Conformational study of monomeric 2,3-butanediols by matrix-isolation infrared spectroscopy and DFT calculations

The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argon and xenon matrixes were studied, allowing the identification of two different conformers for each compound. These conformers were characterized by a +/-gauche arrangement around the O-C-C-O dihedral angle, thus enabling the establishment of a very weak intramolecular hydrogen bond of the O...H-O type. No other forms of these compounds were identified in matrixes, despite the fact that these four conformers had calculated relative energies from 0 to 5.1 kJ mol(-1) and were expected to be thermally populated from 50 to 6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained in terms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compounds during deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to be less than 4 kJ mol(-1) and are easily overcome in matrixes within the family of conformers with the same heavy atom backbone. The barriers for intramolecular rearrangement of the O-C-C-O dihedral angle in both diastereomers were calculated to range from 20 to 30 kJ mol(-1). Interconversions between the latter conformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers, and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G theory. Additional MP2/6-311++G calculations of energies and vibrational frequencies were performed on the most relevant conformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules were also obtained based on theoretical structural data and from vibrational frequencies (using well-established empirical correlations). The obtained values for -DeltaH for both diastereomers of 2,3-butanediol amount to ca. 6-8 kJ mol(-1).     

EPR studies of amine radical cations. Part 2. Thermal and photo-induced rearrangements of propargylamine and allylamine radical cations in low-temperature freon matrices

Matrix EPR studies and quantum chemical calculations have been used to characterize the consecutive H-atom shifts undergone by the nitrogen-centered parent radical cations of propargylamine (1b*+) and allylamine (5*+) on thermal or photoinduced activation. The radical cation rearrangements of these unsaturated parent amines occur initially by a 1,2 H-atom shift from C1 to C2 with pi-bond formation at the positively charged nitrogen; this is followed by a consecutive reaction involving a second H-atom shift from C2 to C3. Thus, exposure to red light (lambda > 650 nm) converts 1b*+ to the vinyl-type distonic radical cation 2*+ which in turn is transformed on further photolysis with blue-green light (lambda approximately 400-600 nm) to the allene-type heteroallylic radical cation 3*+. Calculations show that the energy ordering is 1b*+ > 2*+ > 3*+, so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5*+ undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t1/2 of approximately 1 h to yield the distonic alkyl-type iminopropyl radical cation 6*+; this thermal reaction is attributed largely to quantum tunneling, and the rate is enhanced on concomitant photobleaching with visible light. Subsequent exposure to UV light (lambda approximately 350-400 nm) converts 6*+ by a 2,3 H-shift to the 1-aminopropene radical cation 7*+, which is confirmed to be the lowest-energy isomer derived from the ionization of either allylamine or cyclopropylamine. Although the parent radical cations of N, N-dimethylallylamine (9*+) and N-methylallylamine (11*+) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9*+ leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10*+. The first step of this transformation now involves the migration of a hydrogen atom to C2 of the allyl group from one of the methyl groups (rather than from C1); the reaction is then completed by the cyclization of the generated MeN + (=CH2) CH2CH2CH2* distonic radical cation, possibly in a concerted overall process. In contrast to the ubiquitous H-atom transfer from carbon to nitrogen that occurs in the parent radical cations of saturated amines, the alternate rearrangements of either 1b*+ or 5*+ to an ammonium-type radical cation by a hypothetical H-atom shift from C1 to the ionized NH2 group are not observed. This is in line with calculations showing that the thermal barrier for this transformation is much higher (approximately 120 kJ mol-1) than those for the conversion of 1b*+ --> 2*+ and 5*+--> 6*+ (approximately 40-60 kJ mol-1).     

Variations of wettability and protein adsorption on solid siliceous carriers grafted with poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide), a thermally responsive polymer, was end-grafted to mercaptopropyl derivatives of silica gel, plane glass sheets and glass capillary tubing by free radical polymerization of the monomer in 1,4-dioxane at 100 degrees C. The polymer monolayer attached to the glass carriers provided them with thermally controlled wettability registered by two independent methods: direct measurements of the water contact angle and capillary rise. The water contact angle changed from 54+/-3 degrees to 68+/-3 degrees in the temperature range from 20 to 50 degrees C. The polymer grafting to silica gel (pore diameter 100 A, particle size 5 microm) resulted in 15-30-fold reduction in protein adsorption on the carrier at 35 degrees C. Adsorption isotherms of myoglobin indicate completely different characters of the protein adsorption to silica gel and its polyNIPAM-grafted derivative. Cooling of the grafted carrier containing adsorbed myoglobin to 9 degrees C led to a partial release of the protein to the contacting solution, whereas heating of the system to 35 degrees C resulted in reversible binding of the protein. Adsorption of myoglobin on polyNIPAM-coated silica was ca. 2-fold higher at 35 than at 9 degrees C, most probably due to steric repulsion displayed by the swollen copolymer at the lower temperature.     

Conductive wiring of immobilized photosynthetic reaction center to electrode by cytochrome C

The photosynthetic reaction center (RC) found in photosynthetic bacteria is one of the most advanced photoelectronic devices developed by nature. However, after immobilization on the electrode surface, the efficiency of electron transfer (ET) between the RC and the electrode is relatively low. This inefficiency has limited the possibility of using the RC for technological applications. Here we show that photoinduced electron transfer between the immobilized RC and a gold electrode can be increased by several tens-fold by incorporation of cytochrome c into the RC-self-assembled monolayer (SAM)-electrode complex. The effect does not depend on the initial redox state of the cytochrome and seems to be the result of the formation of a complex between the RC and the cytochrome c serving as an ET wire. This observation opens the possibility for electrochemical analysis of the special pair in the RC protein that is deeply buried inside the protein globe and is barely electrically addressable from the electrode surface.     

Designer dendrimers: branched oligosulfonimides with controllable molecular architectures

The synthesis of "designer" dendrimers and dendrons with sulfonimide units at every branching point is reported. The synthesis is based on a series of (regio)selective functionalization reactions of amines and sulfonamides allowing precise control of the dendrimers' shape, the number of branches in each generation, and their peripheral decoration with functional groups. In principle, structurally different branches can be incorporated at any position within the dendrimer structure at will. Structurally perfect symmetrical and two-faced "Janus"-type dendrimers, as well as dendrimers and dendrons with intended interstices were synthesized on a preparative scale and fully characterized. Oligosulfonimide dendrons of various generations bearing an aryl bromide functional group at their focal points were attached to a p-phenylene core with the aid of Suzuki cross-coupling reactions resulting in dendrimers with photoactive terphenyl cores. The structure and the high purity of all dendritic sulfonimides were confirmed by means of (1)H and (13)C NMR, electrospray ionization mass spectrometry (ESI-MS), and elemental analysis. The utility of MALDI-TOF mass spectrometry for the analytical characterization of these dendrimers was evaluated in comparison to electrospray ionization. Two model branched oligosulfonimides were characterized in the solid state by single-crystal X-ray analysis. Reaction selectivities and conformation of sulfonimide branching points were rationalized by DFT calculations.     

High yield syntheses of stable, singly bonded Pd2(6+) compounds

A general method for the syntheses of dipalladium compounds having a singly bonded Pd26+ core and the formula R,S-cis-Pd2(C6H4PPh2)2(O2CR)2Cl2 is described. When the alkyl group in the carboxylate ligands is an electron donating group, the compounds are stable and the yields high. The Pd-Pd distances for the diamagnetic compounds with R = CF3 and CMe3 are 2.5434(4) and 2.5241(9) A, respectively. Calculations at the DFT level suggest that the electronic configuration is sigma2pi4delta2delta*2pi*4. These represent rare examples of palladium(III) compounds.     

Solution-phase single quantum dot fluorescence resonance energy transfer

We present a single particle fluorescence resonance energy transfer (spFRET) study of freely diffusing self-assembled quantum dot (QD) bioconjugate sensors, composed of CdSe-ZnS core-shell QD donors surrounded by dye-labeled protein acceptors. We first show that there is direct correlation between single particle and ensemble FRET measurements in terms of derived FRET efficiencies and donor-acceptor separation distances. We also find that, in addition to increased sensitivity, spFRET provides information about FRET efficiency distributions which can be used to resolve distinct sensor subpopulations. We use this capacity to gain information about the distribution in the valence of self-assembled QD-protein conjugates and show that this distribution follows Poisson statistics. We then apply spFRET to characterize heterogeneity in single sensor interactions with the substrate/target and show that such heterogeneity varies with the target concentration. The binding constant derived from spFRET is consistent with ensemble measurements.     

Highly convergent three component benzyne coupling: the total synthesis of ent-clavilactone B

The first total synthesis of (+)-clavilactone B, a potent antifungal agent and novel tyrosine kinase inhibitor, is described. The absolute configuration of clavilactones has been unambiguously established by using Sharpless asymmetric epoxidation to generate the enantiomerically pure substrate. The strategy highlights the use of a powerful and convergent three-component benzyne coupling with a methylallyl Grignard and a chiral epoxy-aldehyde to generate two C-C bonds and install the carbon skeleton of clavilactone. Oxidative lactonization, ten-membered ring construction by ring closing metathesis, and oxidation gave clavilactone B.     

Spatial hydration structures and dynamics of phenol in sub- and supercritical water

The hydration structures and dynamics of phenol in aqueous solution at infinite dilution are investigated using molecular-dynamics simulation technique. The simulations are performed at several temperatures along the coexistence curve of water up to the critical point, and above the critical point with density fixed at 0.3 g/cm3. The hydration structures of phenol are characterized using the radial, cylindrical, and spatial distribution functions. In particular, full spatial maps of local atomic (solvent) density around a solute molecule are presented. It is demonstrated that in addition to normal H bonds with hydroxyl group of phenol, water forms pi-type complexes with the center of the benzene ring, in which H2O molecules act as H-bond donor. At ambient conditions phenol is solvated by 38 water molecules, which make up a large hydrophobic cavity, and forms on average 2.39 H bonds (1.55 of which are due to the hydroxyl group-water interactions and 0.84 are due to the pi complex) with its hydration shell. As temperature increases, the hydration structure of phenol undergoes significant changes. The disappearance of the pi-type H bonding is observed near the critical point. Self-diffusion coefficients of water and phenol are also calculated. Dramatic increase in the diffusivity of phenol in aqueous solution is observed near the critical point of simple point-charge-extended water and is related to the changes in water structure at these conditions.     

Synthesis and characterisation of hybrid carbon-alumina support

Hybrid carbon-coated alumina supports have been synthesised using 4,4′-methylenebis(phenylisocyanate) as carbon precursor. The adsorption of 4,4′-methylenebis(phenylisocyanate) on the alumina support is irreversible, the resulting organic moiety can undergo pyrolysis under elevated temperature with the formation of carbon coating on the alumina support. Carbon loading in the synthesised materials and thus a degree of coverage of the alumina surface with carbon layer can be increased by repetition of 4,4′-methylenebis(phenylisocyanate) adsorption-pyrolysis cycles. The carbon coating does not substantially influence the pore structure of the initial alumina support. Upon increasing the carbon loading, the carbon coating becomes more uniform with respect to carbon localisation both on the internal and the external surface of the alumina support. The carbon coating on an alumina support can be discriminated from carbonaceous deposits due to a difference in the steady-state surface charging of the samples. Moreover, carbonaceous surface species which associated with CO, CO and OCO groups in carbon coating can also be identified.