Sugar (3): Starch, Difference between Amylose, Amylopectin

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Although human ancestors had been cooking starchy plant tubers since ancient times, research for starch did not begin until 19th century. In 1811, the German chemist Krichoff discovered that dilute sulfuric acid made potato starch taste sweet. It was soon proven to be glucose produced by starch hydrolysis. In the 1940s, the Swiss chemists K. H. Meyer and T. Schoch discovered that starch contains two types of polysaccharides: amylose and amylopectin

Amylose vs. Amylopectin: Similarities

Both amylose and amylopectin are polysaccharides, and glucose is the fundamental unit. A portion of glucose from photosynthesis is transformed into starch and stored in plant tissues, particularly abundant in seeds and tubers. When plants require energy, these starches are hydrolyzed by enzymes into glucose. They can also be decomposed into glucose by dilute acids in the laboratory. Both amylose and amylopectin share the chemical formula (C₆H₁₀O₅)n, and n is the degree of polymerization (DP) ranging from hundreds to several hundred thousand. Helical structure is their common common characteristic.

Amylose vs. Amylopectin: Differences

Structure of Amylose

The hundreds to thousands of glucose molecules linked by 1,4-glycosidic bonds form a long straight chain of amylose. Hydrogen bonds from hydroxyl groups twist the chain into a helix, and each turn of the helix contains six glucose residues. Hydrophobic hydrogen atoms are located inside the helix, while hydrophilic hydroxyl groups are on the outside. Generally, it is unbranched, but a minority of amylose has limited branches due to 1,6-glycosidic bonds, and about one branch per several hundred glucose residues. Most of these are short chains with a DP of around ten, but a few have long chains with a DP of several hundred. Because branches are very rare (less than 1%), their structure and properties are similar to amylose with only straight-chain. Amylose typically makes up 20-30% of starch, but it is particularly abundant in some genetically modified plants and certain natural plants. The high amylose maize and wrinkled peas have about 70% amylose in their starch. The size of amylose varies significantly in different plants. In cereal amylose, the DP is about 300-1200, while in potato it can be as high as around 4000.

Structure of Amylopectin

In addition to amylose, natural starch also contains about 70% amylopectin where the glucoses are not only linked by 1,4-glycosidic bonds, but also branched by 1,6-glycosidic bonds. On average, a branch point occurs every 25-30 glucose units, making amylopectin completely different from amylose and somewhat similar to a tree branch. These chains are divided into A, B, and C types. A-chains are unbranched, B-chains are highly branched, and there is only one C-chain with one reducing end in an amylopectin molecule. C-chains have a wide range of DP from 15 to 120, and 40 is the most common. C-chains serve as the backbone from which some long B-chains grow (DP>37, 60-80 is very common, and some exceeding 100). Some shorter B-chains and A-chains grow from the long B-chain.

All A-chains and some B-chains are located on the outer side of the amylopectin and entangled into left-handed double helices. The branches close to the trunk are long, while those on the outside are short. The average DP of A-chain ranges from 6-12, and short B-chains have a DP of about 13-24. Researchers have also found super-long chains composed of hundreds or even thousands of glucose units, such as indica rice, wheat and buckwheat. Furthermore, the distribution of short chains with DP 6–17 is very characteristic of amylopectin from different samples and has been referred to as the 'fingerprint' of the sample.

The branched structure makes amylopectin significantly larger than amylose. Its DP is 10 to 100 times greater than amylose. It means a molecule contains thousands to several hundred thousand glucose residues. Even within the same plant, DP varies greatly, and the DP of most amylopectin is very large. Some starch produced by photosynthesis is amylose, but some plants do not synthesize them at all. For example, waxy corn and rice contain almost only amylopectin (they are commonly called waxy because of the waxy appearance of the seed endosperm).

Amylose vs Amylopectin: Physical and chemical Properties

Both types of starch change the color of iodine solution: amylose exhibits blue, whereas amylopectin displays a purplish-red hue. Most of amylose resides in the amorphous regions of starch granules and is loosely connected. When its solution cools, an elastic gel forms. On the other hand, the robust double-helix in amylopectin makes it less soluble in water. Its solution is more viscous and form a non-elastic soft gel when cooled. This difference is attributed to the three-dimensional network formed by amylose. Consequently, branches are often chemically removed from amylopectin to make it more like amylose.

Both α-amylase and β-amylase can hydrolyze starch, but they only cleave the 1,4-glycosidic bonds. These enzymes break down amylose into glucose and maltose. However, they partially degrade amylopectin into glucose, maltose and limit dextrin, due to the absence of active sites near the branching points. The γ-amylase is required to destroy the branching points, although it hydrolyzes the 1,6-glycosidic bonds at a slower rate.

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