Muta-rotation

If only one, or none, of the forms is known in the crystalline state, polarimetry does not yield any useful results. It was not even certain, for example, before the advent of n.m.r. spectroscopy, whether the one known crystalline form of D-ribose is the a- or the / -pyranose its muta-rotational change is small, but complex.5... 

The situation just discussed for glucose is described as a simple muta-rotation. In such cases, as already indicated (see Figure 3 for example),... 

To answer the question above, whether the observed optical muta-rotation of galactose could be explained by the three-component model constrained to experimental ti/2% the optical rotatory values from the computations listed in Table II were plotted vs. time. The three plots did not differ significantly from each other, in spite of the variation in the ratio ki/ks. The plot for k /kz = 2 is compared with the experimental values in Figure 5. The observed rapid initial drop in rotation is not duplicated in the computed plot. No reasonable combination of... 

There are several reasons for reservations about applying the computer extrapolation of crystal structure data for carbohydrates. One is that much of the crystal structure data refer to unsubstituted sugars which are only soluble in hydroxylic or polar solvents where the conformational analysis may be complicated by hydrolysis, isomerism (muta-rotation) (12), or stereospecific solvent interactions which require a more sophisticated model. However, assuming that such chemical changes do not occur or can be suppressed, there still remain questions to be answered before the conformation observed in the crystal can be accepted as a close enough approximation to that of one or more of the rotomers which may predominate in the solution state. (a-L-Sorbose gives an example of the coexistence of two primary alcohol rotameric... 

It was originally proposed by Lowry, and later argued again by Swain, that since the function of the acid (or base) in these reactions is to facilitate the shift of a proton from one part of the substrate molecule to another (i.e., a prototropic shift), that a mechanism that involves simultaneous removal and addition of a proton might provide the most rapid path. In such case, the reactions should be expected to involve the simultaneous attack of acid and base on the proper parts of the molecule. In the muta-rotation of a-D-glucose the process can be pictured as... 

The problem of the acid-catalyzed hydrolysis of the carbohydrate orthoesters was brought nearer to the final solution by Pacsu s experiments on the hydrolysis of maltose methyl 1,2-orthoacetate. Since two adjacent hydroxyl groups on the same side of the plane are necessary for the formation of orthoester derivatives, the maltose methyl orthoacetate must have an -configuration. Hydrolytic experiments with very dilute hydrochloric acid confirmed this. Two consecutive reactions took place at a pH of 4. In the first reaction, the original specific rotation, Co ]d - -103.7 in pure water, increased to -1-134.6° within two minutes. The latter figure corresponds to the specific rotation of a-maltose 2-acetate. The second reaction k = 0.0095) corresponded to the downward muta-rotation of a-maltose 2-acetate. When the hydrogen-ion concentration,... 

Figure 1.20 Possible transition states in the catalysis of tetramethyl glucose muta-rotation by 2-pyridone, and calculated transition state for formic acid catalysis of 2-hydroxytetrahydropyran ring opening. In the calculated transition state, the proton is largely transferred to the endocyclic oxygen, the endocyclic C-O bond has started to break, but the hydroxyl proton is not transferred to the catalyst, i.e. the reaction is concerted but not synchronous.

The use of NMR techniques, rather than polarimetry, to monitor muta-rotation, as with the spontaneous reaction, allowed accelerations of particular conversions of sugars to be directly monitored. Saturation difference measurements indicated that three proteins from Escherichia coli, RbsD, FucU sic) and YiiL, of previously unknown function, had mutarotase activity.RbsD interconverted the (3-pyranose and (3-furanose forms of ribose, without any... 

Bentley and Bhate140 conducted a meritorious study of the muta-rotations of D-glucose and D-galactose in the presence and absence of the enzyme mutarotase from Penicillium notatum under a variety of conditions. For measurements of the water-catalyzed and mutarotase-catalyzed reactions at 23-24°, they found the following... 

The nature of the action of the mutarotase from P. notatum has been investigated extensively by Bentley and Bhate,140 and compared with the acid-, base-, and solvent-catalyzed reactions (see also, p. 31). Through use of lsO on C-l, it was shown that dehydrogenations do not occur on C-l. In addition, no dehydrogenation occurred at carbon-bound hydrogen atoms a single-displacement mechanism was thus eliminated. The enzyme probably transfers a proton, in a process similar to that usually involved in nonenzymically catalyzed muta-rotations. 

The osotriazoles, which possess sharp melting points and show no muta-rotation, have been used for the characterization of osazones. Owing... 

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