Retinal in bacteriorhodopsin and related molecular models investigated with Raman spectroscopy and density functional theory calculations

A first‐principles assisted study of the Raman spectrum associated with the photoactive chromophore in bacteriorhodopsin (bR) allowed the elucidation of the effective coupling existing between the π electrons of the retinylidene chromophore and the local environment of the Schiff base. The role of the counter‐ion on the electronic and vibrational structure of the chromophore is investigated by means of density functional theory (DFT) calculations. Several molecular models of the retinylidene chromophore interacting with different counter‐ions facing the Schiff base have been considered. The counter‐ion induces a strong modulation of the position of both the UV–vis absorption maximum and the strong Raman active collective CC stretching mode, related to the effective conjugation coordinate (ECC). Experimental Raman and UV–vis absorption data are interpreted in the light of these theoretical findings. The data collected in this paper provide an interesting accumulation of points along the optical gap versus ECC wavenumber relationship. Copyright © 2011 John Wiley & Sons, Ltd.     

Resonance Raman contribution to the D band of carbon materials: modeling defects with quantum chemistry

Polycyclic aromatic hydrocarbons (PAHs) are employed to model the Raman features that are generally associated with sp(2) nanostructures in carbon materials or with disorder and defects in graphitic materials. To this end molecular parameters (geometry changes upon electronic excitation, vibrational normal modes, and displacement parameters) are computed with semiempirical quantum-chemical methods for a series of PAHs ranging from 6 to 384 carbon atoms, and Raman intensities are evaluated according to Albrecht's formalism restricted to the A term. The computed preresonance and resonance Raman intensities are compared with available experimental data for hexa-peri-hexabenzocoronene and for pyrene. For the latter compound, simulations carried out at semiempirical and ab initio levels of theory are shown to be of comparable quality. Finally, the collection of displacement parameters computed for the sample of conjugated molecules is used to model the effect of disorder and defects in the Raman response of a carbon material containing sp(2) islands. It is shown that the computed D-band frequency dispersion, with respect to excitation wavelength, reproduces closely the experimental data measured for sp(2) hybridized carbon materials.     

Differential pulse voltammetric determination of tin in the presence of noble metals

A voltammetric method for the determination of tin is proposed to minimise interferences from noble metals that are commonly encountered with other analytical techniques. Strong distortions of voltammetric peaks are observed in the presence of platinum. On the basis of a full investigation, the formation of an intermediate Sn(II)–Pt mixed chloro-complex at the electrode surface is identified as being responsible for the platinum interference, as it competes with the normal Sn(IV)→Sn(0)_Hg reduction. The use of a higher scan rate prevents the relatively low reaction kinetics and thus gets rid of this interference. No problems are encountered with other noble metals such as Pd, Ir, Re, Rh and Ru when using the modified method, although a baseline subtraction is necessary for the latter one. The proposed method is validated with real Pt–Sn catalysts.     

Purification and characterization of two xylanases from alkalophilic and thermophilic Bacillus licheniformis 77-2

The alkalophilic bacteria Bacillus licheniformis 77-2 produces significant quantities of thermostable cellulase-free xylanases. The crude xylanase was purified to apparent homogeneity by gel filtration (G-75) and ionic exchange chromatography (carboxymethyl sephadex, Q sepharose, and Mono Q), resulting in the isolation of two xylanases. The molecular masses of the enzymes were estimated to be 17 kDa (X-I) and 40 kDa (X-II), as determined by SDS-PAGE. The K _m and V _max values were 1.8 mg/mL and 7.05 U/mg protein (X-I), and 1.05 mg/mL and 9.1 U/mg protein (X-II). The xylanases demonstrated optimum activity at pH 7.0 and 8.0–10.0 for xylanase X-I and X-II, respectively, and, retained more than 75% of hydrolytic activity up to pH 11.0. The purified enzymes were most active at 70 and 75°C for X-I and X-II, respectively, and, retained more than 90% of hydrolytic activity after 1 h of heating at 50°C and 60°C for X-I and X-II, respectively. The predominant products of xylan hydrolysates indicated that these enzymes were endoxylanases.     

Spectroscopic behaviour, bond properties and charge distribution in methoxy groups in hydrofluoroethers: the effect of neighbouring CF2 group

A theoretical investigation is presented aimed to the interpretation of the spectroscopic behaviour of the methoxy group in molecules belonging to the class of hydrofluoroethers. The simulation of infrared and Raman spectra of four different stable conformers of CH₃–O–CF₂–CF₂–O–CH₃ and the comparison with the experimental spectra allow to propose a vibrational band assignment in the CH stretching region. This clarifies the role of the CF₂ group in determining the electronic properties and spectroscopic parameters of methyl CH bonds when back-donation of electronic charge take place from oxygen.     

Invariant syzygies for the Hermitian Dirac operator

This paper is devoted to the algebraic analysis of the system of differential equations described by the Hermitian Dirac operators, which are two linear first order operators invariant with respect to the action of the unitary group. In the one variable case, we show that it is possible to give explicit formulae for all the maps of the resolution associated to the system. Moreover, we compute the minimal generators for the first syzygies also in the case of the Hermitian system in several vector variables. Finally, we study the removability of compact singularities. We also show a major difference with the orthogonal case: in the odd dimensional case it is possible to perform a reduction of the system which does not affect the behavior of the free resolution, while this is not always true for the case of even dimension. A. Damiano is a postdoctoral fellow at the Eduard Čech Center and is supported by the relative grants. D. Eelbode is a postdoctoral fellow supported by the F.W.O. Vlaanderen (Belgium).