Conservation Science

Textiles 75

Fibroin is largely composed of three amino acids, glycine (45%), alanine
(29%) and serine (12%); it can be divided into two main components, the crys-
talline and the amorphous regions, which are in the approximate ratio 3:2. The
crystalline regions are formed of a hexapeptide motif (shown below), folded
into an anti-parallel -sheet secondary structure, held together be extensive
hydrogen bonding (Figure 14) and strongly aligned with the fibre axis.

• Gly-Ala-Gly-Ala-Gly-Ser-
  or

N

N

N

N

N

O

O

O

O O

O

H

H

H

H

H

R

H

N N

R

R

H O

OH

N N N N N

RO

H OH

N

N N N N N

R

HO

R

• Intermolecular
  Hydrogen Bond


• Me or CH 2 OH

R

R

NNNN

H

H H

H

H

H

H H

H

H

H

H

H

O H

OO

O

O

O

O

O

O

O

O

O

R O

R R

R R

R

Figure 14Hydrogen bonding in silk -sheets

N

H

N

H

N

H

N

H

N

H

N

OOOOOOH

HO

n

The crystalline close packing of this region is facilitated by the small, non-
bulky side-chains of the residues ( H, Me or CH 2 OH); see Figure 15. The
unit cell is built from four protein chains, with abbeing in the plane of the
-sheets and blying along the fibre axis.
The highly crystalline nature of fibroin leads to the characteristic properties
of silk fibres: a good mechanical strength and limited extensibility, arising from
the extensively inter-bonded, fully extended nature of the protein chains, and