INTRODUCTION Carbohydrates

1 INTRODUCTION

The Germans first and foremost introduced the word ‘kohlenhydrates’ which was later on coined to carbohydrates. The name obviously suggests that these compounds are essentially the hydrates of carbon. In reality, all carbohydrates comprise of carbon, hydrogen and oxygen; whereas, the last two elements are found to exist in the same proportions as in water ( i.e., H₂O – 2:1). However, it has been observed that there are certain compounds that do conform to the said ‘hydrate rule’ i.e., maintain the ratio of H and O (2:1) but do not belong to the category of carbohydrates, for instance:

(i) Formaldehyde [HCHO] 2:1

(ii) Acetic Acid [CH₃COOH] 2:1

(iii) Lactic Acid [C₃H₆O₃] 2:1

Besides, there exist such compounds that evidently show the chemical properties of carbohydrates but do not necessarily abide by the above mentioned ‘hydrate rule’, for example:

In view of the above cited glaring examples with regard to various anomalies the terminology ‘Carbohydrates’ has still been retained to represent not only the sugars but also those substances that are related to them basically in structure and other characteristic features.

Invariably, the carbohydrates belong to the chemical class of the aldehydes, ketone alcohols, and also the condensation polymers of these partially oxidized polyalcohols collectively known as ‘Polysaccharides’ or ‘Oligosaccharides’.

Glycan is the generic term for polysaccharide and in the systematic nomenclature the latter is assigned a suffix “-an”. Generally, the polysaccharide may be classified into two broad heads, namely:

(a) Homoglycan: The polysaccharide is termed as homoglycan when it contains only one type of monosaccharide unit, and

(b) Heteroglycan: The polysaccharide is known as heteroglycan when it involves more than one kind of monosaccharide unit.

However, a more accurate and precise demarkation of polysaccharides essentially makes use of nomenclature that includes, first the type of monosaccharide building unit, and secondly, the exact position and configuration of the glycosidic linkage involved.

Examples:

(i) Homoglycan: e.g., Cellulose. It may also be expressed as β-1, 4 –D-glycan by virtue of the following reasons, namely:

* Prevailing attached unit is D-glucose,

* D-glucose bears the β-configuration at the anomeric C-atom (i.e., C-1),

* C-1 is linked to C-4 of the next identical unit of D-glucose.

(ii) Heteroglycan: e.g., D-gluco-D-mannose. It is made up of D-glucose and D-mannose. The two altogether different monosaccharides usually show up in an orderly manner. In this particular instance, the diheteroglycan is composed of two different types of monosaccharides that has been arranged in an alternating and regular fashion.

It is worhwhile to mention at this juncture that plant kingdom provides a variety of complex polysaccharrides, such as: cellulose, starch, dextran, inulin and the like. These complex polysaccharides yield the respective sugar residues upon hydrolysis, for example:

Pentosans Hydrolysis-> Pentoses, Arabinose, Xylose, Ribose;

Hexosans Hydrolysis-> Hexoses, Glucose, Fructose;

Fructan Hydrolysis-> Inulin that results Fructose;

Glucan Hydrolysis-> Strach that gives Glucose.

Nevertheless, the starch and sugars find their abundant applications not only as food or food supplements, but also as indispensable adjuvants in the formulation of a wide range of pharmaceutical products all over the globe.

REFERENCES

Ashutosh Kar (2003), Pharmacognosy and Pharmaco biotechnology, 2nd Edition

‘Handbook of Medicinal Herbs’ (2001), J.A. Duke, CRC-Press, London, 1st Edn.

William Charles Evans (2002), Trease and Evans Pharmacognosy 15th Edition by: Trease, Bailliere Tindall; Evans.

Ramstad (1956), E., ‘Modern Pharmacognosy’, McGraw Hill, London.