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Reversible Cleavage (reversible + cleavage)

Distribution by Scientific Domains

Chemistry	75%

Selected Abstracts

The kinetics of enhanced spin capturing polymerization: Influence of the nitrone structure

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2009
Edgar H. H. Wong
Abstract Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number-average molecular weight, Mn, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N - tert -butyl-,-phenylnitrone (PBN), N -methyl-,-phenylnitrone (PMN), and N -methyl-,-(4-bromo-phenyl) nitrone (pB-PMN), addition rate coefficients, kad,macro, in a similar magnitude to the styrene propagation rate coefficient, kp, are found with spin capturing constants CSC (with CSC = kad,macro/kp) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol,1 were deduced for kad,macro, congruent with a decreasing CSC with increasing temperature. Almost constant Mn over up to high monomer to polymer conversions is found when CSC is close to unity, while increasing molecular weights can be observed when the CSC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098,1107, 2009 [source]

The structure of l -rhamnulose-1-phosphate aldolase (class II) solved by low-resolution SIR phasing and 20-fold NCS averaging

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2002
Markus Kroemer
The enzyme l -rhamnulose-1-phosphate aldolase catalyzes the reversible cleavage of l -rhamnulose-1-phosphate to dihydroxyacetone phosphate and l -lactaldehyde. It is a homotetramer with an Mr of 30,000 per subunit and crystallized in space group P3221. The enzyme shows a low sequence identity of 18% with the structurally known l -fuculose-1-phosphate aldolase that splits a stereoisomer in a similar reaction. Structure analysis was initiated with a single heavy-atom derivative measured to 6,Å resolution. The resulting poor electron density, a self-rotation function and the working hypothesis that both enzymes are C4 symmetric with envelopes that resemble one another allowed the location of the 20 protomers of the asymmetric unit. The crystal-packing unit was a D4 -symmetric propeller consisting of five D4 -symmetric octamers around an internal crystallographic twofold axis. Presumably, the propellers associate laterally in layers, which in turn pile up along the 32 axis to form the crystal. The non-crystallographic symmetry was used to extend the phases to the 2.7,Å resolution limit and to establish a refined atomic model of the enzyme. The structure showed that the two enzymes are indeed homologous and that they possess chemically similar active centres. [source]

Purification, crystallization and preliminary X-ray crystallographic study of the l -fuculose-1-phosphate aldolase (FucA) from Thermus thermophilus HB8

ACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 12 2005
Jeyaraman Jeyakanthan
Fuculose phosphate aldolase catalyzes the reversible cleavage of l -fuculose-1-phosphate to dihydroxyacetone phosphate and l -lactaldehyde. The protein from Thermus thermophilus HB8 is a biological tetramer with a subunit molecular weight of 21,591,Da. Purified FucA has been crystallized using sitting-drop vapour-diffusion and microbatch techniques at 293,K. The crystals belong to space group P4, with unit-cell parameters a = b = 100.94, c = 45.87,Å. The presence of a dimer of the enzyme in the asymmetric unit was estimated to give a Matthews coefficient (VM) of 2.7,Å3,Da,1 and a solvent content of 54.2%(v/v). Three-wavelength diffraction MAD data were collected to 2.3,Å from zinc-containing crystals. Native diffraction data to 1.9,Å resolution have been collected using synchrotron radiation at SPring-8. [source]

The Controlled Formation and Cleavage of an Intramolecular d8,d8 Pt,Pt Interaction in a Dinuclear Cycloplatinated Molecular "Pivot-Hinge"

CHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2009
Chi-Kin Koo Dr.
Abstract The bis(diphenylphosphino)methane (dppm)-bridged dinuclear cycloplatinated complex {[Pt(L)]2(,-dppm)}2+ (Pt2,dppm; HL: 2-phenyl-6-(1H -pyrazol-3-yl)-pyridine) demonstrates interesting reversible "pivot-hinge"-like intramolecular motions in response to the protonation/deprotonation of L. In its protonated "closed" configuration, the two platinum(II) centers are held in position by intramolecular d8,d8 Pt,Pt interaction. In its deprotonated "open" configuration, such Pt,Pt interaction is cleaved. To further understand the mechanism behind this hingelike motion, an analogous dinuclear cycloplatinated complex, {[Pt(L)]2(,-dchpm)}2+ (Pt2,dchpm) with bis(dicyclohexylphosphino)methane (dchpm) as the bridging ligand, was synthesized. From its protonation/deprotonation responses, it was revealed that aromatic ,,, interactions between the phenyl moieties of the ,-dppm and the deprotonated pyrazolyl rings of L was essential to the reversible cleavage of the intramolecular Pt,Pt interaction in Pt2,dppm. In the case of Pt2,dchpm, spectroscopic and spectrofluorometric titrations as well as X-ray crystallography indicated that the distance between the two platinum(II) centers shrank upon deprotonation, thus causing a redshift in its room-temperature triplet metal,metal-to-ligand charge-transfer emission from 614 to 625,nm. Ab initio calculations revealed the presence of intramolecular hydrogen bonding between the deprotonated and negatively charged 1-pyrazolyl-N moiety and the methylene CH and phenyl C,H of the ,-dppm. The "open" configuration of the deprotonated Pt2,dppm was estimated to be 19,kcal,mol,1 more stable than its alternative "closed" configuration. On the other hand, the open configuration of the deprotonated Pt2,dchpm was 6,kcal,mol,1 less stable than its alternative closed configuration. [source]