An Organization Device for Visualizing Mechanisms and Regiochemistry Rationales in Electrophilic Aromatic Substitution

Design and testing of a hand-held device, consisting of two sheets of card stock, presenting electrophilic aromatic substitution (EAS) data in an organized fashion, and designed to facilitate learning electrophilic aromatic substitution reactions is reported. The device includes a large amount of information with similar data grouped, and the groups are juxtaposed in order to facilitate pattern recognition and differentiation. This, in turn, facilitates visualization, retention, understanding, and use of the data presented. One sheet is placed over the other in order to visualize the mechanism between the substituted aromatic compound selected and the chosen reagent. It shows two aspects of EAS on separate pages and, then, demonstrates interactively how they interrelate. The two aspects are (1) the substitution itself, including the identity of the electrophile, the reagent(s) needed to generate it, and the substituent in the product and (2) the electronic effects of the groups in the aromatic compounds upon the intermediate(s) and the structure of the product. The effect of the use of the Electrophilic Aromatic Substitution Tool (also known as EAS-at-a-Glance) on the test performance of students enrolled in undergraduate organic chemistry was determined by a post-test-only control-group study. The subject samples were assigned to a control group and three device groups, differing with respect to use of the device. Results show that the EAS Tool enabled a better student performance (by 12.3% to 17.3%, depending upon the method of use), and that the best method of use is as an out-of-class supplement.     

Fractals of graphene quantum dots in photoluminescence of shungite

Viewing shungite as loosely packed fractal nets of graphene-based (reduced graphene oxide, rGO) quantum dots (GQDs), we consider photoluminescence of the latter as a convincing proof of the structural concept as well as of the GQD attribution to individual rGO fragments. We study emission from shungite GQDs for colloidal dispersions in water, carbon tetrachloride, and toluene at both room and low temperatures. As expected, the photoluminescence of the GQD aqueous dispersions is quite similar to that of synthetic GQDs of the rGO origin. The morphological study of shungite dispersions shows a steady trend of GQDs to form fractals and to drastically change the colloid fractal structure caused by the solvent exchange. Spectral study reveals a dual character of the emitting centers: individual GQDs are responsible for the spectra position while the fractal structure of GQD colloids ensures high broadening of the spectra due to structural inhomogeneity, thus causing a peculiar dependence of the photoluminescence spectra on the excitation wavelength. For the first time, photoluminescence spectra of individual GQDs were observed in frozen toluene dispersions, which paves the way for a theoretical treatment of the GQD photonics.     

Correlation of relative rates of PdCl(2) oxidation of functionalized acyclic alkenes versus alkene ionization potentials, HOMOs, and LUMOs.

Investigations of the title reaction, carried out by plotting logs of the relative reaction rates vs IPs, vs HOMOs, and vs LUMOs, reveal multiple nearly parallel lines of correlation with small negative slopes in each. Overall, the natural grouping into monosubstituted and disubstituted alkenes gives better correlations than that obtained by using all alkenes. Comparison with analogous plots for other reactions indicates that the mechanism for this reaction has similarities to that for hydroboration, the major difference being that the lines in the plots for hydroboration have positive slopes, indicating an electrophilic rate-determining step involving the pi electrons, while those for the title reaction have small negative slopes, indicating a nucleophilic rate-determining step. Of the two reaction mechanisms proposed for the title reaction, only one has a nucleophilic attack at the complexed alkene as the rate-determining step, and therefore, this work supports that reaction mechanism.     

A new procedure for measuring peripheral compression in normal-hearing and hearing-impaired listeners.

Forward-masking growth functions for on-frequency (6-kHz) and off-frequency (3-kHz) sinusoidal maskers were measured in quiet and in a high-pass noise just above the 6-kHz probe frequency. The data show that estimates of response-growth rates obtained from those functions in quiet, which have been used to infer cochlear compression, are strongly dependent on the spread of probe excitation toward higher frequency regions. Therefore, an alternative procedure for measuring response-growth rates was proposed, one that employs a fixed low-level probe and avoids level-dependent spread of probe excitation. Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at a test frequency of 1 kHz, where the short 1-kHz probe was fixed in level at about 10 dB SL. The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. The TMCs were obtained for an on-frequency masker (1 kHz) and for other maskers with frequencies both below and above the probe frequency. From these measurements, input/output response-growth curves were derived for individual ears. Response-growth slopes varied from >1.0 at low masker levels to <0.2 at mid masker levels. In three subjects, response growth increased again at high masker levels (>80 dB SPL). For the fixed-level probe, the TMC slopes changed very little in the presence of a high-pass noise masking upward spread of probe excitation. A greater effect on the TMCs was observed when a high-frequency cueing tone was used with the masking tone. In both cases, however, the net effects on the estimated rate of response growth were minimal.     

Discretization orders for protein side chains

Proteins are important molecules that are widely studied in biology. Since their three-dimensional conformations can give clues about their function, an optimal methodology for the identification of such conformations has been researched for many years. Experiments of Nuclear Magnetic Resonance (NMR) are able to estimate distances between some pairs of atoms forming the protein, and the problem of identifying the possible conformations satisfying the available distance constraints is known in the scientific literature as the Molecular Distance Geometry Problem (MDGP). When some particular assumptions are satisfied, MDGP instances can be discretized, and solved by employing an ad-hoc algorithm, named the interval Branch & Prune. When dealing with molecules such as proteins, whose chemical structure is known, a priori information can be exploited for generating atomic orderings that allow for the discretization. In previous publications, we presented a handcrafted order for the protein backbones. In this work, we propose 20 new orders for the 20 side chains that can be present in proteins. Computational experiments on artificial and real instances from NMR show the usefulness of the proposed orders.     

3-D echo planar (1)HMRS imaging in MS: metabolite comparison from supratentorial vs. central brain

To determine if metabolite ratios as measured by 3-dimensional echo planar spectroscopy imaging (3D-EPSI) from central brain regions of interest (ROI) centered at the corpus callosum reflect imaging metrics of large volumes of supratentorial brain (STB) from patients with multiple sclerosis. METHODS: 48 MS patients with relapsing-remitting, secondary progressive, and primary progressive disease underwent a 3D-EPSI sequence covering large volumes of STB. Metabolite ratios were first estimated from all voxels within a STB mask using a linear regression of N-acetylaspartate (NAA) over Creatine (Cr), NAA over choline (Cho) and Cho over Cr. Secondly, spectroscopic voxels from a central brain (CB) ROI centered at the corpus callosum were selected within the STB. Ratios were compared using Bland-Altman regression analysis and Spearman's correlation coefficients between STB versus central brain. Ratios from studied ROIs were correlated with the EDSS and compared to normal controls. RESULTS: Very strong correlations ranging from 0.884 and 0.938 (p < 0.0001) were found for all metabolite ratios between STB versus central brain. NAA/Cr ratios were similarly and negatively correlated with the EDSS across all ROIs, trends ranging from -0.257 to -0.314 (p < 0.1). NAA/Cr from all MS patients was similarly decreased compared to controls across all ROIs (p < 0.01). CONCLUSION: Metabolite ratios from a central brain ROI were statistically equivalent and highly correlated with ratios from the STB. The study of NAA/Cr using (1)HMRS from a central brain ROI centered at the corpus callosum seems to be representative of brainwide axonal changes in patients with MS.     

Three-dimensional magnetic resonance spectroscopic imaging of histologically confirmed brain tumors

The goal of this study was to determine whether presurgical metabolite levels measured by 3D MR Spectroscopic Imaging (MRSI) can accurately detect viable cancer within human brain tumor masses. A total of 31 patients (33 exams, 39 pathology correlations) with brain tumors were studied prior to surgical biopsy and/or resection. The 3D MRSI was obtained with a spatial resolution of 0.2 to 1 cc throughout the majority of the mass and adjacent brain tissue using PRESS-CSI localization. Levels of choline, creatine and NAA were estimated from the locations of the resected tissue and normalized to normal appearing brain tissue. The data were correlated with subsequent histologic analysis of the biopsy tissue samples. Although there were large variations in the metabolite ratios, all regions of confirmed cancer demonstrated significant choline levels and a mean choline/NAA ratio of 5.84 + 2.58 with the lowest value being 1.3. This lowest value is greater than 4 standard deviations above the mean (0.52 +/- 0.13) found in 8 normal volunteers. The choline signal intensities in confirmed cancers were significantly elevated compared to normal appearing brain tissue with a mean ratio of 1.71 +/- 0.69. Spectra with no significant metabolite levels were observed in the non-enhancing necrotic core of the tumor masses. The results of this study indicate that 3D MRSI of brain tumors can detect abnormal metabolite levels in regions of viable cancer and grades and can differentiate cancer from necrosis and/or normal brain tissue.