Un paseo por el bosque

2.1.3' Solubility Properties.

o-Gluoosone is very readily soluble in water and

methanol, less soluble in ethanol; the osone is very sparingly soluble in cold glaoial acetio aoid, dioxan, and acetone, and insoluble in chloroform, ether, light petroleum, and benzene. 2.1.4. Optical Properties.

Solutions of D-glueosone have been shown to exhibit mutarotation. Thus, a aqueous solution of the "froth", shown, analytically, to be a hydrate of o-glucosone, showed an initial specific rotation of -1 0.38^ which increased to a

constant value of >4.21*^ after 130 hours (see Fig. 1.). The exhibition of mutarotation indicates the presence of at least one lactoi ring in the molecule and from the direction of

change in rotation it is suggested that the greater proportion of the "froth" is the 0-anomcr of such a ring structure.

The mutarotations of many sugars follow the first-order equation (Hudson, 19C3J Isbell & Pigman, 1937) and this

conformation makes it probable that the main constituents of the equilibrium solutions of these sugars are the a- and 0- -pyranose modifications. For such sugars a plot of

logCrotatlon at time t - final equilibrium rotation) against time results in a straight line. A number of important

sugars, including o-galaotose, o-talose, and t-ribose, exhibit mutarotations which do not follow the first-order equation, and a "first-order plot" for such sugars results in a curve; the deviation of the curve from a straight line is an indica­ tion of the lack of conformity of the mutarotation with the first-order equation. A "first-order plot" for o-glucosone (Fig. 2.) results in a curve very similar to that obtained by Isbell & Pigman (1937) for the mutarotation of a-o-talose, and demonstrates the complexity of the mutarotation of the osone.

Fig, 1. Muta r o t a t i o n of o-Glucosone at lR° (o, 9-27 in water) + 10 40 — I 0 _{TIME. HOURS} ? -3 0 -50 - 6 0 — 70 • 80 -9.0 -100

Fig. 2. First-order Plot of Mutarotation of p-Glucosone at l8° ( c , 9.27 in w a t e r ). ♦ 10 ♦ 0-7 So-i ^0 so 60 TIME. HOURS 90 100 -0 3 - 0 5 -o-«

105

In general« the mutarotations which cannot be expressed by the first-order equation conform to equations derived on the assump­ tion of three or more components in the equilibrium mixture

(Hiiber & Minsaas, 1926; Smith & Lowry, 1923; Isbell & Pigman, 19)7)# The mutarotation reactions which follow such equations may be considered to consist of two simultaneous or consecutive reactions, one of which is slow and represents the conver­ sions between pyranose isomers, and the other of which is rapid and possibly represents pyranose-furanose interoonversions

(Isbell & Pigman, 19)7# 19)8)+ Thus, the complexity of the mutarotation of o-glucosone, which shows both a fast and a slow reaction (see Fig. 2.), may perhaps be explained in terms of

these types of interconversion» However, consideration must be given to other factors probably contributing to the production of a complex mutarotation.

During the course of mutarotation of the osone solution no change in reducing power or in the content of osone, estimated as D-gluoose 2;4-dinitrophenylosazone (see Part II, 2.2.5») could be detected, and no change in pH was observed. However, paper ohromatogr;plic analysis (see Part II, 2.1.6.) indicated the

occurrence of certain structural interconversions. It was shown that partial concentration of a homogeneous solution of the osone, prepared by ydrolysis of tri-Q-isopropylidene o-r;;lucosonG

hydrate, led to the formation of a second component considered to be a stable polymer of the osone; a chromatogram of a fresh solution of the ’’froth**, obtained by complete concentration, showed the presence not only of the pure osone and the stable polymer but also of a third component of intermediate value. Chromatographic tinalysis during the course of mutarotation showed the gradual disappearance of this central spot and it is proposed tnat this component is an unstable polymer which depolymerises in aqueous solution. The initial presence of this unstable polymer as well as the pure osone and t.e stable polymer in -he system would account for t-ie exhibition of complex mutarotational

behaviour by a solution of the **froth”. Also, as a result of a study of the reotions and ultravfLolet absorption spectrum of solutions of the osone (see Part II, 2.1.5») it is proposed

that o-glucosone hydrate is not stable in solution, a structure containing a free carbonyl group at 0% being formed in small proportion (see Part IX, 4#); such a cinversion would further contribute to the production of a complex mutarotation.

Thus, the complex mutarotation of o-glucosone is not to be ascribed to a simple interoonversion of the pyranose-furanose type but rather to the ready polymerisation of the highly

reactive molecule, and the instability in solution of the hydrated form of the monomer.

2.1.5* Spectrophotometric Analysis.

In view of the results of the work of Bednarcayk & ^archlewmki (19)8) on the absorption of ultraviolet light by solutions of crude o-gXucosone (see Tart I, 2.1.5#) similar investigations have been carried out by the present author on the pure osone in aqueous solution at different p:î levels.

Chromatographically homogeneous solutions of o-glucosone, prepared by hydrolysis of tri-O-isopropylidene o-glucosone

hydrate with O.lN-sulphuric acid, were examined with the Unioam ultraviolet spectrophotometer, model S.500; the results are pre anted graphically - see Fig. ). Curve A represents the absorption spectrum of a 0.3/b solution of the osone in 0 04h sulphuric acid, i.e. a solution of pH 1.5; an absorption maxi­ mum was observed at a wavelength of and a minimum at 25C%yi* Neutralisation of this solution with ion-exchange resin gave a solution of pH /.O whose absorption spectrum is represented by curve B; it may be seen that both absorption maximum and mini­ mum were displaced towards the shorter wavelengths, being at 277 and 244nyu.respectively. Such a displacement could be accounted for by the necessary alteration in slit width used with the instrument as well as the decrease in concentration of

107

.

?ig. 3. Ultraviolet Absorption Spectra of Oolu Lions of u-Glucosone

0 6 0-5 04

E

Aqueous solution of u~çluco s o n e , pH l o ₁

B: Aqueous solution of D-g’l u c o s o n e , pH 7.0 C: Aqueous solution of D-g luc oso ne, pH 12.5

the solution due to retention of the 6sone on the ion-exchange resln, apart from possible effeots of the increase in pH value. The neutral solution was made alkaline (pH 12.$) by the addition of sodium hydroxide solution and after standing at room temper­ ature for 10 minutes examined in the spectrophotometer. The absorption spectrum of this alkaline solution (curve 0), showing an absorption maximum at 3lBmyu. and a minimum at is almost identical with that described by Stacey & Turton (1946) for kojic aoid in alkaline solution and for the product of alkaline hydrolysis of their specimen of 2:3:4:6-tetra-0-aoetyl o-gluo- ooone hydrate - see Table 1.

Compound. Max. ( ^&.) Min. Reference.

LrGorbose 273 ₂₄₃ B. & M. (1933)