Dor Ben-Amotz

Water structural transformation at molecular hydrophobic interfaces

Hydrophobic hydration is considered to have a key role in biological processes ranging from membrane formation to protein folding and ligand binding. Historically, hydrophobic hydration shells were thought to resemble solid clathrate hydrates, with solutes surrounded by polyhedral cages composed of tetrahedrally hydrogen-bonded water molecules. But more recent experimental and theoretical studies have challenged this view and emphasized the importance of the length scales involved. Here we report combined polarized, isotopic and temperature-dependent Raman scattering measurements with multivariate curve resolution (Raman-MCR) that explore hydrophobic hydration by mapping the vibrational spectroscopic features arising from the hydrophobic hydration shells of linear alcohols ranging from methanol to heptanol. Our data, covering the entire 0–100 °C temperature range, show clear evidence that at low temperatures the hydration shells have a hydrophobically enhanced water structure with greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water. This structure disappears with increasing temperature and is then, for hydrophobic chains longer than ∼1 nm, replaced by a more disordered structure with weaker hydrogen bonds than bulk water. These observations support our current understanding of hydrophobic hydration, including the thermally induced water structural transformation that is suggestive of the hydrophobic crossover predicted to occur at lengths of ∼1 nm (refs 5, 9, 10, 14).

Quantitative vibrational imaging by hyperspectral stimulated Raman scattering microscopy and multivariate curve resolution analysis.

Spectroscopic imaging has been an increasingly critical approach for unveiling specific molecules in biological environments. Toward this goal, we demonstrate hyperspectral stimulated Raman loss (SRL) imaging by intrapulse spectral scanning through a femtosecond pulse shaper. The hyperspectral stack of SRL images is further analyzed by a multivariate curve resolution (MCR) method to reconstruct quantitative concentration images for each individual component and retrieve the corresponding vibrational Raman spectra. Using these methods, we demonstrate quantitative mapping of dimethyl sulfoxide concentration in aqueous solutions and in fat tissue. Moreover, MCR is performed on SRL images of breast cancer cells to generate maps of principal chemical components along with their respective vibrational spectra. These results show the great capability and potential of hyperspectral SRL microscopy for quantitative imaging of complicated biomolecule mixtures through resolving overlapped Raman bands.

Optical imaging of metastatic tumors using a folate-targeted fluorescent probe

We describe the use of a tumor targeting ligand, the vitamin folic acid, to deliver an attached fluorescent probe to both primary and metastatic tumors overexpressing the folate receptor. Upon laser excitation, derived images of normal tissues generally show little or no fluorescence, whereas images of folate receptor-expressing tumors display bright fluorescence that can be easily distinguished from adjacent normal tissue. Furthermore, metastatic tumor loci of submillimeter size can also be visualized without the aid of image processing or enhancement. The sharp distinction between tumor and normal tissues provided by this technique could find application in the localization and resection of tumor tissue during surgery or in the enhanced endoscopic detection and staging of cancers.

Evaluation of folate conjugate uptake and transport by the choroid plexus of mice

Purpose. Because the choroid plexus (CP) is enriched in cell surface folate receptors, an investigation was initiated to evaluate whether folate receptor-mediated transcytosis might be exploited to deliver folate conjugates into the brain.Methods. Balb/c mice were injected with radioactive and fluorescent conjugates of folate to measure and image their uptake by the CP.Results. Retention of a radioactive folate conjugate, folate-diethylenetriaminepentaacetic acid (DTPA)-111In, into the brain of balb/c mice was observed, although repeated injections or prolonged release via an osmotic pump of the compound did not result in increased brain uptake. Uptake of an 125I-labeled anti-folate receptor antibody into the brain was very low, and no competition was observed with unlabeled antibody. Imaging of brain thin-sections and whole brain tissue from a mouse injected with folate-fluorescein revealed strong fluorescence in the CP, but virtually no where else in the brain.Conclusions. Both fluorescence and radioimaging results demonstrate specific uptake of small molecular weight folate conjugates into CP cells of the murine brain, but no significant transport of the molecules across the CSF. Furthermore, no uptake of larger folate-linked proteins by choroid plexus cells is observed, suggesting folate conjugate size may strongly influence access to CP folate receptors.

Solvent and pressure‐induced perturbations of the vibrational potential surface of acetonitrile

Raman‐scattering studies at both ambient pressures and in a high‐pressure diamond‐anvil cell are used to measure gas‐to‐liquid vibrational frequency shifts of three normal modes of acetonitrile, CH3CN (ν1, CH stretch; ν2, CN stretch; and ν4, CC stretch) dissolved in various solvents (methylenechloride, chloroform, carbontetrachloride, toluene, pyridine, acetone, and methanol). The results are compared with calculated repulsive and attractive solvation force‐induced perturbations of polyatomic vibrational potential surfaces. Repulsive solvation forces are modeled using recently developed analytical ‘‘hard‐fluid’’ expressions for heteronuclear two‐cavity distribution functions in hard‐sphere fluids, while attractive forces are assumed to contribute a van der Waals(linearly density‐dependent) mean field. Results for the CN and CC stretches of acetonitrile compare favorably with theoretical predictions, while the CH stretch appears to experience a nonlinearly density‐dependent attractive frequency shift at hi...

Enhanced Chemical Classification of Raman Images in the Presence of Strong Fluorescence Interference

Raman spectra and spectral images containing severe fluorescence interference are analyzed by using a variety of correlation and classification algorithms, both before and after preprocessing with the use of the Savitzky-Golay second-derivative (SGSD) method (and other related methods). Spectral correlation coefficient, principal component, and minimum Euclidean distance analyses demonstrate superior suppression of background and noise interference in Raman spectra when using SGSD preprocessing. The tested spectra include fluorescence interference that is more intense than the Raman features of interest and also contains broad background peaks that vary in shape and intensity from sample to sample. The high chemical information content of the SGSD-processed Raman spectra is demonstrated by using quantitative comparisons of correlation coefficients in a series of synthetic Raman spectra with either different or identical large backgrounds. The practical utility of SGSD in chemical image classification is illustrated by using an experimental Raman image of sugar microcrystals on substrates with large interfering background signals. The functional equivalence of SGSD and other windowed preprocessing algorithms is discussed.

Rapid Micro-Raman Imaging Using Fiber-Bundle Image Compression:

A new technique for rapid Raman imaging and chemical analysis of micro-composites and biomaterials, with potential applications in real-time robotic vision, automated manufacturing, and medical imaging, is described and demonstrated. The key feature of this new instrument is a fiber-optic bundle used to compress two-dimensional images onto a one-dimensional fiber stack, which serves as the entrance slit of an imaging optical spectrograph. Thus a complete Raman spectrum is simultaneously collected from every point within a sample in a single scan of a charge-coupled-device (CCD) detector. The method is demonstrated by using Raman imaging of a microscopic mixed-salt sample. Its efficiency relative to alternative Raman imaging methods is quantitatively evaluated, and potential applications in other spectral imaging measurements are discussed. Index Headings: Raman spectroscopy; Spectral imaging; Chemical imaging; Fiber optics; Chemical sensor.

Perturbations of Water by Alkali Halide Ions Measured using Multivariate Raman Curve Resolution

Polarized Raman spectroscopy, combined with multivariate curve resolution (MCR), is used to measure the influence of dilute alkali halide ions on the OH stretch vibrational band of water. The frequency and integrated intensity of the resulting hydration shell OH bands are found to increase with increasing anion size. Comparisons of results obtained from salt solutions in H(2)O and HOD/D(2)O imply that ion-water interactions reduce the influence of resonance coupling on the OH stretch band of H(2)O. Polarized Raman results indicate that the hydration shell of F(-) gives rise to a nearly perfectly polarized OH stretch band, while large anions produce larger depolarization.

Chemical mapping of elemental sulfur on pyrite and arsenopyrite surfaces using near-infrared Raman imaging microscopy

Abstract Near-infrared Raman imaging microscopy (NIRIM) was used to produce chemical images of the distribution of elemental sulfur on oxidized pyrite and arsenopyrite surfaces. Analysis using Savitsky–Golay filtering permits an unambiguous identification of surface products even in the presence of broad background signals. Rather than forming a continuous, passivating layer at the mineral surface, the NIRIM images reveal that elemental sulfur forms in isolated patches on the order of tens of microns in diameter. The potential implications of this strongly heterogeneous distribution of chemical products for geochemical modeling of acid mine drainage (AMD) are discussed.

Unveiling Electron Promiscuity

Although the wave-like proclivity of electrons for delocalization is familiar to every student of chemistry, it seems that electrons may have less respect for atomic and molecular boundaries than one might have considered proper. The boundaries in question include those between H-bonded dimers and within hydrated clusters, as well as those of aqueous cavities, colloidal suspensions, and macroscopic air-water and oil-water interfaces. Unveiling the promiscuous behavior of electrons at such frontiers may both raise eyebrows and demand acknowledgment.