Quantum Monte Carlo study of the CO interaction with a dimer model surface for Cr(110)

The chemisorption of CO on a Cr (110) surface is investigated using the quantum Monte Carlo method in the diffusion Monte Carlo (DMC) variant and a model Cr₂CO cluster. The present results are consistent with the earlier ab initio HF study with this model that showed the tilted/near-parallel orientation as energetically favoured over the perpendicular arrangement. The DMC energy difference between the two orientations is larger (1.9 eV) than that computed in the previous study. The distribution and reorganization of electrons during CO adsorption on the model surface are analysed using the topological electron localization function method that yields electron populations, charge transfer and clear insight on the chemical bonding that occurs with CO adsorption and dissociation on the model surface.     

Characterization of aromatic-thiol π-type hydrogen bonding and phenylalanine-cysteine side chain interactions through ab initio calculations and protein database analyses

In this study, the aromatic-thiol π hydrogen bonding and phenylalanine-cysteine side chain interactions are characterized through both molecular orbital calculations on a C₆H₆-HSCH₃ model complex and database analyses of 609 X-ray protein structures. The aromatic-thiol π hydrogen bonding interaction can achieve a stabilization energy of 2.60 kcal mol^?1, and is stronger than the already documented aromatic-hydroxyl and aromatic-amino hydrogen bonds. However, the occurrence of the aromatic-thiol hydrogen bond is rather rare in proteins. This is because most of the thiol groups participate in the formation of either disulphide bonds or stronger S—H…O (or N) ‘normal’ hydrogen bonds in a protein environment. Interactions between the side chains of phenylalanine and cysteine residues are characterized as the phenyl(Phe)(HSCH₂-)(Cys) interaction. The bonding energy for such interactions is approximately 3.71 kcal mol-¹ and is achieved in a geometric arrangement with an optimal phenyl(Phe)-(HS-)(Cys) π-type hydrogen bonding interaction. The interaction is very sensitive to the orientation of the two lone electron pairs on the sulphur atom relative to the π electron cloud of the phenyl ring. Accordingly, the interaction configurations that can accomplish a significant bonding energy exist only within a narrow configurational space. The database analysis of 609 experimental X-ray protein structures demonstrates that only 268 of the 1620 cysteine residues involve such phenylalanine-cysteine side chain interactions. Most of these interactions occur in the form of π (aromatic)-lone pair(sulphur) attractions, and correspond to a bonding energy less than 1.5 kcal mol^?1. A few were identified as the aromatic-thiol hydrogen bond with a bonding energy of 2.0–3.6 kcal mol^?1.     

Nanoscaled ZnO films used as enhanced substrates for fluorescence detection of dyes

The ability of nanoscaled ZnO films to enhance fluorescence was studied.We found that the fluorescence intensities of Cy5,rhodamine 6G,and fluorescein can be enhanced about 10-fold on nanoscaled ZnO films as compared to that on glass substrates.The lifetimes of all samples were measured,and no obvious change in lifetime was observed for dyes on different substrates.The mechanism for the nanoscaled ZnO film enhanced fluorescence appears to be different from that for the metal-fluorophore systems.     

Highly Efficient Detection of Single Fluorophores in Blood Serum Samples with High Autofluorescence

Single molecule detection (SMD) is usually performed on surface-immobilized molecules with diffraction-limited observation volumes, typically with confocal optics to suppress background from the sample and instrument. In this paper we consider the more difficult task of detecting single fluorophores in the presence of a substantial fluorescence background. We determined that for a readily accessible macroscopic observation volume of 1 pL that the background from undiluted blood serum was approximately equal to 2700 Cy5 molecules, and the background from whole blood equal to about 14 000 Cy5 molecules in whole blood. These high backgrounds appear to preclude the possibility of SMD of Cy5 molecules. However, we show that the signal-to-noise ratio (SNR) in high background samples can be increased dramatically by reduction of the observed volume. We were able to detect single surface-bound Cy5-labeled DNA (Cy5-DNA) oligomers in diluted blood serum with an SNR near 40. We also examined freely diffusing Cy5-DNA in blood serum. The data showed that single Cy5-DNA molecules could be detected even in the undiluted serum. We further investigated the SNR on silver island films. We found that the fluorescence signal was greatly enhanced in the presence of metallic nanostructures showing a larger SNR in the application tested. These results suggest the possibility of clinical assays based on SMD in blood serum and possibly whole blood. Increased SNR near metallic nanostructure could probably overcome the need for diffraction-limited volumes and enhance our ability to do in situ SMD.     

Fluorescence Anisotropy Controlled by Light Quenching*

We demonstrated that fluorescence anisotropy can be effectively decreased or increased in the presence of light quenching, depending on relative polarizations of excitation and quenching pulses. For parallel light quenching, anisotropy decreases to 0.103 and z-axis symmetry is preserved. In the presence of perpendicular light quenching, the steady-state anisotropy of a pyridine-2-glycerol solution increases from 0.368 for an unquenched sample to 0.484 for a quenched one. We show that the angular distribution of transition moments loses z-axis symmetry in the presence of perpendicular light quenching. In these cases we used more general definitions of anisotropy. Induced by light quenching, anisotropy can be applied in both steady-state and time-resolved measurements. In particular, the systems with low or no anisotropy can be investigated with the proposed technique.     

Fluorescence intensity and anisotropy decays of the intrinsic tryptophan emission of hemoglobin measured with a 10-GHz fluorometer using front-face geometry on a free liquid surface

We measured the intensity and anisotropy decays of the intrinsic tryptophan emission from hemoglobin solutions obtained using a 10-GHz frequency-domain fluorometer and a specially designed cuvette which allows front-face excitation on a free liquid surface. The cuvette eliminates reflections and stray emissions, which become significant for low-intensity fluorescence such as in hemoglobin. Three lifetimes are detectable in the subnanosecond range. The average lifetime of hemoglobin emission is ligand dependent. The measured values of average lifetimes are 91, 174, and 184 ps for deoxy-, oxy-, and carboxyhemoglobin, respectively. Fluorescence anisotropy decays of oxy-, deoxy-, and carbonmonoxyhemoglobin can be fitted with up to three correlation times. When three components are used, the floating initial anisotropyr _o is, in each case, higher than the steady-state anisotropy of tryptophan in vitrified solution. For deoxy hemoglobin it is close to 0.4. The data are consistent with an initial loss of anisotropy from 0.4 to about 0.3 occurring in the first 2 ps.     

Fluorescence Spectral Properties of Indocyanine Green on a Roughened Platinum Electrode: Metal-Enhanced Fluorescence

The interactions of fluorophores with noble metal particles can modify their emission spectral properties, a relatively new phenomenon in fluorescence. We subsequently examined indocyanine green (ICG), which is widely used in medical testing and imaging, in close proximity to an electrically roughened platinum electrode. The emission intensity and lifetimes were decreased about 2-fold on the roughened surface as compared to a smooth Pt surface, and the photostability about the same. Platinum does not appear promising for metal enhanced fluorescence, at least for long wavelength fluorophores.     

Spectroscopic and Photophysical Characterization of Fluorescent Chemosensors for Monosaccharides Based on N-Phenylboronic Acid Derivatives of 1,8-Naphthalimide

Spectroscopic and photophysical properties of two fluorescent probes for monosaccharides are presented. Probes are based on the N-phenyl-1,8-naphthalimide structure having the boronic acid group [R-B(OH)₂] in ortho in one case, and meta in the other case, positions of the N-phenyl group. Formation of the anionic form of the boronic acid group [R-B(OH)^– ₃] induced a substantial decrease of the steady-state fluorescence of both compounds. Because no change in the fluorescence lifetime from the neutral to the anionic forms is observed, static quenching resulting from photoinduced electron transfer from the anionic form of the boronic acid is used to explain the decrease of the emission intensity. Both compounds show substantial decreases of their fluorescence intensity in the presence of sugars. In addition, this decrease of the fluorescence intensity is associated with an increase of the fluorescence lifetime for the ortho derivative while no effect on the lifetime is observed for the meta derivative. Both photoinduced electron transfer and steric hindrance are discussed to correlate the observed results.     

A Non-linear Singular Perturbation Problem Arising in the Study of Chemical Flow Reactors

This paper presents a method of obtaining the complete asymptoticsolution of boundary value problems of the form for x [0,1] where b(x) is strictly positive andfor small and positive. Physically, the problem arises in determiningthe steady-state concentration of a substance in a chemicalflow reactor. A "two-variable" expansion procedure is used.