Substituent effects in the fragmentation reactions of acetanilides and phenyl acetates

The mass spectra of a group of m- and p-substituted acetanilides were measured to determine the effect of the substituents, if any, on the rate of C₂H₂O expulsion (rearrangement) vs. the rate of [CH₃CO]⁺ formation (simple cleavage). Our results agree with observations of others in that substituents which raise the ionization potential of the aromatic ring appear to localize, on the average, less charge density on this locus, and conversely. The atomic composition of the substituent, however, irrespective of its position, seems to determine the relative rates of the competing reaction. Although in several cases Z_m/Z_p values were close to unity, rearrangement of isomeric molecular ions to a common species is shown not to occur.     

LAMMA mass spectra of β-diketone,bis(benzoylacetone)ethylenediimine and bis(salicylidene)ethylenediimine complexes of some transition metals

Complexes of 2,4-pentanedione (acetylacetone, acac), [Cu(acac)₂], [VO(acac)₂] and [CO(acac)₃], and the chromium(III) derivative of 3-methyl-2,4-pentanedione (methylacetylacetone, meac), [Cr(meac)₃], the ligands bis(benzoylacetone)ethylenediimine and bis(salicylidene)ethylenediimine, and their cobalt(II), nickel(II) and copper(II) chelates were analysed by laser desorption mass spectrometry (LAMMA) and compared to electron impact (EI) results. The positive ion LAMMA spectra generally reveal mostly small fragments, although metal cationization peaks are seen for most complexes. Negative ion LAMMA produce carbon clusters and some structurally important fragments.     

Triplet-triplet energy transfer in the tryptophyl-tyrosinate dipeptide

Direct evidence for intermolecular triplet-triplet energy transfer in the dipeptide tryptophyl-tyrosine is presented. The methods of steady state ODMR and transient ODMR yield rates for the triplet-triplet energy transfer process of order 10 s^?1.     

Density-functional theory calculations of optical rotatory dispersion in the nonresonant and resonant frequency regions

The complex linear response function, which can be employed for calculations of second-order molecular properties in regions of strong absorption, is here extended to encompass the mixed electric-dipole-magnetic-dipole polarizability. The mixed electric-dipole-magnetic-dipole polarizability determines the optical rotation and, when absorption is taken into account, the full anomalous optical rotatory dispersion (ORD) spectra of chiral molecules can be calculated using first-principle quantum-chemical methods. Gauge-origin independence of the results is ensured through the use of London atomic orbitals. To illustrate the importance of taking the absorption process properly into account, we here apply this methodology to the study of the anomalous ORD of hydrogen peroxide, 3R-methylcyclohexanone, 4R-1,1-dimethyl-[3]-(1,2)-ferrocenophan-2-on, and the D(2) isomer of the C(84) fullerene.     

ROSEOFLAVIN AS A BLUE LIGHT RECEPTOR ANALOG: SPECTROSCOPIC CHARACTERIZATION

Abstract— The blue light absorption band of roseoflavin is polarized along the axis roughly connecting N₃-C₈ positions. A weak, second π→π* transition with a polarization angle of ca. 25° is hidden under the short wavelength side of the blue absorption band. The excited state of roseoflavin is somewhat more basic than the ground state, by a 1.5 p K _a unit. The fluorescence quantum yield and lifetime of roseoflavin are substantially lower than those of other flavins, thus making it kinetically less efficient as a blue light photoreceptor.     

Presentations for groups acting on simply-connected complexes

New expansions for global semigroup theory are developed. Many expansions have a left and a right version, each with specific (dual) properties; e.g., the Rhodes expansions ?^$\text{L}$, resp. ?^$\text{R}$, have unambiguous $\text{L}$-resp. $\text{R}$-order. In applications one sometimes needs expansions having both properties simultaneously; these can be constructed by alternately applying the left and the right expansion (possibly infinitely often) while keeping the same set of generators. Thus one obtains an expansion which is invariant under application of the old two expansions and thus has the properties of both (e.g., one obtains -+ with

, and so -+ has unambiguous $\text{L}$-and $\text{R}$-order). It is proved that, in the case of the Rhodes expansion, the new expansion is ‘close’ to the original semigroup; in particular (and this is the main result of the paper), ?⁺_A is finite (resp. finite $\text{J}$-above) if S is finite (resp. finite$\text{J}$-above).     

Molecular dynamics simulations of the dinuclear zinc-beta-lactamase from Bacteroides fragilis complexed with imipenem

Herein, we present results from MD simulations of the Michaelis complex formed between the dizinc beta-lactamase from B. fragilis and imipenem. We considered two catalytically important configurations, which differ in the presence or absence of a hydroxide bridge connecting the two zinc ions in the active site. The structural and dynamical effects induced by substrate binding, the specific roles of the conserved residues and the zinc-bound water molecules, the near attack conformers of the Michaelis complex, and so forth, are discussed in detail. The relative stability of the two configurations was estimated from QM linear scaling calculations on the enzyme-substrate complex combined with Poisson-Boltzmann electrostatic calculations and normal mode calculations. Importantly, we find that the two configurations have similar energies, indicating that these two structures could readily be interchanged, thereby facilitating catalysis. The configuration with the hydroxide bound to the two zinc ions is predicted to be the resting form of the enzyme, while the configuration without the bridge is the reactive form that was found to place the hydroxide in position to attack the carbonyl of the beta-lactam ring. Thus, we propose that the enzyme initiates catalysis by converting from the hydroxide bridge form into the configuration that lacks the hydroxide bridge. This interconversion increases the nucleophilicity of the hydroxide ion and exposes it to the beta-lactam carbonyl, which ultimately facilitates nucleophilic attack. The implications of the observed modes of binding, the possible influence of mutating the Lys184 and Asn193 residues on substrate binding, and the reaction mechanism are also discussed in detail.     

Determination of the enantiomeric composition of guest molecules by chemometric analysis of the UV-visible spectra of cyclodextrin guest-host complexes

Multivariate statistical techniques were applied to the UV spectra of a series of solutions at pH 12 containing a fixed concentration (30 mM) of beta-cyclodextrin (beta-CD) and a fixed concentration (15 mM) of 2-phenylglycine (phi-Gly) with various known enantiomeric compositions. Multivariate correlation of the spectral data for the solutions containing the phi-Gly/beta-CD guest-host complexes with the known enantiomeric composition of the phi-Gly samples was accomplished by partial-least-squares regression. When the multivariate model was used to predict the enantiomeric purity of a test set of samples over the mol fraction range of 0.5-0.9 R-phi-Gly, the average magnitude of the relative errors in the mol fraction determination of enantiomeric composition was 3%. A plot of the enantiomeric composition predicted by the model versus the known enantiomeric composition of the calibration set gave a straight line with a correlation coefficient of 0.955, a slope of 1.05, and an offset of 5.61 x 10-4.     

Destruction of carbonyl and hydroperoxide groups in oxidised polypropylene: Effect on photo-oxidation

The effect of destroying carbonyl and hydroperoxide groups in oxidised polypropylene on the subsequent rate of photo-oxidation has been examined using infra-red spectroscopy. The results show that carbonyl groups dominate the rate of photo-oxidation in severely oxidised polymer. In mildly oxidised polymer hydroperoxide groups control the rate, but to a much smaller extent. Destruction of the photo-active carbonyl and hydroperoxide groups in the unoxidised and oxidised polymers by prior photolysis in an inert atmosphere gave rise to some interesting and complex effects on subsequent photo-oxidation. The results indicate that although carbonyl and hydroperoxide groups may control the rates of photo-oxidation of thermally oxidised/processed polymer, their importance as primary photo-initiators is highly questionable. Oxygen-polymer charge transfer complexes appear to be the more likely photo-initiators.