Silk
consist of two fundamental proteins sericin and fibroin which is emitted by
silkworm. 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

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Chemical structure:

 

Both the proteins sericin and fibroin
have 18 similar amino acids
such as glycine, alanine and serine in variable amounts. Sericin is the sticky material surrounding
fibroin and fibroin is the structural centre of silk. Fibroin comprises of mainly
amino acids: Gly-Ser-Gly-Ala-Gly-Ala and forms beta-pleated sheets known as  ?-keratin1. There are many different silk
polymorphs which generally seen in (silk I ) water soluble state and comes in
glandular state before crystallization ,(silk II) which is often seen in
spun  silk state and air/water assembled
interfacial silk usually in helical structure (silk III). Silk I is usually
exposed to heat or physical heat spinning to convert it to silk II, it can be
easily done as silk II structure consist of ?- sheet secondary structure. Silk
I in aqueous condition when exposed to methanol or potassium chloride, the
surface of the ?-sheet structure is asymmetrically divided into hydrogen side
chains and methyl side chains. Hydrogen bonds and van der Waals forces
interacts with the methyl group and hydrogen groups to make the inter-stacking
sheets of crystal to be thermodynamically stable2. Silk II structure at the
later stage deny water and becomes less or completely not soluble in several
solvents very mild acidic and basic conditions.

The
structure represents a tight packing of stacked sheets of hydrogen bonded in an
anti-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 where
the hydrogen bond network are situated . Fibroin contains a high proportion of
three ?- amino acids (G; Gly, 45%, R=H), alanine (A;
Ala, 29%, R=CH3), and serine (S; Ser, 12%, R=CH2OH) the
approximate 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 one
hydrogen and, so it is not sterically constrained. Alanine and serine has many
interceded hydrogen bonds and are strong and resistant to breaking. The less
crystalline forming regions are known as linkers which consists of fibroin
heavy 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 acid
residues found in crystalline region. Primary sequence of proteins is highly
repetitive which provides homogeneity in the secondary structure. Primary
sequence generates hydrophobic proteins which are in natural co-polymer block
design. The interspace is filled with many hydrophobic and hydrophilic domains,
large hydrophobic domains interspace with smaller hydrophilic domains to
bolster the assembly of silk and improves the strength and resiliency of fibre.