Carbohydrates

CARBOHYDRATES

Modified starches are amenable to both chemical and biochemical modification.

Both types of modifications are used commercially to improve the properties of starches and expand their range of uses.

(Modified Starch: slows down the absorption of carbohydrates.

Slow carbs: Modified starches are often categorized as ‘slow carbs’ because they are digested and absorbed more slowly than regular starches.

Increased Fat Oxidation: Slower absorption can lead to increased fat oxidation, which might be beneficial for weight management or athletic performance.

Increased GI(gastrointestinal) Problems: It’s also associated with the use of modified starches.

Reduced carbohydrate Delivery: Modified starches may reduce the delivery of carbohydrates to the muscles, which could potentially impact athletic performance.

No Improvement or Reduced Performance: in some cases, using modified starches might not improve or could even reduce athletic performance.)(this is only for understanding)

Monosaccharides

• Carbohydrates contain chiral carbon atoms. 

• A chiral carbon atom is one that can exist in two different spatial arrangements or configurations. 

• Chiral carbon atoms have four different groups attached to them.

• The two different arrangements of the four groups in space (configurations) are non superimposable mirror images of each other. 

• In other words, one is the reflection of the other. That one would see in a mirror, with everything that is on the right in one configuration on the left in the other and vice versa.

D-Glucose

• D-glucose is both a polyalcohol (an organic compound containing multiple hydroxyl groups) and an aldehyde(contains a carbonyl group at the end of a carbon chain).

• An Aldehyde is a type of organic compound in which a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom, and a single bond with another atom or group of atoms (designated R in general chemical formulas and structure diagrams)

• Glucose is classified as an aldose, a designation for sugars containing an aldehyde group.

• The suffix -ose signifies a sugar; the prefix ald signifies an aldehyde group. 

• D-glucose can be represented in an open or vertical, straight-chain fashion known as an acyclic structure. In this form, the aldehyde group (carbon atom 1) is at the top, and the primary hydroxyl group (carbon atom 6) is at the bottom.

• D-glucose can also exist in a cyclic form, where the aldehyde group reacts with the hydroxyl group on carbon atom 5 to form a ring structure. This cyclic form is called glucopyranose.

• All with a left-hand positioned hydroxyl group on the highest numbered chiral carbon atom are designated L-sugars. 

• L-sugars are less numerous and less abundant in nature than are the D-forms, but nevertheless, have important biochemical roles.

Monosaccharide Isomerization 

Isomerization: Isomerization is a process where a polyatomic ion or molecular fragment is transformed into a different chemical structure while maintaining the same molecular formula.

OR

Molecules with the same molecular formula but different structural arrangements are called isomers.

• Simple sugars with a carbonyl group (C=O) at the end (aldoses) or in the middle (ketoses) are isomers of each other if they have the same number of carbon atoms. For example, a hexose (6 carbons) and a hexulose (6 carbons with a middle carbonyl) are isomers.

• Aldoses and ketoses can be interconverted through isomerization reactions.

• The carbonyl group is a polar functional group, meaning it has a partial positive and partial negative charge.

• Isomerization of monosaccharides (simple sugars) involves the movement of the carbonyl group and an adjacent hydroxyl group (OH).

• By  this  reaction,  an  aldose is  converted  into  another  aldose (with the  opposite  configuration  of  C-2)  and  the  corresponding  ketose,  and  a  ketose  is  converted  into  the  corresponding  two aldoses. 

• Therefore,  by  isomerization,  D-glucose,  D-mannose,  and  D-fructose  can  be  interconverted. 

• Isomerization  can  be  catalyzed  by  either  a  base  or  an  enzyme.

Monosaccharide Reactions

• All carbohydrate molecules have hydroxyl groups available for reaction. 

• Simple monosaccharides and most other low-molecular-weight carbohydrate molecules also have carbonyl groups available for reaction. 

• Reactions are as follows

1.Oxidation to Aldonic Acid & Aldonolactones

• An aldose is defined as a monosaccharide whose carbon skeleton has an aldehyde group. 

• Aldoses are readily oxidized to aldonic acids by oxidation of the aldehyde group to a carboxyl/carboxylate group. 

• The reaction is commonly used for quantitative determination of sugars. 

• One of the earliest methods for detection and measurement of sugars employed the Fehling solution.

• Fehling solution is an alkaline solution of copper(II) that oxidizes an aldose to an aldonate and in the process is reduced to copper(I), which precipitates as brick-red Cu2O. 

• Variations (the Nelson– Somogyi and Benedict reagents) are still used for determining amounts of reducing sugars in foods and other biological materials.

• In the process of oxidizing the aldehyde group of an aldose to the salt of a carboxylic acid group, the oxidizing agent is reduced, that is, the sugar reduces the oxidizing agent; thus, aldoses are called reducing sugars. 

• Ketoses are also termed reducing sugars because, under the alkaline conditions of the Fehling test, ketoses are isomerized to aldoses. 

• The Benedict reagent, which is not alkaline, will react with aldoses, but not with ketoses.

       2.Reduction of Carbonyl Groups

• When a reactant loses electrons during a reaction, it is called oxidation. When a reactant accumulates electrons during a reaction, it is called reduction.  

• Hydrogenation is the addition of hydrogen to a double bond. 

• When applied to carbohydrates, it entails addition of hydrogen to the double bond between the oxygen atom and the carbon atom of the carbonyl group of an aldose or ketose. 

• Hydrogenation of D-glucose is readily accomplished with hydrogen gas under pressure in the presence of Raney nickel as a catalyst

• The product  is D-GLUCITOL, commonly known as sorbitol, the -itol suffix denoting a sugar alcohol (an alditol). 

• Alditols are also known as polyols and polyhydroxy alcohols.

• Sorbitol is widely distributed in plants, ranging from algae to higher plants, where it is found in fruits and berries; but the amounts present are generally small. 

• It is about half as sweet as sucrose, is sold both as a syrup and as crystals, and is used as a general humectant, that is, a substance that will hold/retain moisture in a product.

D-Mannitol

• D-mannitol can be obtained by hydrogenation of D-mannose.

• Commercially, it is obtained along with sorbitol from hydrogenolysis of sucrose. 

• It is a product of hydrogenation of the d-fructose component of sucrose and of isomerization of D-glucose, which can be controlled by the alkalinity of the solution undergoing catalytic hydrogenation. 

• D-mannitol, unlike sorbitol, is not a humectant. 

• Rather, it crystallizes easily and is only moderately soluble.

• It has been used as a non sticky coating on candies. 

• It is 65% as sweet as sucrose and is used in sugar-free chocolates, pressed mints, cough drops, and hard and soft candies

Xylitol

• Xylitol is produced from hydrogenation of D-xylose obtained from hemicelluloses, especially from birch trees. 

• Its crystals have a high negative heat of solution.

• This endothermic heat of solution of crystalline xylitol produces a cooling effect in the mouth. 

• This cooling effect makes xylitol desirable as an ingredient in mint candies and in sugarless chewing gum. 

• Its sweetness is about equal to that of sucrose.

•  Xylitol is anti cariogenic because it is not metabolized by the microflora of the mouth that produce dental plaques.