Silkconsist of two fundamental proteins sericin and fibroin which is emitted bysilkworm. Silk fibroin is produced by sedentary silkworm Bombyx mori and from spiders(Nephila clavipes and Araneus diadematus) it is also a natural protein. R=H, glycine R=CH3, alanine R=CH2OH, serine Chemical structure: Both the proteins sericin and fibroinhave 18 similar amino acidssuch as glycine, alanine and serine in variable amounts. Sericin is the sticky material surroundingfibroin and fibroin is the structural centre of silk. Fibroin comprises of mainlyamino acids: Gly-Ser-Gly-Ala-Gly-Ala and forms beta-pleated sheets known as ?-keratin1. There are many different silkpolymorphs which generally seen in (silk I ) water soluble state and comes inglandular state before crystallization ,(silk II) which is often seen inspun silk state and air/water assembledinterfacial silk usually in helical structure (silk III).
Silk I is usuallyexposed to heat or physical heat spinning to convert it to silk II, it can beeasily done as silk II structure consist of ?- sheet secondary structure. SilkI in aqueous condition when exposed to methanol or potassium chloride, thesurface of the ?-sheet structure is asymmetrically divided into hydrogen sidechains and methyl side chains. Hydrogen bonds and van der Waals forcesinteracts with the methyl group and hydrogen groups to make the inter-stackingsheets of crystal to be thermodynamically stable2. Silk II structure at thelater stage deny water and becomes less or completely not soluble in severalsolvents very mild acidic and basic conditions. Thestructure represents a tight packing of stacked sheets of hydrogen bonded in ananti-parallel chain of protein. Hydrogen bonds are formed in between of each chains,and the side chains form opposite sides(above and below) of the plane surface wherethe hydrogen bond network are situated . Fibroin contains a high proportion ofthree ?- amino acids (G; Gly, 45%, R=H), alanine (A;Ala, 29%, R=CH3), and serine (S; Ser, 12%, R=CH2OH) theapproximate molar weight of these amino acids is 3:2:1 while, the remaining 13%consist of Tyrosine, valine, aspartic acid etc.
Glycine has a high proportion(50%) which allows it to tight packing this is because its R-group has only onehydrogen and, so it is not sterically constrained. Alanine and serine has manyinterceded hydrogen bonds and are strong and resistant to breaking. The lesscrystalline forming regions are known as linkers which consists of fibroinheavy chain they are situated in between 42-44 amino acid residues in length.All linkers do have identical amino acid residues which are charged amino acidresidues found in crystalline region.
Primary sequence of proteins is highlyrepetitive which provides homogeneity in the secondary structure. Primarysequence generates hydrophobic proteins which are in natural co-polymer blockdesign. The interspace is filled with many hydrophobic and hydrophilic domains,large hydrophobic domains interspace with smaller hydrophilic domains tobolster the assembly of silk and improves the strength and resiliency of fibre.