Savory - Arachnida 1977

33
Chemical Arachnology
In the study of arachnology there are encountered a number of substances,
the chemical constitution of which is of unusual interest. Among
these are chitin, silk, venom, hormones and pheromones.
CHI TIN
A considerable proportion of the exoskeleton of an arachnid has been
believed to consist of chitin, a resistant material that is unaffected by air
or water and is insoluble in caustic alkalis. It is dissolved by strong
sulphuric or hydrochloric acid, forming a solution from which dilution
or neutralization causes it to be deposited in a changed form as a white
precipitate.
If into glucose molecule, CH 2 0H(CH.OH) 4 CHO, the amino radical
NH 2
is introduced, the product is glucosamine, CH 2 0H(CH.OH) 3
CH.NH 2
.CHO. If next one ofthe hydrogen atoms of the amino radical
is replaced by the acetyl radical, CH 3 , the compound so formed is
acetyl glycosamine, CH 2 0H.(CHOH) 3 CH.NH(CH 2 CO).CHO. Finally,
a polymer of this is chitin, to which the following structural formula
has been assigned :
33. CHEMICAL ARACHNOLOGY 283
The structural formula displays the presence of the peptide bond
CO.NH, which is characteristic of all protein molecules. Hence chiti~
?as been doubtfu_lly described as a polypeptide. Nor, it may be added, is
lt synonymous w1th the comparable substance cuticulin.
SILK
No material is more characteristic of Arachnida than is silk, which is
produced by spiders, pseudoscorpions and Acari. Its striking properties
and its differences from the familiar silk of Bombyx mori haYe made a
determination of its composition desirable, despite the manipulative
difficulties involved.
A thread of unspun commercial or "true" silk consists of a core of the
pr~t~in fibro.in covered with a sheath or outer layer of another protein,
sencm. A sp1der's thread has no coat of sericin; the fibroin of which it
is almost wholly made is very closely allied to the fibroin of true silk
but is not identical with it. The silk from either animal is stained yello~
by zinc chlor-iodide and dissolves in hot caustic potash or in cold concentrated
sulphuric acid, but whereas true silk resists the action of
chromic acid, acetic acid or ammonia, all these substances cause spider
silk to swell or contract.
~he firs~ publis?ed analysis of spider-silk was due to Fischer ( 1907).
This mentiOned nme constituents, most which were amino acids of
\vhich the chief was alanine at 23·4°/.J·
'
It was soon realized, however, that the silk from different species is
slightly different, as also is the silk from different glands of the same
species, and this was included in an analysis by Peakall. This precision
was taken a stage further by Anderson (1971), who pointed out that
analysis of a \Vhole gland, by including the cells of the gland itself, introduced
an avoidable inaccuracy. He therefore analysed the contents
of the lumen only. His results mentioned 16 constituents, ten of which
were not included bv Fischer.
A general impression of whole matter may be gained by comparing
the figures of these three analyses, but limited to the chief compounds
common to the two most recent sets of figures.
When strong acids hydrolyse this molecule, it is decomposed into
acetic acid and glucosamine,