Stereochemical studies on the formation of melanin by monophenol monooxygenase

The oxidation of tyrosine by monophenol monooxygenase (tyrosinase: EC 1.10.3.1) to melanin has been studied by a combination of ultraviolet, circular dichroism, and nuclear magnetic resonance techniques. It is demonstrated that the chiral intermediate (dopachrome) is generated stereoselectively in this enzymic reaction.     

光学活性金属卟啉的磁手性效应研究

磁光学活性与自然光学活性均可用介质对左右圆偏振光的吸收之差来表示 .但自然光学活性和磁光学活性的物理机制是截然不同的 ,前者源于镜象不能互相重叠介质的非定域光学响应 ( nonlocaloptical response) ,而后者则是由于介质的时间反演对称性 ( time- reversal symmetry)被磁场打破所致 .理论分析表明 ,当介质的两种光学活性同时存在时 ,将会出现一个新的附加光学效应 ,这种磁光学活性与自然光学活性之间的交叉效应称为磁手性效应 ( magneto- chiral effect)或磁手二色性 ( magneto-chiral dichroism) [1,2 ] .磁手性效应通常很弱 ,直到 1…     

Interplay between size, composition, and phase transition of nanocrystalline Cr(3+)-doped BaTiO3 as a path to multiferroism in perovskite-type oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.     

Determination of Aristocularine Enantiomers by Dispersive Liquid–Liquid Microextraction and Chiral High-performance Liquid Chromatography

Here is reported a novel analytical approach for the extractive separation and determination of enantiomeric ratios of aristocularine in bovine serum albumin. The results demonstrate suitable analytical performances. The separation was performed by chiral high-performance liquid chromatography with a 5-µm column using a mobile phase of 1:1 n-hexane:ethanol at a flow rate of 0.7?mL?min^?1 with ultraviolet–visible absorption, circular dichroism, and polarimetric detection. The enantiomers were eluted at 13.2 and 15.6?min for (+) and (?)-aristocularine, with a resolution of 1.58 and a separation factor of 1.27. The analytical parameters for the dispersive liquid–liquid microextraction were optimized; under these conditions, the extraction recoveries were from 88.6% to 93.9% for a two-step extraction. The precision, reported as the percent relative standard deviation, had values from 2.9% to 3.2% for 0.5?µg?mL^?1 of analyte for five replicate measurements using ultraviolet–visible absorption and circular dichroism detection. The limits of detection were between 0.05 and 0.08?µg?mL^?1 with enrichment ratios up to a value of 12.     

Complex polarization propagator calculations of magnetic circular dichroism spectra

It is demonstrated that the employment of the nonlinear complex polarization propagator enables the calculation of the complete magnetic circular dichroism spectra of closed-shell molecules, including at the same time both the so-called Faraday A and B terms. In this approach, the differential absorption of right and left circularly polarized light in the presence of a static magnetic field is determined from the real part of the magnetic field-perturbed electric dipole polarizability. The introduction of the finite lifetimes of the electronically excited states into the theory results in response functions that are well behaved in the entire spectral region, i.e., the divergencies that are found in conventional response theory approaches at the transition energies of the system are not present. The applicability of the approach is demonstrated by calculations of the ultraviolet magnetic circular dichroism spectra of para-benzoquinone, tetrachloro-para-benzoquinone, and cyclopropane. The present results are obtained with the complex polarization propagator approach in conjunction with Kohn-Sham density functional theory and the standard adiabatic density functionals B3LYP, CAM-B3LYP, and BHLYP.     

Circular Dichroism and Isotropy – Polarity Reversal of Ellipticity in Molecular Films of 1,1’-Bi-2-Naphtol

We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1’-Bi-2-naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.     

DIFFRACTION AND HOLOGRAPHY WITH PHOTOELECTRONS AND FLUORESCENT X-RAYS

We consider studies of the atomic and magnetic structure near surfaces by photoelectron diffraction and by the holographic inversion of both photoelectron diffraction data and diffraction data involving the emission of fluorescent x-rays. The current status of photoelectron diffraction studies of surfaces, interfaces, and other nanostructures is first briefly reviewed, and then several recent developments and proposals for future areas of application are discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low energy electron diffraction and scanning tunneling microscopy, to the epitaxial growth of oxides and metals is considered. Several new avenues that are being opened up by third-generation synchrotron radiation sources are also discussed. These include site-resolved photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements of surface reactions with chemical-state resolution, and circular dichroism in photoelectron angular distributions from both non-magnetic and magnetic systems. The addition of spin to the photoelectron diffraction measurement is also considered as a method for studying short-range magnetic order, including the measurement of surface magnetic phase transitions. This spin sensitivity can be achieved through either core-level multiplet splittings or circular-polarized excitation of spin-orbit-split levels. The direct imaging of short-range atomic structure by both photoelectron holography and two distinct types of x-ray holography involving fluorescent emission is also discussed. Both photoelectron and x-ray holography have demonstrated the ability to directly determine at least approximate atomic structures in three dimensions. Photoelectron holography with spin resolution may make it possible also to study short-range magnetic order in a holographic fashion. Although much more recent in its first experimental demonstrations, x-ray fluorescence holography should permit deriving more accurate atomic images for a variety of materials, including both surface and bulk regions.     

Synchrotron radiation circular dichroism spectroscopy of proteins and applications in structural and functional genomics

The technique of Synchrotron Radiation Circular Dichroism (SRCD) spectroscopy and its advantages over conventional circular dichroism spectroscopy are described in this tutorial review, as well as recent applications of the technique in structural and functional genomics.Circular dichroism (CD) spectroscopy is a well-established method in biological chemistry and structural biology, but its utility can be limited by the low flux of the light source in the far ultraviolet and vacuum ultraviolet wavelength regions in conventional CD instruments. The development of synchrotron radiation circular dichroism (SRCD), using the intense light of a synchrotron beam, has greatly expanded the utility of the method, especially as a tool for both structural and functional genomics. These applications take advantage of the enhanced features of SRCD relative to conventional CD: the ability to measure lower wavelength data containing more electronic transitions and hence more structural information, the higher signal-to-noise hence requiring smaller samples, the higher intensity enabling measurements in absorbing buffers and in the presence of lipids and detergents, and the ability to do faster measurements enabling high throughput and time-resolved spectroscopy.This article discusses recent developments in SRCD instrumentation, software, sample preparation and methods of analyses, with particular emphasis on their applications to the study of proteins. These advances have led to new applications in structural genomics (SG), including the potential for fold recognition as a means of target selection and the examination of membrane proteins, a class of proteins usually excluded from SG programmes. Other SG uses include detection of macromolecular interactions as a screen for complex formation, and examination of glycoproteins and sugar components. In functional genomics (FG) new applications include screening for ligand binding as a means of identifying function, and examination of structural differences in mutant proteins as a means of gaining insight into function.     

Circular dichroism effect for linear molecules induced by a resonant circularly polarized pumping optical field

In this paper we propose and discuss the laser-induced circular dichroism (LICD) effect, which is expected to occur in linear molecules pumped by a strong circular resonant light beam. The effect is to be detected via the absorption of a weak circularly polarized probe beam on another transition. Analogous to the external magnetic field in magnetic circular dichroism the resonant circular polarized pumping optical field can induce the nonzero antisymmetric rotational polarizabilities of a linear molecule, and cause the LICD effect. LICD contains three distinct contributions from M-dependent splittings of the sublevels mid R:JM due to the ac Stark effect, from the differences of Boltzmann statistical distributions among the ground state sublevels mid R:JM due to the ac Stark splittings, and from the changes of occupation probability in rotational sublevels mid R:JM due to the pumping effect. The fundamental formulas for the above three terms of LICD have been deduced by the density matrix method. As an example, the LICD for CO molecules have been calculated. The results indicate that in comparison with the rotationally resolved magnetic vibrational circular dichroism experiment, LICD may be measurable and form a basis of a different kind of CD spectroscopy.     

Spectroscopic and computational studies on iron and manganese superoxide dismutases: nature of the chemical events associated with active-site pKs

A combined spectroscopic/computational approach has been utilized to explore the chemical origins of the active-site pKs of the structurally homologous Fe- and Mn-dependent superoxide dismutases (SODs). Absorption, circular dichroism, magnetic circular dichroism, and variable-temperature, variable-field magnetic circular dichroism spectroscopic experiments have permitted us to determine electronic transition energies and polarizations, as well as ground-state spin Hamiltonian parameters. These experimental data have been used in conjunction with semiempirical intermediate neglect of differential overlap/spectroscopic parametrization configuration interaction (INDO/S-CI) computations for evaluating hypothetical active-site models for the high-pH species generated by density functional theory (DFT) geometry optimizations. Our experimental and computational data indicate that both reduced FeSOD and oxidized MnSOD do not bind hydroxide at high pH; rather, the active-site pK for these two species is attributed to deprotonation of a second-sphere tyrosine. Conversely, our data obtained on oxidized FeSOD indicate that hydroxide binding is responsible for the observed active-site pK for this species. Intriguingly, in the Fe-substituted form of MnSOD this identical chemical event occurs at a significantly lower pH. Overall, our results suggest an important role for second-sphere amino acids in tuning the active sites' interaction with small anions and bring into question the assumption that these homologous enzymes operate by the same molecular mechanism.     

XMCD for monitoring exchange interactions. The role of the Gd 4f and 5d orbitals in metal-nitronyl nitroxide magnetic chains

We report here the X-ray magnetic circular dichroism (XMCD) study at the Gd M(4,5)- and L(2,3)-edges of two linear magnetic chains involving Gd(III) cations bridged by nitronyl nitroxide radicals. This spectroscopy directly probes the magnetic moments of the 4f and 5d orbitals of the gadolinium ions. We compare macroscopic magnetic measurements and local XMCD signals. The M(4,5)-edges results are in agreement with the J values extracted from the fits of the SQUID magnetic measurements. The L(2,3)-edges signals show that the electronic density in the Gd 5d orbitals depends on the neighbors of the gadolinium cations. Nevertheless, the 5d orbitals do not seem to play any role in the superexchange pathway between radicals through the metal ion proposed to explain the particular magnetic exchange interactions between the radicals in these chains.     

Core electron transitions as a probe for molecular chirality: natural circular dichroism at the carbon K-edge of methyloxirane

We have measured for the first time the X-ray natural circular dichroism (XNCD) of a chiral molecule in an isotropic medium. In this condition the only surviving term contributing to CD is the cross product between the electric dipole and the magnetic dipole transition moments. The non-zero value of the magnetic dipole transition moment in a 1s-to-valence electron transition is attributed to contribution of valence states to core molecular orbitals. These results open the way to a "local" chiral molecular analysis and to the study of stereochemically selected photochemical processes.     

Hamaker Constants of Systems Involving Water Obtained from a Dielectric Function That Fulfills the f Sum Rule

Spectroscopic methods provide a powerful tool for studying the properties of proteins at interfaces. The protein accumulated in one adsorbed layer is frequently less than the minimum mass of protein required by a detection method. In such a case (as is the case in circular dichroism spectroscopy) the sorbent material is usually supplied as dispersion. However, light scattering by the dispersed particles often interferes with the measurement of the circular dichroism of the protein. Therefore, there is a strong need for an experimental setup that enables these measurements to be made using flat surfaces. An example of such a setup is the multiplate quartz cell presented here. The potential of this multiplate quartz cell is shown by some preliminary circular dichroism measurements of IgG adsorbed on different types of surfaces. Copyright 2000 Academic Press.     

Radiation-induced chemical evolution of biomolecules

Chemical evolution in glycilglycine (Gly2) films irradiated with 146 nm vacuum ultraviolet light was studied. It is found that quantum efficiency of chemical evolution from Gly2 to glycilglycilglycine (Gly3) is smaller than that to glycilglycilglycilglycine (Gly4) due to the multiple step of reaction. Furthermore, we have carried out measurement of soft X-ray natural circular dichroism spectra for serine and alanine films in the energy region of oxygen 1s transition and we report the splitting of 1s→π* transitions.     

Absorption and magnetic circular dichroism spectra of matrix isolated Ta atoms

The absorption and magnetic circular dichroism (MCD) spectra of Ta atoms isolated in an argon matrix at 14 K have been measured. Over sixty-five MCD bands have been observed and used together with gas phase intensity and energy pattern data to reassign the matrix spectrum. The observed matrix shift (average: 765 cm^?1) to the blue is smaller than determined by previous workers. From this study it is suggested that the assignment of matrix-isolated atomic species can be considerably aided by the determination of their magnetic circular dichroism spectra.     

Electronic circular dichroism spectra from the complex polarization propagator

The complex linear polarization propagator approach has been applied to the calculation of electronic circular dichroism spectra of 3R-chloro-1-butyne, 3R-methylcyclopentanone, 3S-methylcyclohexanone, 4R-1,1-dimethyl-[3]-(1,2)ferrocenophan-2-on, S-3,3,3',3'-tetramethyl-1,1'-spirobi[3H,2,1]-benzoxaselenole, and the fullerene C84. Using time-dependent Kohn-Sham density functional theory, it is shown that a direct and efficient evaluation of the circular dichroism spectrum can be achieved. The approach allows for the determination of the circular dichroism at an arbitrary wavelength thereby, in a common formulation and implementation, covering the visible, ultraviolet, and x-ray regions of the spectrum. In contrast to traditional methods, the entire manifold of excited states is taken into account in the calculation of the circular dichroism at a given wavelength.     

Facile synthesis and high optical activity of poly(1‐pentyne)s carrying amino‐acid pendant groups

1‐Pentynes containing different amino acid moieties and pendant terminal groups {HC?C(CH₂)₂CONHC(R′)HCO₂CH₃, where R′ = CH₃, CH₂CH(CH₃)₂, CH₂C₆H₅, and HC?C(CH₂)₂CONHC[CH₂CH(CH₂)₃]HCO₂‐(1R,2S,5R)‐(+)‐menthol} have been designed and synthesized. The polymerizations of the monomers are effected by organorhodium catalysts, giving soluble polymers with moderate molecular weights in satisfactory yields. The structures and properties of the polymers have been characterized and evaluated with infrared, nuclear magnetic resonance, thermogravimetric analysis, circular dichroism, and ultraviolet analyses. All the polymers are thermally stable (≥300 °C) and show strong circular dichroism signals at ～310 nm because of the helicity of the polyene backbone. The circular dichroism and ultraviolet absorptions of the polymers can be tuned with a solvent. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6190–6201, 2006     

Theory of electronic circular dichroism lineshapes

A microscopic, quantum field theory of lineshapes for electronic circular dichroism spectra is presented. A simple, model Hamiltonian for a single impurity in a crystal is considered. In this formalism, electron-photon coupling terms contribute directly to the magnetic transition dipole moment. Lineshape functions for absorbance and circular dichroism spectra are derived. Electronic circular dichroism spectra contain vibronic contributions which do not appear in absorbance spectra. This treatment does not require perturbation theory to obtain the vibrational contribution to the circular dichroism lineshape.     

Systematic assignment of the configuration of flexible natural products by spectroscopic and computational methods: the bistramide C analysis

[reaction: see text] The combination of NMR NOE, chemical shift, and J-coupling measurements with molar rotation and circular dichroism (CD) determinations, including RI-DFT BP86/aug-cc-pVDZ calculations, reduced a candidate pool of 1024 possible stereoisomers of (+)-bistramide C to a single absolute configuration assignment for the 10 stereogenic carbons of the marine natural product.