified as the excited singlet (S 1) electronic state of naphthalene.

The time-resolved resonance raman spectrum of pulse radiolytically produced phenoxyl radical

The time-resolved resonance-enhanced Raman spectrum of phenoxyl radical has been observed at microsecond times in pulse radiolysis experiments. Raman b

Infrared and resonance enhanced Raman spectra of some metal vic-dioximes with short hydrogen bonds

Infrared and enhanced Raman spectra were recorded of bis(2,3-butanonedioximato-N,N′) nickel (II) and palladium (NiDMG, PdDMG) as well as the i.r. spectra of bis(ethanedialdioximato-N,N′) nickel (II) (NiHG), bis(2,3-pentanonedioximato-N,N′) nickel (II) (NiEMG), and of bis(2,3-butanonedioximato-N,N′) copper (II) (CuDMG). The mutual exclusion of i.r. and Raman active modes of NiDMG and PdDMG prove the existence of an inversion centre whereas the coupling of the in-plane OH deformation with two skeletal modes of CN stretching character indicates that the molecular symmetry is likely to be C 2 h only. The B u OHO stretching mode has been identified in the nickel complexes with a strong, broad band near 900 cm −1. The resonance Raman enhancement is observed mainly with skeletal vibrations of CN character and, less, with those having NO character. The effects of high pressure upon i.r. spectra were observed. The possible symmetry of the hydrogen bond is discussed.

Identification of enzyme coupling sites with aromatic diazonium salts—A resonance Raman study

The coupling reaction of diazonium salts of aromatic compounds with the aromatic residues of proteins results in chromophoric covalent derivatives which yield strong resonance enhanced Raman spectra. The protein residues modified by these coupling reactions have been identified using the ν(NN) and ν(N-φ) vibrational bands in the resonance Raman spectra. Previous studies have established that diazoarsanilic acid couples with carboxypeptidase at tyrosine 248. The resonance Raman spectrum of arsanilazocarboxypeptidase was compared with spectra of arsanilazotyrosine and arsanilazohistidine model compounds; the results are consistent only with coupling at a tyrosine residue. This confirmation of the previously established site of modification establishes the utility of resonance Raman spectroscopy as a tool for identification of the site of covalent modification. To further investigate this approach, the diazonium salt of sulfanilamide (a site-specific reagent) was used to prepare a covalent coupling derivative of bovine carbonic anhydrase. The coupling reaction appears to have a stoichiometry of 1:1 and results in nearly complete loss of sulfanilamide binding capability and esterase activity. Comparison of the pH dependence of the resonance Raman spectra of sulfanilazocarbonic anhydrase with the spectra of sulfanilazotyrosine, sulfanilazohistidine, and sulfanilazotryptophan suggests that histidine is the site of modification of this new carbonic anhydrase derivative.

Carotenoids as raman-active probes of erythrocyte membrane structure

1. 1. Erythrocyte ghosts exhibit resonance-enhanced Ramand bands at 1530 cm −1 and 1165 cm −1 attributable to ν(-CC-) and ν(C-C), respectively, of the conjugated polyene chains in carotenoids. In lipid extract of ghosts, these resonance-enhanced bands lie at 1527 and 1158 cm −1. The spectra indicate the presence of membrane-bound β-carotene. 2. 2. The resonance-enhanced Raman spectrum of β-carotene in lecithin liposomes is identical to that obtained with hexane or chloroform solutions. 3. 3. Increasing proportions of cholesterol in cholesterol-lecithin liposomes up to a cholesterol: phospholipid molar ratio of 0.8–0.9 drastically decreases the intensity of both resonance-enhanced bands. 4. 4. In ghosts the carotenoid bands respond to membrane perturbations. Trypsinization, lysolecithin treatment and reduction of pH increase the intensities of the 1530 and 1165 cm −1 bands. In contrast, a decrease in the intensity of both bands follows equilibration of ghosts for 15 min at approx. 50 °C or addition of (0.1%) sodium dodecyl sulfate. 5. 5. We suggest that pertubants known to change lipid-protein interactions in erythrocyte membranes modify the microenvironment and/or configuration of the membrane-bound carotenoid.

The copper coordination group in "blue" copper proteins: evidence from resonance Raman spectra.

Tunable dye laser excitation in the intense similar to 600-nm absorption band of azurin, plastocyanin, and ceruloplasmin provides resonance enhanced Raman spectra. They consist of a complex set of bands, at least three or four in number, between 350 and 473 cm-1, which are assignable to Cu-N or Cu-O bond stretching, and a weak band near 270 cm-1, which probably arises from Cu-S stretching. A weak band at 765 cm-1 found in plastocyanin may arise from C-S stretching. Analysis of the Raman intensity pattern, as well as of the nature of the resonant electronic transition, leads to a model of the "blue" copper site involving approximately trigonal-bipyramidal coordination, with a sulfur and two nitrogen ligands in the equatorial plane, and less strongly bound nitrogen or oxygen ligands at axial positions. This arrangement would be well poised for stabilization of Cu(I) upon reduction.