Homomorphs pyranoses

It has been noted92 that, in aqueous solutions of the D-glycero-L-hep-toses, the a- to y -pyranose ratio is somewhat higher than that for the homomorphous hexoses, whereas, for the D-gh/cero-D-heptoses the ratio is the same, or even slightly lower. No explanation is apparent for this observation. 

Two biologically important ketoaldonic acids should be mentioned here. N-Acetylneuraminic acid (5-acetamido-3,5-dideoxy-D-gh/cero-D-gaiocto-nonulosonic acid) is homomorphous with 3-deoxy-g(uco(and monno)-heptulose, and therefore, in solution, would be expected to be overwhelmingly in the / -D -pyranose form (14). Actually, although the... 

Deoxy-D-manno-2-octulosonic acid109 ( KDO ) is homomorphous with 3-deoxy-D-gaiacfo(and falo)-heptulose a preponderance of the a-pyranose form (15), accompanied by substantial proportions of the two furanose forms, would therefore be expected. Actually, there is, again, a small proportion of the / -pyranose form present. The composition found110 in a 0.72 M solution of the ammonium salt at 28° is 64 6 20 10 and, in a 0.18 M solution, 60 11 20 9. 

As would be expected from the identity of the configurations of the pyranose or furanose rings, the members of each homomorphous series show marked chemical and physical similarities ), and it is often possible to predict the properties of unknown members from those of the basic type. The greatest differences, as might be anticipated, are found between the pentoses and the corresponding hexoses. It appears that enzymes which hydrolyze the hexoside members of each series also hydrolyze the glycosides of the other members of each series (4 ). Thus, the enzyme a-mannosidase of almond emulsin hydrolyzes the a-lyxosides as well as the a-mannosides. 

At least in alkaline solution, glucosamine may not exist in the pyranose form 278). The rate of oxidation of glucosamine by hypoiodous acid coincides closely with that of the galactose-arabinose homomorphous group of aldoses rather than that of the glucose-xylose group. 

The C-n.m.r. spectra of nine heptoses and all the heptuloses have been measured and most of the resonances assigned, and hence the furanose and pyranose compositions were determined. For the heptoses the compositions were similar in the main to those for the homomorphous hexoses, while the compositions of the heptuloses varied from 100% o -pyranose for the D-gluco isomer to mainly furanose, e.g, ... 

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