L-Arabinosides

Fig. 7.—Predicted (- -) and Fragment (—) Circular Dichroism Spectra /3-L-Arabinose Calculated from (a) a-D-Xylose, and (b) Methyl /3-L-Aiabinoside a-L-Arabinose Calculated from (c) j3-D-Xylose, and (d) Average Calculated for /3-L-Arabinose Methyl a-L-Arabinoside Calculated from (e) Methyl /3-D-Xyloside, and (f) Methyl /3-L-Arabinoside. (Redrawn from Ref. 6.)...

This enzyme [EC 3.2.1.55], also called arabinosidase, catalyzes the hydrolysis of terminal nonreducing a-L-arabinofuranoside residues in a-L-arabinosides. Actual substrates include a-L-arabinofuranosides, a-L-arabi-nans containing (1,3)- and/or (l,5)-linkages, arabinoxy-lans, and arabinogalactans. It should be noted that some /3-galactosidases and )3-D-fucosidases will also hydrolyze a-L-arabinosides. 

Chart V summarizes the reactions of 2,3- and 3,4-anhydropento-pyranosides. Buchanan and coworkers have shown144 that the methyl 2,3-anhydro-a(and /3)-lyxosides, and 3,4-anhydro-/3-L-arabinoside adopt that half-chair conformation (5H0) in which the anomeric meth-oxyl group is axially attached this indicates the importance of the ano-... 

Honeyman147 reported the synthesis of methyl 2,3-anhydro-/3-L-lyxopyranoside, and claimed that, on alkaline hydrolysis, this gives a 2 1 ratio of L-xyloside L-arabinoside, but these results could not be substantiated by Buchanan and R. Fletcher,68 who recorded different constants for the epoxide and for the disulfonic ester claimed147 to be the starting material for its synthesis. The same epoxide has been synthesized by Reist and coworkers,160 and its properties are in agreement with Buchanan and Fletcher s results. Ethyl or methyl 3,4-anhydro-/3-L-ribopyranoside undergoes substitution at C-4 in all the reactions thus far studied. Neither half-chair conformation would seem to be clearly favored, but the specificity observed can be rationalized by considering the steric and polar interactions that may arise in the transition state substitution at C-3 in conformation 58 would involve marked interactions between the nucleophile and... 

The configuration of the products was established by periodate oxidation of the D compound, and reduction of the resulting dialdehyde to an optically active diol whose sign and magnitude of optical rotation ([a] ° +2.5° in water) correspond to that of the diol ([a]n° 46.8° in water) obtained by similar treatment of methyl a-L-arabinopyrano-side. The bis(p-nitrobenzoates) of the two diols exhibited similar relationships. Hence, it appears likely that the configuration at C-2 of the anhydride is the same as that at C-l of the L-arabinoside. The cis relationship of the hydroxyl groups was retained from the arabinal started with. 

This L-arabinosidase acts on L-arabinosides of low molecular weight, such as the synthetic substrates already mentioned, and L-arabino-oligo-saccharides. 

The side-chain L-arabinosyl groups of wheat-flour L-arabino-D-xylan are almost completely split off by the purified a-L-arabinofuranosidase11 from A. niger Kl. Similar results were reported36 for an enzyme preparation from Pectinol R-10. In contrast, the a-L-arabinofuranosidase from Pectinol 59-L hydrolyzes only 18% of the L-arabinosidic linkages of wheat L-arabino-D-xylan.37... 

Terminal L-arabinosidic linkages in L-arabinose conjugates are also hydrolyzed by the enzyme. The enzyme of R. fiava releases L-arabinose from the polysaccharide of the water shield (Brasenia schreberi J. F. Gmel)38 and from the cotyledon of Tora bean (Phaseolus vulgaris).39 Some 70 to 80% of the side chains of the arabinoxylan in rice cell-wall are composed of L-arabinose. When the a-L-arabinofuranosidase from R. 

A further method of synthesizing glyculosides is by brominolysis of 1,2-diol dibutylstannylene acetals. Thus, the /f-L-arabinoside derivative 221 gave the ketone 222, by oxidation of the axial oxy group.439... 

J. S. Burton, W. G. Overend, and N. R. Williams, Branched-chain-sugars. Part II. The introduction of branching into methyl 3,4-0-isopropylidene-/ -l-arabinoside and the synthesis of L-hamamelose, J. Chem. Soc., (1965) 3433-3445. 

Di-O-methyl-D-galactose, 4-O-methyl-D-mannose, methyl /3-L-arabinoside 0.32... 

Various methyl 2,3-anhydroribosides have been made. Thus Honey-man171 treated methyl 2-tosyl-3,4-diacetyl-/3-L-arabinoside (CXVIII) with sodium methoxide to obtain an amorphous anhydride which, in view of the well-known mechanism of the reaction of tosyl derivatives with alkali,172 was assigned structure CXIX. Mukherjee and Todd,173... 

Arabinosecarboxylic acid, I, 3, 4 Arabinoside. See also Arabinofuranoside and Arabinopyranoside, L-Arabinoside, methyl 2,3-dimethyl-, II, 245... 

EC 3.2.1.55 a-N-arabinofuranosidase Hydrolysis of terminal non-reducing o-L-arabinofu-ranoside residues in o-L-arabinosides... 

Hemicellulase Hydrolysis of P-l,4-glucans, a-L-arabinosides, P-D-mannosides, 1,3-P-D-xylans, and other polysaccharides, yielding polysaccharides of lower molecular weight. 

This enzyme catalyzes the hydrolysis of P D-galactosides and a-L-arabinosides. It is best known for its action in hydrolyzing lactose and is, therefore, also known as lactase. The enzyme is widely distributed and occurs in higher animals, bacteria, yeasts, and... 

The exo and endo isomers of benzylidene acetals of the carbohydrate series can be obtained separately. Their behavior on reduction with HzAlCl accords with the analysis outlined above frequently only one product is obtained. Thus, the 1,2,4,6-di-O-benzylidene derivatives of glucose (42) and (43 R = H or Bn) cleave rapidly with 1 equiv. of H2AICI at 0 °C (30 min for 42 and 10 min for 43) at the marked bonds." Here, at least, it is clear that steric effects outweigh polar ones. The 2,3-endo and -exo isomers of the a-D-mannoside series (44) and (45) likewise cleave mainly equatorial (78 22) and axial (96 4), respectively, when treated with 1 equiv. of H2AICI at room temperature. The same behavior is seen with the methyl 2,3-benzylidene-a-L-rhamnosides," the methyl 3,4-0-benzylidene-P-L-arabinosides" and... 

Thus, refluxing methyl P-D-glucoside two equivalents of tributyltin oxide, [(C4Hg)3Sn]20 and molecular sieves (3 A) in chloroform for 2-3 h followed by treatment with two equivalents of bromine at 0 °C furnishes methyl p-D-3-dehydro-3-ketoglucoside in more than 90% yield. Similarly, methyl p-D-xyloside gives the 3-oxo compound, whereas methyl a-D-glu-coside, a-D-galactoside, a-D-xyloside, and p-L-arabinoside yield 70-80% of the corresponding 4-keto derivatives [734],... 

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