Synthesis and Evaluation as Irreversible Glycosidase Inhibitors of Mono- and Oligo(glycosylthio)benzoquinones

The mono(glucosylthio)hydroquinone 2 was prepared by S-glycosidation of 2-mercaptobenzene-1,4-diol and by addition of the acetylated 1-thioglucose 3 to benzo-1,4-quinone (Scheme 1). The second, higher yielding procedure was adopted for the preparation of a range of (glucosylthio)hydroquinones. Addition of 3 to 2-chlorobenzo-1,4-quinone, followed by oxidation gave the 1-thioglucosides 7 and 12 (1.3:1), while addition of HCl to the (glucosylthio)quinone 4 and oxidation gave mainly 12 (Scheme 1). Similarly, the bis(glucosylthio)hydroquinone 33 was obtained from 3 and 4 (Scheme 4), and the (cellobiosylthio)hydroquinone 18 from the thiol 16 and benzo-1,4-quinone (Scheme 2). Addition of the 4-thioglucoside 21 to benzo-1,4-quinone (→ 22 ) and to 4 was followed by oxidation to yield the mono(glucosylthio)quinone 23 and the disubstituted quinones 24 and 25 , respectively (Scheme 3). A mixture 24 / 25 was also obtained from the addition of 3 to 23 . The tris(glucosylthio)hydroquinone 36 was obtained by addition/elimination to the dichloroquinone 29 or the dimesylate 31 , which was prepared in a simplified way (Scheme 4). The tetrakis(glucosylthio)hydroquinone 37 was obtained from 3 and chloranil, followed by reduction. The acylated hydroquinones were deprotected (→ 5, 9, 14, 19, 27, 34 , and 38 ), and oxidized to the corresponding quinones ( 6, 10, 15, 20, 28, 35 , and 40 ). The (glucosylthio)quinones 6, 15, 20, 28 , and 35 were tested as time-dependent inactivators of a retaining β-1,4-glucosidase from Agrobacterium faecalis (Abg), which has a strong exo-glucosidase action (Table 1). Similarly, compounds 20, 28 , and 35 were tested with a cellulase from Cellulomonas fimi (Cex) which degrades cellulose and cellooligosaccharides by hydrolysis of a cellobiose unit from the nonreducing terminus. The most effective inactivators for Abg were 6, 15 , and 35 , which inactivated this enzyme with similar second-order rate constants. (Glycosylthio)quinone 28 was the worst inactivator and did not show normal saturation behaviour. Inactivation of Cex by the (glycosylthio)quinones was 3–500 times slower than that of Abg. The three inactivators 20, 28 , and 35 had approximately the same efficacy with Cex, suggesting that they bind to this enzyme in a similar mode. Further, the K_i values observed are very similar to K_m values measured for aryl cellobiosides, implying that they bind at the active site.     

Synthesis and evaluation of potential inhibitors of chondroitin AC lyase from Flavobacterium heparinum

Chondroitin AC lyase from Flavobacterium heparinum degrades chondroitin sulfate glycosaminoglycans via an elimination mechanism, resulting in disaccharides or oligosaccharides with Delta4,5-unsaturated uronic acid residues at their nonreducing end. The syntheses and testing of two potential inhibitors of this lyase are described. Methyl O-(2-acetamido-2-deoxy-beta-D-galactopyranosyl)-(1-->4)-alpha-L-threo-hex-4-enopyranoside, 1, has the trigonal geometry at C5 of the uronic acid moiety expected at the transition state, yet retains the "leaving group" sugar moiety. Surprisingly, compound 1 showed no inhibition of the enzyme. The novel 5-nitro sugar, phenyl (5S)-5-nitro-beta-D-xylopyranoside, 2, is a monosaccharide nitro analogue of the natural substrate, with C5 being a carbon acid of pK(a) 8.8. The rate of reprotonation of the anion generated at this center is sufficiently low that the anion of 2 can be used directly in initial steady-state velocity measurements without significant interference from the conjugate carbon acid. The anion of compound 2 was found to be a competitive inhibitor with a K(i) value of 5 mM, whereas the conjugate acid had a K(i) value of 35 mM.     

N-Alkylated Derivatives of 1,5-Dideoxy-1,5-iminoxylitol as β-Xylosidaseand β-Glucosidase Inhibitors

Summary.  A range of lipophilic derivatives of the D-xylosidase inhibitor 1,5-dideoxy-1,5-iminoxylitol was synthesized by N-alkylation of the parent compound with different alkyl halides. Inhibitory activities of the products obtained were measured with β-glucosidase from Agrobacterium sp., a family 1 enzyme, as well as with β-xylosidase from Thermoanaerobacterium saccharolyticum belonging to family 39 of the glycohydrolases. Whereas the former enzyme, which has low β-xylosidase activity, was only moderately inhibited, with K _i values in the millimolar range, good inhibition was observed with the latter one. Inhibitors with terminally functionalized N-alkyl chains open up opportunities for novel applications as affinity ligands as well as for various types of tagging for diagnostic purposes. Received October 25, 2001. Accepted November 19, 2001     

Formal hydride transfer mechanism for photoreduction of 3-phenylquinoxalin-2-ones by amines. Association Of 3-phenylquinoxalin-2-one with aliphatic amines

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin-2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a bathochromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ and [HNQ-amine], both MeNQ and HNQ are photoreduced efficiently on irradiation in the presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(-) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxalin-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XNQ in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mechanism for the reversible photoreduction.     

Dinucleotide junction cleavage versatility of 8-17 deoxyribozyme

We conducted 16 parallel in vitro selection experiments to isolate catalytic DNAs from a common DNA library for the cleavage of all 16 possible dinucleotide junctions of RNA incorporated into a common DNA/RNA chimeric substrate sequence. We discovered hundreds of sequence variations of the 8-17 deoxyribozyme--an RNA-cleaving catalytic DNA motif previously reported--from nearly all 16 final pools. Sequence analyses identified four absolutely conserved nucleotides in 8-17. Five representative 8-17 variants were tested for substrate cleavage in trans, and together they were able to cleave 14 dinucleotide junctions. New 8-17 variants required Mn2+ to support their broad dinucleotide cleavage capabilities. We hypothesize that 8-17 has a tertiary structure composed of an enzymatic core executing catalysis and a structural facilitator providing structural fine tuning when different dinucleotide junctions are given as cleavage sites.     

ABSORPTION AND EMISSION STUDIES OF ELECTRONIC STATES OF N-ARYLBENZAMIDES

Abstract— Absorption and emission spectra of several N-arylbenzamides have been measured. The quantum yields for their fluorescence were found to be dependent on matrix viscosity and temperature. Singlet-triplet splittings for these compounds were determined from their emission spectra and found to be abnormally small for π. π* states (˜ 1500 cm^-1). Indeed, the phosphorescence maxima of N-arylbenzamides occur slightly to the blue relative to their fluorescence maxima. Intersystem crossing efficiencies were determined for several of these compounds and are consistent with S₁→ S ₀ radiationless decay.     

A TEM study of Ni ordering in the Ni6Se5−xTex, 0<x<∼1.7, system

The Ni₆Se_5−xTe_x, 0<x<∼1.7, system has been carefully investigated via electron diffraction and TEM imaging. They reveal a somewhat disordered modulated superstructure phase arising from Ni ion ordering within an essentially well-defined chalcogen sub-structure. As x, and the underlying parent substructure cell dimensions increase, the incommensurate primary modulation wave-vector q characteristic of this Ni ion ordering quickly swings from close to for x=0 towards for x?0.5. A lock-in to would formally transform the underlying parent Bmmb (a_p, b_p, c_p) structure into a P1a1 (a_s=2a_p, b_s=b_p, c_s=a_p+c_p) superstructure phase.