The stability of detergent formulation ingredients, particularly in liquids where the components are in intimate contact,
is a major consideration. Borates inhibit the loss of activity of several useful detergent components.
Enzymes
Several classes of enzymes are now included in both powder and liquid detergents to assist in the removal of organic matter:
for example, proteases act upon protein-based stains such as egg, mucus and blood; lipases deal with fatty deposits like
butter; amylases tackle starchy stains; and cellulases contribute to whitening and depilling of fabrics.
Unlike many enzymes the subtilisins, proteases from Bacillus sp. bacteria, are fairly stable toward, for example, organic
solvents, anionic surfactants and high pH.
However, in liquid detergent compositions, proteases have inherent stability problems. As well as digesting the stains
they are meant to, they also self-digest. This ‘cannibalism’ results in a shorter shelf-life and progressively deteriorating
performance.
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Fig C1: molecular model of the subtilisin enzyme Savinase with phenylethyl-boronic acid (in pink) bound to the active site. |
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However, self-digestion can be controlled by the inclusion of a small percentage (<2wt.%) of boric acid which at higher
concentrations forms polyborates that similarly inhibit self-digestion.
In liquid detergents boric acid temporarily locks into the active site of the enzyme. The active catalytic site for stain
removal in subtilisins comprises three amino acids: serine, histidine and aspartate at positions 221, 64 and 32, respectively,
on the peptide chain.
In the wash, the boric acid is released allowing the enzyme to fulfil its function.
Borate-polyol complexes (see pH effects section) also act as enzyme stabilization systems and have been found to be
effective in preventing lipases, amylases and cellulases (themselves proteins) being attacked by proteases.
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Enlarge Image |
| Fig C2: schematic of boric acid complexing with serine and histidine of the active site of a subtilisin |
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