Glucose acetal formation

Lee etal. determined the concentration of glucose, acetate, formate, lactate, and phenylalanine simultaneously in two E. coli bioreactions using Raman spectroscopy.33 The bioreactor was modified to have a viewing port, and spectra were collected through the port window. This enabled researchers to sterilize the reactor and ensure that it would never be contaminated by contact with the instrument. 

Sucrose is a disaccharide that is composed of a unit of glucose (acetal form) and a unit of fructose (ketal form) linked through C-1 of glucose and C-2 of fructose, i.e. a 1,2 link. In sucrose, neither glucose nor fructose can exist in open chain form because of the formation of acetal and ketal as shown below. As a result, sucrose is not a reducing sugar, and does now exhibit mutarotation. The specific rotation [a]D of sucrose is +66°. 

Using glucose as a model substrate, hydrogen production is accompanied with either acetate formation (Eq. (15.1) or butyrate formation (Eq. (15.2) (Miyake, 1998). In the acetate fermentation 4 ATPs are produced whereas, 3 ATPs are produced in the butyrate fermentation. Thus, it seems that the acetate fermentation is energetically more favorable than the butyrate fermentation. However, acetate and butyrate fermentations are commonly carried... 

Acetal formation, demonstrated in the formation of the disaccharide sucrose, common table sugar. The reaction between the hydroxyl groups of the monosaccharides glucose and fructose produces the acetal sucrose. The bond between the two sugars is a glycosidic bond. 

When glucose reacts with an alcohol to form a glucoside, the major product becomes the a-glucoside. Since acetal formation is reversible, the a-glucoside must be more stable than the j8-glucoside. The preference of certain substituents bonded to the anomeric carbon for the axial position is called the anomeric effect. 

Figure 4a. Major metabolic pathways in M. ammoniaphilum involving glucose, acetate, and glutamate. Glucose labeled at C-1 produces [3- C] pyruvate via the Embden-Meyerhof pathway (EMP) and unlabeled pyruvate via the hexose monophosphate shunt (HMS). [3- C] pyruvate enters the tricarboxylic acid (TCA) and glyoxylate shunt (GS) cycles as [3- C] oxaloacetate and/or [2- C] acetate and can result in the formation of [2- C] glutamate, [4- C] glutamate, and [2,4A C] glutamate via a-ketoglutorate formed in 1/3 of a turn of the TCA cycle. Formation of glutamate after one or more turns of the TCA cycle will tend to randomize the label because of the formation of the symmetrical intermediates succinate...

The simplest way to fix glucose in the pyranose form is to trap it as an acetal. Acid-catalysed condensation with an alcohol, methanol, for example, gives an acetal and, remarkably, the acetal has an axial OR group. Acetal formation is under thermodynamic control (Chapter 11) so the axial compound must be the more stable. This is because of the anomeric effect—so-called because this C atom is called the anomeric position and the acetal diastereoisomers are called anomers. The effect is a bonding interaction between the axial lone pair on the oxygen atom in the ring and the c orbital of the OMe group. 

Fig. 14 Modified van Krevelen plot for the dehydration of sorbitol (after hydrogenation of glucose), lactic acid, 1,4-butanediol, and n-butanol. DH = dehydration, HG = hydrogenation. The effect of hydration of an aldose (hemi-acetal formation with water) is also shown...

OH groups of the D-glucoside. Figure 22.43 shows the reactions one can use to obtain each of the alcohol groups of a-D-glucopyranoside independently available for further modification. The reactions used for protecting are typically a combination of acetal formation, alkylation, acylation, silylation, and/or reduction. Keep in mind that every carbohydrate is different. So, for example, the method that works for D-glucose may not work for D-mannose. 

---