formaldehyde - Sciencemadness Dot Org

222 FORMALDEHYDE
anism by which they are produced has received comparatively little
attention.
It is generally accepted that formaldehyde reacts most readily with the
free amino groups of proteins. However, it can also react with the amido
nitrogen of the peptide linkage 59 . Reactions are apparently similar in
many respects to those involving simple amines and amides. According to
Gustavson*, data obtained in studies of the action of formaldehyde on
collagen and deaminated collagen are in accord with the formation of
methylene cross-links between peptide chains and micellar units. Theis 114
attributes the Thermal stability of formaldehyde-tanned leather to methylene
bridges connecting adjacent polypeptide chains. Reaction of formaldehyde
with free amino groups is not limited to the formation of methylene
amine linkages, but may also involve the production of triformals 116 .
This is indicated in the following equation; in which the letter P designates
a protein radical to which is attached an amino group:
CH2—O
/ \
P—iNH. -I- SCH.Ofeq) —-* P—N CH2 + H20
\ /
It is probable that simple methylol derivatives are the primary products
formed in all protein-formaldehyde reactions.
Since the formation of methylene bridges between protein molecules
results in a product of increased molecular weight, increased hardness and
reduced water-sensitivity would naturally be expected in the reaction
product. Furthermore, since such reactions result in the removal of reactive
hydrogen atoms from the protein molecule, the stability of formaldehyde-treated
proteins to heat, enzymes, and chemical reagents is increased.
As illustrated hi the case of gelatin 39 , the isoelectric point of proteins is
lowered by reaction with formaldehyde. X-ray diffraction patterns are
also modified 16 .
On heating casein with an excess of acid formaldehyde, degradation of the
protein molecule takes place with the formation of primary, secondar3 r , and
tertiary methylamines equivalent to from 12 to 40 per eent of the total
protein nitrogen. When this reaction is carried out under pressure at 180 °C
with formaldehyde solution containing 5 per cent acetic acid, trimethylamine
is obtained 131 . These reactions are similar to those which take place
when simple amine salts are reacted with formaldehyde.
Proteins containing formaldehyde give characteristic colors both with
sulfuric acid alone and in the presence of nitrous acid 47 - 130 . These tests
are extremely sensitive.
Formaldehyde-treated proteins are particularly stable to the action of