The simple carbohydrates and the glucosides / by E. Frankland Armstrong.

  • Edward Frankland Armstrong
Date:
1910
Catalogue details

Licence: In copyright

Credit: The simple carbohydrates and the glucosides / by E. Frankland Armstrong. Source: Wellcome Collection.

    24/132 (page 8)
    for a short time and then observing 'the optical rotatory power of the solution before and after the addition of a drop of ammonia, which hastens the rate of the isomeric change, and therefore has the effect of establishing equilibrium almost immediately. As a glucose of high initial rotatory power was obtained from a-methyl glucoside, and one of low initial rotatory power from the /3-glucoside, it is clear that a- and ,8-glucose correspond respectively to the a- and /3-glucoside. It remains to establish the nature of Tanret’s 7-glucose, which he, as well as Simon and Lippmann, regarded as a third isomeride, ascrib- ing the mutarotation of a- and /3-glucose to their complete conversion into the isomeric aldehyde. The change in rotatory power of glucose was shown to be a process of reversible isomeric change by Lowry in 1S99. Lowry subsequently (1903) concluded that not only are a- and /8-glucoses isodynamic com- pounds, but that Tanret’s 7-glucose is a mixture in which these two compounds are present in equilibrium. On concentration of the solution of such an equilibrated mixture, a point is reached when one of the constituents crystallises out from the saturated liquid. The mixture in solution is consequently thrown out of equilibrium ; but as this happens a change takes place spon- taneously to restore the equilibrium—/3 passing into a, or vice versA. A solution of glucose containing a- and /3-forms can therefore be made to yield wholly a- or wholly /3-glucose on concentration, according to the temperature at which crystallisation takes place. The a-form. which is then the less soluble, is that obtained at lower temperatures ; but above 98°, the /8-form, being the less soluble at the higher tem- perature, alone separates. Were the change into aldehyde complete, as Simon and Lippmann suggest, it would be impossible by mere crystallisation to convert this into a-glucose. Tanret (1905) has accepted the conclusion that there are but two isomerides of glucose, corresponding to the a- and /3-methyl glucosides, and that his supposed third modification is an equilibrated mixture of these two forms. He has calculated from the rotatory power [a]D+ 110° of the pure a- and [a]D + 19° of the pure /3-form that the proportion in which these are in equilibrium is a = 37 per cent., /3 = 63 per cent, in a 10 per cent, solution, and a — 40, /3 = 6o per cent, in a concentrated aqueous solution. By means of solubility determinations Lowry finds 52 Per cent, of the a-form to be present in saturated solutions of glucose in methyl alcohol: the proportion of a decreases as the amount of water increases, amounting to 40 per cent, in the mixture EtOH + H20. He does not,