Starch Structure

Sources for starch

Starch is the major carbohydrate reserve in plant tubers and seed endosperm where it is found as granules , each typically containing several million amylopectin molecules accompanied by a much larger number of smaller amylose molecules. By far the largest source of starch is corn (maize) with other commonly used sources being wheat, potato, tapioca and rice. Amylopectin (without amylose) can be isolated from 'waxy' maize starch whereas amylose (without amylopectin) is best isolated after specifically hydrolyzing the amylopectin with pullulanase . Genetic modification of starch crops has recently led to the development of starches with improved and targeted functionality.

Structural unit

Starch consists of two types of molecules, amylose (normally 20-30%) and amylopectin (normally 70-80%). Both consist of polymers of α-D-glucose units in the ⁴C₁ conformation. In amylose these are linked -(1" width="22" height="10">4)-, with the ring oxygen atoms all on the same side, whereas in amylopectin about one residue in every twenty or so is also linked -(1" width="22" height="10">6)- forming branch-points. The relative proportions of amylose to amylopectin and -(1" width="22" height="10">6)- branch-points both depend on the source of the starch, for example, amylomaizes contain over 50% amylose whereas 'waxy' maize has almost none (~3%) .

Molecular structure

Amylose and amylopectin are inherently incompatible molecules; amylose having lower molecular weight with a relatively extended shape whereas amylopectin has huge but compact molecules. Most of their structure consists of α-(1" width="22" height="10">4)-D-glucose units. Although the α-(1" width="22" height="10">4) links are capable of relatively free rotation around the (φ) phi and (ψ) psi torsions, hydrogen bonding between the O3' and O2 oxygen atoms of sequential residues tends to encourage a helical conformation. These helical structures are relatively stiff and may present contiguous hydrophobic surfaces.

Amylose

Amylose molecules consist of single mostly-unbranched chains with 500-20,000 α-(1" width="22" height="10">4)-D-glucose units dependent on source (a very few α-1" width="22" height="10">6 branches and linked phosphate groups may be found , but these have little influence on the molecule's behavior ). Amylose can form an extended shape (hydrodynamic radius 7-22 nm ) but generally tends to wind up into a rather stiff left-handed single helix or form even stiffer parallel left-handed double helical junction zones. Single helical amylose has hydrogen-bonding O2 and O6 atoms on outside surface of the helix with only the ring oxygen pointing inwards.

Hydrogen bonding between aligned chains causes retrogradation and releases some of the bound water (syneresis). The aligned chains may then form double stranded crystallites that are resistant to amylases. These possess extensive inter- and intra-strand hydrogen bonding, resulting in a fairly hydrophobic structure of low solubility. The amylose content of starches is thus the major cause of resistant starch formation.

Single helix amylose behaves similarly to the cyclodextrins by possessing a relatively hydrophobic inner surface that holds a spiral of water molecules, which are relatively easily lost to be replaced by hydrophobic lipid or aroma molecules. It is also responsible for the characteristic binding of amylose to chains of charged iodine molecules (for example, the polyiodides; chains of I₃^- and I₅^- forming structures such as I₉^3- and I₁₅^3-; note that neutral I₂ molecules may give polyiodides in aqueous solution and there is no interaction with I₂ molecules except under strictly anhydrous conditions) where each turn of the helix holds about two iodine atoms and a blue color is produced due to donor-acceptor interaction between water and the electron deficient polyiodides.

Amylopectin

Amylopectin is formed by non-random α-1" width="22" height="10">6 branching of the amylose-type α-(1" width="22" height="10">4)-D-glucose structure. This branching is determined by branching enzymes that leave each chain with up to 30 glucose residues. Each amylopectin molecule contains a million or so residues, about 5% of which form the branch points. There are usually slightly more 'outer' unbranched chains (called A-chains) than 'inner' branched chains (called B-chains). There is only one chain (called the C-chain) containing the single reducing group.

A-chains generally consist of between 13-23 residues . There are two main fractions of long and short internal B-chains with the longer chains (greater than about 23-35 residues) connecting between clusters and the shorter chains similar in length to the terminal A-chains .

Each amylopectin molecule contains up to two million glucose residues in a compact structure with hydrodynamic radius 21-75 nm . The molecules are oriented radially in the starch granule and as the radius increases so does the number of branches required to fill up the space, with the consequent formation of concentric regions of alternating amorphous and crystalline structure.

B - shows the organization of the amorphous and crystalline regions (or domains) of the structure generating the concentric layers that contribute to the “growth rings“ that are visible by light microscopy. C - shows the orientation of the amylopectin molecules in a cross section of an idealized entire granule.

D - shows the likely double helix structure taken up by neighboring chains and giving rise to the extensive degree of crystallinity in granule. There is some debate over the form of the crystalline structure but it appears most likely that it consists of parallel left-handed helices with six residues per turn. An alternative arrangement of interconnecting clusters has been described for some amylopectins .

Some amylopectin (for example, from potato) has phosphate groups attached to some hydroxyl groups, which increase its hydrophilicity and swelling power. Amylopectin double-helical chains can either form the more open hydrated Type B hexagonal crystallites or the denser Type A crystallites, with staggered monoclinic packing, dependent on the plant source of the granules . Type A, with unbroken chain lengths of about 23-29 glucose units is found in most cereals.

Type B, with slightly longer unbroken chain lengths of about 30-44 glucose units is found in banana, some tubers such as potato and high amylose cereal starches. There is also a type C structure, which is a combination of types A and B and found in peas and beans. Starch granule architecture has been recently described