Arabinose spectra

The NMR spectrum of the dry sample showed broad unresolved peaks that correspond to a typical mixture of 4-O-methyl-D-glucuronic acid, L-arabinose and D-xylose, and proteins (Oliveira et al., 2010) (Figure 5). 

Besides the peaks mentioned above, the mass spectrum of n-arabinose diethyl dithioacetal tetraacetate reveals fragments obtained by cleavage of C-2-C-3, C-3-C-4, and C-4-C-5 bonds. 

A melting point of 82°-83°C and [ ]D20 —85° - —100° were reported for 2,3-di-O-methyl-D-arabinose by Verheijden and Stoffyn (20). These authors preparation must have been largely or wholly an a-anomer, whereas ours is preponderantly ft. In view of the rarity of crystalline furanoses, it is presumed to be the / -pyranose, and in the PMR spectrum run at 0°C on a sample freshly dissolved in DoO the anomeric proton gave a signal, with the appearance of an unresolved doublet, at t 4.50. This chemical shift and a small value of Jit2, would be expected for H-l of a / -arabinopyranose in the 1C conformation. 

Despite the broad spectrum of distribution, the catalytic properties of the enzyme in the different species are remarkably similar. The same four sugars (D-glucose, D-galactose, D-xylose, and L-arabinose) are substrates for the enzyme from all sources (Table VI). Some changes, however, have taken place, and the marginal activity for maltose found in the lower species (67) has been lost in the mammals, whereas the relative activity towards the pentoses has become enhanced. The Km values and turnover numbers for the substrates of purified beef kidney mutarotase are given in Table V. 

Fig. 5, —The Low-field Portion of the N.m.r. Spectrum of Tetra-O-acetyl-D-arabinose Diethyl Dithioacetal (195) in Chloroform-d at 100 MHz.

Isobarbaloin appears to be an isomer of barbaloin, and degradative experiments give the same results as with barbaloin thus, Ldger - has shown that, on prolonged, acid treatment, isobarbaloin gives aloe-emodin and D-arabinose. It yields aloe-emodin anthrone on treatment with aqueous borax, and consumes 2 moles of periodic acid per mole. The ultraviolet spectrum of isobarbaloin is identical with that of barbaloin, showing that it, too, has an anthrone structure. The infrared spectrum of isobarbaloin shows only minor differences from that of barbaloin. From these data, it seems likely that isobarbaloin and barbaloin differ only in their stereochemistry. 

Hi with Rha. H2 ( H,/H2=2.3Hz). The remaining proton spins of the rhamnose residue (H3 through He) were also delineated from the COSY spectrum enabling the assignment of Rha. He through Rha. H3. These rhamnose proton assgnments were subsquently confirmed by the double and triple RCT spectra. The two arabinose anomeric protons were overlapped, but distinguished from cross-sections of COSY spectrum at the frequencies of Ara. H2 and Ara . 

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