Home  About us  Contact
 

Slow Conversion (slow + conversion)

Distribution by Scientific Domains
Life Sciences75%

Selected Abstracts

The Low-temperature Photochemistry of s - cis Acyclic 1,3-Dienes,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2002
Michael Squillacote
ABSTRACT In this article we have examined the very low-temperature photochemistry of three acyclic 1,3-dienes. We have used high-temperature deposition techniques combined with matrix isolation to create samples enriched with the thermally meta-stable s - cis form. This technique has allowed us to examine the separate photochemistry of the s - cis and s - trans conformers. Our results suggest the presence and the absence of barriers on the excited-state surface. In particular, we have found that the electrocyclic closure and s - cis,s - trans photochemical isomerization stops at 15 K for 2,3-dimethyl-1,3-butadiene-d10. The closure occurs at higher temperatures in solution but is slowed by a deuterium isotope effect. The s - trans conformer of EE-2,4-hexadiene shows almost no photoreactivity in a matrix under 254 nm irradiation, but the s - cis conformer is rapidly converted to ZE-2,4-hexadiene (ZE-HXD). The photoreactivity of ZE-HXD is similar in that there is a relatively quick conversion of the s - cis conformer under these conditions, with only a very slow conversion of the s - trans to photoproducts. [source]

Structure of the T109S mutant of Escherichia coli dihydroorotase complexed with the inhibitor 5-­fluoroorotate: catalytic activity is reflected by the crystal form

ACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 3 2007
Mihwa Lee
Crystals of a single-point mutant (T109S) of Escherichia coli dihydroorotase (DHOase) with diminished activity grown in the presence of l -dihydroorotate (l -DHO) are tetragonal, with a monomer in the asymmetric unit. These crystals are extremely unstable and disintegrate shortly after formation, which is followed by the growth of orthorhombic crystals from the remnants of the tetragonal crystals or at new nucleation sites. Orthorhombic crystals, for which a structure has previously been reported [Thoden et al. (2001), Biochemistry, 40, 6989,6997; Lee et al. (2005), J. Mol. Biol.348, 523,533], contain a dimer of DHOase in the asymmetric unit; the active site of one monomer contains the substrate N -carbamyl- l -aspartate (l -CA-asp) and the active site of the other monomer contains the product of the reaction, l -DHO. In the subunit with l -­DHO in the active site, a surface loop (residues 105,115) is `open'. In the other subunit, with l -CA-asp in the active site, the loop folds inwards, forming specific hydrogen bonds from the loop to the l -CA-asp. The tetragonal crystal form can be stabilized by crystallization in the presence of the inhibitor 5-fluoroorotate (FOA), a product (l -DHO) mimic. Crystals of the complex of T109S DHOase with FOA are tetragonal, space group P41212, with unit-cell parameters a = b = 72.6, c = 176.1,Å. The structure has been refined to R and Rfree values of 0.218 and 0.257, despite severe anisotropy of the diffraction. In this structure, the flexible loops are both in the `open' conformation, which is consistent with FOA, like l -­DHO, binding at both sites. The behaviour of the T109S mutant crystals of DHOase in the presence of l -DHO is explained by initial binding of l -DHO to both subunits, followed by slow conversion to l -CA-asp, with consequent movement of the flexible loop and dissolution of the crystals. Orthorhombic crystals are then able to grow in the presence of l -DHO and l -CA-asp. [source]

Solid-phase synthesis and characterization of N -methyl-rich peptides

CHEMICAL BIOLOGY & DRUG DESIGN, Issue 2 2005
M. Teixidó
Abstract:, A library of peptides required for a project investigating the factors relevant for blood,brain barrier transport was synthesized on solid phase. As a result of the high N -methylamino acid content in the peptides, their syntheses were challenging and form the basis of the work presented here. The coupling of protected N -methylamino acids with N -methylamino acids generally occurs in low yield. (7-azabenzotriazol-1-yloxy)-tris(pyrrolidino)phosphonium hexafluorophosphate (PyAOP) or PyBOP/1-hydroxy-7-azabenzotriazole (HOAt), are the most promising coupling reagents for these couplings. When a peptide contains an acetylated N -methylamino acid at the N-terminal position, loss of Ac- N -methylamino acid occurs during trifluoroacetic acid (TFA) cleavage of the peptide from the resin. Other side reactions resulting from acidic cleavage are described here, including fragmentation between consecutive N -methylamino acids and formation of diketopiperazines (DKPs). The time of cleavage is shown to greatly influence synthetic results. Finally, high-performance liquid chromatography (HPLC) profiles of N -methyl-rich peptides show multiple peaks because of slow conversion between conformers. [source]

Modeling of Product Removal during Enzymatic Conversions by Using Affinity Molecules

BIOTECHNOLOGY PROGRESS, Issue 6 2007
Daniël G. R. Halsema
The feasibility of using magnetic particles for in-line product isolation during enzymatic conversion was studied. A comparison was made between a process based on magnetic particles and a conventional adsorption column. The enzymatic reaction was described by two consecutive first-order reactions (synthesis and subsequent hydrolysis), while the adsorption of substrate and product was described by multicomponent Langmuir isotherms. The yield as well as synthesis/hydrolysis ratio were calculated for various system characteristics. The results show that magnetic particles are very effective when the affinity with the particles is specific and for enzymatic conversions involving low ratios of the rate of synthesis versus the rate of hydrolysis. For slow conversions and for low specific affinity molecules column separations are more appropriate. [source]