Chemistry

Monosaccharides share the same functional groups, but their isomeric forms often exhibit differences in chemical reactions. Disaccharides exhibit a similar range of reactions to monosaccharides owing to the presence of similar functional groups. Oligosac-charides generally exhibit properties similar to those of monosaccharides and disaccharides with similar functional groups, but some oligosacchar-ides with nine monosaccharide units may exhibit similar properties to polysaccharides. In general, polysaccharides show slower reaction rates because of steric effects.

Solubility

Monosaccharides, disaccharides, and oligosaccharides have similar solubilities. Overall, they are very soluble in water. Sucrose is extremely soluble in water, while lactose is soluble to a lesser extent. Furthermore, they are insoluble in nonpolar organic solvents. They exhibit limited solubility in pure alcohols but are very soluble in aqueous alcohol solutions (70-80% v/v), and therefore these solutions are widely used for extraction and analysis. Oligosaccharides are less soluble than monosaccharides in aqueous alcohol solutions, and their solubility decreases as the number of monosaccharide units increases.

In general, polysaccharides form colloidal solutions in water, while some other polymers are extremely insoluble in water and require prior treatment with acid, alkali, or organic solvents to get them to dissolve. For example, ^-1,4-mannans and glucans (e.g., cellulose) are very insoluble owing to hydrogen bonding between parallel chains. On the other hand, arabinoxylans are readily soluble in water, because the arabinosyl chains inhibit hydrogen bonding. Galactomannans are also readily soluble in water, producing viscous solutions, and are used as food additive gums. The a-linked glucans (e.g., amylose and amylopectin) have completely different solubilities. The glucan a-1,4-amylose is very soluble in warm water and forms colloidal solutions. When the amylose chains cool down, they form an amylose gel, which subsequently forms an insoluble crystalline material. Amylopectins are also very soluble in hot water but do not form an insoluble crystalline material to the same degree as amylose.

Reducing Properties

Monosaccharides are powerful reducing agents to a range of metals in alkaline solution, owing to the presence of aldo and keto groups. The extent of reduction varies among different monosaccharides. Disaccharides and oligosaccharides have the same reducing properties, except for sucrose, in which both hemi-acetal groups are combined. Polysaccharides usually contain one reducing group at the terminal end of the polymer chain and, as a result, have lower reducing properties.

Reactions in Acidic Solutions

When heated in strong acidic solutions, monosac-charides dehydrate and condense into a range of furans. The resulting furans condense with several reagents to generate colored products; hence the presence of monosaccharides and their derivatives can be verified. Under weaker acidic conditions, fructose is labile.

Reactions in Alkaline Solutions

In weak alkaline solutions, monosaccharides undergo isomerization of the aldose-keto group (enolization). In stronger alkaline solutions, they produce a series of degradation compounds, namely saccharinic acids. In the presence of ammonia, amino acids, and proteins, they condense repeatedly to generate a series of highly colored products (Maillard reaction); this reaction is used in the food industry to produce caramel colors.

Hydrolysis

Acid Disaccharides and oligosaccharides in mild acidic conditions are hydrolyzed to their constituent monosaccharides. The fructofuranosyl linkages of the fructooligosaccharides are quite susceptible to acid hydrolysis. Polysaccharides are also hydrolyzed to their constituent monosaccharides by acid hydrolysis, but the conditions necessary for complete hydrolysis depend on the solubilities of the polymers. The majority of polysaccharides (e.g., starch) are completely hydrolyzed under weak acidic conditions. However, cellulose requires treatment with strong acid for several hours prior to hydrolysis and subsequent heating under weak acidic conditions for the completion of the reaction. The uronans are very resistant to complete acid hydrolysis, and disaccharides of aldobiuro-nic acids are generally produced. Acid hydrolysis of polysaccharides results in extensive losses of their monosaccharide constituents.

Enzymatic Disaccharides are hydrolyzed in specific enzymatic solutions, and, therefore, this is a useful method for the analysis of sugar mixtures. Oligosac-charides are also susceptible to enzymatic hydrolysis. The maltooligosaccharides can be rapidly hydrolyzed by glucosidase enzymes.

Polysaccharides are more efficiently hydrolyzed to their monosaccharide constituents using specific enzymes. Fungal enzymes act specifically to hydro-lyze different polysaccharides. The a-1,4 glycosidic linkages in starch can be hydrolyzed by various a amylases (e.g., salivary and pancreatic), producing maltose and isomaltose. The ft-1,6 glycosidic linkages in amylopectin are not as easily hydrolyzed and require the presence of pullulanase - a fungal enzyme - to complete the hydrolysis.

Ester Formation

Monosaccharides contain hydroxyl groups and react with acids to form a variety of esters. The phosphate esters play a main role in carbohydrate metabolism. For example, the first step of glycolysis involves the production of the glucose-6-phosphate ester in a reaction catalyzed by the enzyme glucokinase in the presence of adenosine triphosphate. The uronic acids react with alcohols to form esters. The methyl esters of uronic acids are the most important in determining the physical properties of the uronans.

The presence of additional hydroxyl groups in dis-accharides and oligosaccharides increases the number of sites for esterification reactions. Sucrose reacts with fatty acids to produce nondigestible esters, which have similar properties to the triacylglycerols.

The polysaccharide galacturonans, which are composed of an a-1,4 galacturonic acid chain with integrated rhamnose units, form salts with cations and may be esterified with methoxyl groups.

Substitution

Monosaccharides undergo substitution reactions with methyl iodide to produce methyl ether derivatives. These compounds have been used to identify the structure of polymers, because the sites of nonmethyl substituted groups are indicative of the branch points after hydrolysis. Monosaccharides undergo acetylation, which occurs on the free or the reduced molecule to produce acetylated alditols. These volatile compounds have been used to identify sugar mixtures by GLC. The presence of additional hydroxyl groups in disacchar-ides and oligosaccharides increases the number of sites for substitution reactions.

Addiction To Nutrition

Addiction To Nutrition

Get All The Support And Guidance You Need To Be A Success At Beating Addictions With Nutrition. This Book Is One Of The Most Valuable Resources In The World When It Comes To A Definitive Guide To Unchain Addiction The Smarter And Healthy Way.

Get My Free Ebook


Post a comment