Frictional dynamics of fluorine-terminated alkanethiol self-assembled monolayers

The frictional dynamics of fluorine-terminated alkanethiol (S(CH2)8CF3) self-assembled monolayers (SAMs) on gold are studied using molecular dynamics simulations. The simulations treat the interactions between two SAMs on flat surfaces. The structure and frictional behavior are investigated as a function of applied pressure (200 MPa to 1 GPa) for a shear velocity of 2 m/s and compared to methyl-terminated alkanethiol SAMs. The maximum adhesive pressure between the SAMs is 220 MPa for both end groups. In agreement with experiments on the molecular scale, the shear stress and the coefficient of friction for CF3-terminated alkanethiols are larger than for CH3-terminated alkanethiols. The main source for the difference is primarily the tighter packing of the fluorinated terminal group resulting in a higher degree of order. The molecular scale coefficient of friction is correlated with the degree of order among all the systems.     

Structure of the asparagine-linked sugar chains of porcine kidney and human urine cerebroside sulfate activator protein

The specific sugar residues and their linkages in the oligosaccharides from pig kidney and human urine cerebroside sulfate activator proteins (saposin B), although previously hypothesized, have been unambiguously characterized. Exhaustive sequential exoglycosidase digestion of the trimethyl-p-aminophenyl derivatives, followed by either matrix-assisted laser desorption/ionization and/or mass spectrometry, was used to define the residues and their linkages. The oligosaccharides were enzymatically released from the proteins by treatment with peptidyl-N-glycosidase F and separated from the proteins by reversed-phase high-performance liquid chromatography (HPLC). Reducing termini were converted to the trimethyl-p-aminophenyl derivative and the samples were further purified by normal-phase HPLC. The derivatized carbohydrates were then treated sequentially with a series of exoglycosidases of defined specificity, and the products of each digestion were examined by mass spectrometry. The pentasaccharides from pig kidney and human urine protein were shown to be of the asparagine-linked complex type composed of mannose-alpha 1-6-mannose-beta 1-4-N-acetylglucosamine-N-acetylglucosamine(alpha 1-6-fucose). This highly degraded structure probably represents the final product of intra-lysosomal exoglycosidase digestion. Oligosaccharide sequencing by specific exoglycosidase degradation coupled with mass spectrometry is more rapid than conventional oligosaccharide sequencing. The procedures developed will be useful for sequencing other oligosaccharides including those from other members of the lipid-binding protein class to which cerebroside sulfate activator belongs. (c) 2000 John Wiley & Sons, Ltd.     

Synthesis,characterization, and star polymer assembly of boronic acid end‐functionalized polycaprolactone

Boronic acid end‐functionalized polycaprolactone (PCL) polymers were synthesized by ring‐opening polymerization using a pinacol boronate ester‐containing (Bpin) initiator. The polymerization provides access to boron‐terminated polymers (i.e. Bpin‐PCL‐OH) with narrow molecular weight distributions (PDI = 1.09). Postsynthetic manipulation of the polymer's terminal hydroxyl group by copper‐catalyzed azide‐alkyne cycloaddition chemistry provides a series of bis end‐functionalized polymers with significant structural diversity at the termini. Deprotection of the boronate ester end group was accomplished with an acidic solid phase DOWEX resin. The boronate ester deprotection methodology does not result in hydrolysis of the polymeric backbone. The boronic acid‐tipped polymers were converted into star polymer assemblies using thermal dehydration and ligand‐facilitated trimerization. Thermal dehydration of (HO)₂B‐PCL‐OAc to the corresponding boroxine‐based star polymer assembly was inefficient and lead to degradation products. Ligand‐facilitated trimerization using either pyridine or 7‐azaindole as the Lewis base was efficient and mild. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Heats of formation of C(6)H(5)(•), C(6)H(5)(+), and C(6)H(5)NO by threshold photoelectron photoion coincidence and active thermochemical tables analysis

Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C(6)H(5)NO(+)]. Dissociation to C(6)H(5)(+) + NO products was measured over a range of internal energies and rate constants from 10(3) to 10(7) s(-1) and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.607 ± 0.020 eV was derived by using the simplified statistical adiabatic channel model. The thermochemical network of Active Thermochemical Tables (ATcT) was expanded to include phenyl and phenylium, as well as nitrosobenzene. The current ATcT heats of formation of these three species at 0 K (298.15 K) are 350.6 (337.3) ± 0.6, 1148.7 (1136.8) ± 1.0, and 215.6 (198.6) ± 1.5 kJ mol(-1), respectively. The resulting adiabatic ionization energy of phenyl is 8.272 ± 0.010 eV. The new ATcT thermochemistry for phenyl entails a 0 K (298.15 K) C-H bond dissociation enthalpy of benzene of 465.9 (472.1) ± 0.6 kJ mol(-1). Several related thermochemical quantities from ATcT, including the current enthalpies of formation of benzene, monohalobenzenes, and their ions, as well as interim ATcT values for the constituent atoms, are also given.     

Experimental and theoretical studies of the kinetics of the reactions of OH and OD with 2-methyl-3-buten-2-ol between 300 and 415 K at low pressure

The rate constants for the reactions of OH and OD with 2-methyl-3-buten-2-ol (MBO) have been measured at 2, 3, and 5 Torr total pressure over the temperature range 300-415 K using a discharge-flow system coupled with laser induced fluorescence detection of OH. The measured rate constants at room temperature and 5 Torr for the OH + MBO reaction in the presence of O2 and the OD + MBO reaction are (6.32 +/- 0.27) and (6.61 +/- 0.66) x 10(-11) cm3 molecule(-1) s(-1), respectively, in agreement with previous measurements at higher pressures. However, the rate constants begin to show a pressure dependence at temperatures above 335 K. An Arrhenius expression of k0 = (2.5 +/- 7.4) x 10(-32) exp[(4150 +/- 1150)/T] cm6 molecule(-2) s(-1) was obtained for the low-pressure-limiting rate constant for the OH + MBO reaction in the presence of oxygen. Theoretical calculations of the energetics of the OH + MBO reaction suggest that the stability of the different HO-MBO adducts are similar, with predicted stabilization energies between 27.0 and 33.4 kcal mol(-1) relative to the reactants, with OH addition to the internal carbon predicted to be 1-4 kcal mol(-1) more stable than addition to the terminal carbon. These stabilization energies result in estimated termolecular rate constants for the OH + MBO reaction using simplified calculations based on RRKM theory that are in reasonable agreement with the experimental values.     

Enyne metathesis-oxidation sequence for the synthesis of 2-phosphono pyrroles: proof of the "yne-then-ene" pathway

A new tandem reaction sequence has been developed for the synthesis of 2-phosphono pyrroles. The sequence consists of ring-closing enyne metathesis of a substituted aminophosphonate, containing a terminal alkyne and an internal alkene, in combination with in situ oxidation of the produced 3-pyrrolines using tetrachloroquinone. By analyzing the formation of the end and certain byproducts, taking into account the difference in reactivity of different substrates and carefully studying spectroscopic data, it was found that the reaction proceeds by means of the "yne-then-ene" pathway. During the initiation phase, a new ruthenium carbene is formed which continues the propagation cycle.     

Reactions of CF3CH2I+O(3P): competing mechanisms of HF elimination

Ab initio density functional and molecular orbital calculations provide singlet and triplet electronic potential energy surfaces for the reactions of CF3CH2I+O(3P) leading to OI and HF eliminations, reactions which have been the subject of recent experimental studies. A barrier to OI formation occurs on the triplet potential energy surface; there is no reverse barrier to OI formation on the singlet pathway. Findings suggest that two competing pathways may form HF. One is an addition-insertion-elimination process involving insertion of O into the C-I bond. The alternate path involves OI elimination, addition of an O atom to CF3CH2, and subsequent HF elimination. The computed reactant pathways and energetics are discussed in relation to recent experiments.     

Enhancement of phosphoprotein analysis using a fluorescent affinity tag and mass spectrometry

A fluorescent affinity tag (FAT) was synthesized and was utilized to selectively modify phosphorylated serine and threonine residues via beta-elimination and Michael addition chemistries in a 'one-step' reaction. This labeling technique was used for covalent modification of both phosphoproteins and phosphopeptides, allowing identification of these molecular species by fluorescence imaging after solution- or gel-based separation methods. In addition to the strong fluorescence of the rhodamine tag, a commercially available antibody can be used to enrich low-abundance post-labeled phosphopeptides present in complex mixtures. Application of this methodology to phosphorylation-site mapping has been evaluated for a phosphoprotein standard, bovine beta-casein. Initial results demonstrated low femtomole detection limits after fluorescence image analysis of FAT-labeled proteins or peptides.