Conservation Science

82 Chapter 4

Besides acids and alkalis, strong oxidants used to bleach silk also have the
capacity to seriously degrade the fibres. The most significant reactions occur at
tyrosine and threonine side-chains, resulting in their oxidation to acids and the
breaking of surrounding peptide bonds; cross-links are also generated, e.g.
between lysine and tyrosine residues.

Photolytic Damage. Silk is particularly susceptible to damage by radiation,
especially in the ultraviolet region, typified by weakening and embrittlement,
accompanied by yellowing. The explanation for this lies in the relatively high
proportion of aromatic residues in the protein, all of which are prone to photo-
oxidation. The degradation products are generally not well characterised, but
include chromophoric derivatives of tryptophan and tyrosine. Besides side-chain
modification, the radicals generated will affect cleavage of neighbouring peptide
bonds, weakening the fibres (Figure 21). A variety of other photochemical reac-
tionscan also occur, including cross-linking in the amorphous regions, leading
to a loss of flexibility, primarily due to the reactions of photo-activated tyrosine
residues with each other or with lysine. The presence of certain dyes, such as
anthraquinones, will accelerate these various processes, and moisture also has
an influence.
Some weighting agents will act to accelerate the photo-deterioration of
silks too. Metal ions, such as tin, adopt catalytic rôles in degradation reac-
tions, whereas tannin compounds can reduce the pH of the silk to the point
where it becomes particularly susceptible to photodegradation.

Biological Degradation. Microbial (enzymatic) attack can occur on silk,
but it is the water-soluble sericin, that is the more readily depolymerised.

N

O

R

OH

H

O

N

R'

H

H 2 O

OH-

O

N

R'

H

OH

O

R

OH

H 2 N

Figure 20Alkaline hydrolysis of silk

N N

H O

O H

N N

O

OH

O

N

O

O H

NH 3

O

O

O

OH

NH 3

H 2 N

radical

oxidation

H 2 O

Figure 21The radical oxidation and scission of the peptide bond, following
photodegradation