![]() ![]() This sugar is one of the lowest ranking in terms of sweetness, being about one-sixth as sweet as sucrose. Human milk contains about 7.5% lactose, and cow’s milk contains about 4.5%. In fact, the natural synthesis of lactose occurs only in mammary tissue, whereas most other carbohydrates are plant products. Lactose is known as milk sugar because it occurs in the milk of humans, cows, and other mammals. Enzymatic catalysis is usually also very specific.\): An Equilibrium Mixture of Maltose Isomers The term "catalyzed" implies that enzyme speeds up the reaction in both directions, so that both formation and hydrolysis (conversion from acetal to hemiacetal using a molecule of water) are faster with the enzyme. First, it is catalyzed by the enzyme maltase. There are several intriguing features of this conversion. ![]() The formation of maltose from two molecules of glucose is an example of this: For this purpose, the hemiacetal includes the anomeric carbon of a monosaccharide and the alcohol role is played by a specific OH group of a second monosaccharide. We can envision them as being made by the formation of an acetal from a hemiacetal and an alcohol. The linkages between the monosaccharide ring units in disaccharides are acetal linkages. This brings us to the topic of disaccharides. In fructose, both anomeric carbons are in acetal functional groups, so fructose is a non-reducing sugar. Maltose contains a hemiacetal functional group and is a reducing sugar. Consider the disaccharides maltose and fructose. The characterization of sugars as reducing or non-reducing is gives useful clues as to their structures. In a non-reducing sugar, the anomeric carbon is in an acetal. The outcome is that in a reducing sugar the anomeric carbon is in an aldehyde or hemiacetal. In contrast, acetal forms (glycosides) are not reducing sugars, since with base present, the acetal linkage is stable and is not converted to the aldehyde or hemiacetal. A hemiacetal form is thus a reducing sugar. This means that the cyclic hemiacetal form of a sugar will produce an equilibrium amount of the open-chain aldehyde form, which will then reduce the copper(II) to copper (I) and give a positive test. These reagents are used in basic solution, so that hemiacetals and aldehydes are in equilibrium. Sugars which are oxidized by these reagents are called reducing sugars because they reduce the copper(II) to copper(I). The oxidizing agents used in carbohydrate chemistry are typically copper(II) compounds which are reduced to copper(I) oxide. Aldehydes are fairly easy to oxidize to carboxylic acids, while acetals (which have no carbonyl group) are quite difficult to oxidize. There is another important difference between the hemiacetal and acetal linkages in sugars and saccharides, and that is their reaction with mild oxidizing agents. ![]() Often this improves the water solubility of the alcohol and makes it easier to excrete. Besides the di- and polysaccharides we will look at later, it is very common for glucose (or other sugars) and an alcohol to form an acetal linkage. Conditions can be arranged to produce either the alpha or beta stereochemistry in the glycoside. The conversion between an aldehyde and a hemiacetal is catalyzed either by base or by acid. Keep in mind that the conversion between a hemiacetal and an acetal requires an acid catalyst. The acetal products are called "glycosides." If the sugar used is glucose, they are "glucosides." There are several reasonable mechanisms for these conversions and we will not look at them in detail. If we apply this feature of the scheme to a solution of glucose in methanol (with a trace of acid catalyst included), we get: Let's begin by remembering the reaction sequence which links aldehydes and alcohols, hemiacetals, and acetals.įor our purposes, the key feature is the conversion of a hemiacetal and an alcohol to an acetal, with the concurrent release of a molecule of water. We'll find that these acetal linkages are what holds di- and polysaccharides together. In particular, we'll recall how hemiacetals are converted to acetals. Today we'll look in more detail at the chemistry of that hemiacetal linkage. We saw that the major stereochemical features of aldohexoses and aldopentoses are usefully described by Fischer projection formulas, but we learned that the structures of these compounds must also be understood as cyclic hemiacetals. Last time we explored the structural characteristics of monosaccharides. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |