In Part 1 of The Heart of Leather Biodegradability, we looked at the role that bacteria and fungi play in biodegradability and introduced the mechanic of their enzyme machinery in adjusting to the surrounding conditions and available nutrients. In this Part 2, we look at some of the challenges that bacteria and fungi offer during the tanning and preparation phases.
The collagen backbone
The most common collagenase that damage leather is known as the metallopeptidase. These enzymes are metal-centred enzymes and as their name suggests they attack the collagen peptide bond. Figure 2A is a reminder of the peptide bond that is so common in the collagen backbone. The peptide bond is the common site of enzymatic and/or acid/alkaline attack (see Figure 2B). Even the hydrogen bond breaking ability of H3O+ have been implicated in acid destruction
Figure 2. A. The chemical structure of a protein peptide bond. B. The illustration of where the proton or hydroxyl ion attack the delta positive carbonyl group (in the peptide bond).
To gain access to these peptide or side chain groups, the enzymatic or chemical catabolite must be able to reach the peptide group. It is believed that the mechanism of prevention of biodegradation in tanned leather is the shielding of these groups. An interpenetrating network of tanning chemicals makes this access even harder. The precipitation of polyphenol vegetable tanning agents may very well cause an interpenetrating network that proves resistant to the effects of enzymes but does not perform favourably against the effects of acid or alkaline. Red-rot of vegetable tanned leather is due to the degradative effects of acid. Alkaline triggered biodegradation of vegetable tanned leather may very well be ascribed to the chemical stripping of the polyphenols, allowing access to the peptide group.
The enzymes required to breakdown the backbone do not always have to be a collagenase, trypsin (another common protease) can cleave the backbone in the position adjacent to the peptide bond on the carbonyl, as opposed to amide side.
Oxidation
It is well known that the tanning materials can undergo oxidation resulting in their destruction – lowering their ability to shield the peptide group and preventing the breakdown of the leather by biological or chemical means. Vegetable tanning agents can be oxidised as can the collagen. Larsen (1995) showed that atmospheric pollution can be inhibited by acid hydrolysis – so it is not all bad news (Florian, 2006)
The side chains of the collagen can also undergo breakdown, for example the modification of arginine into glutamic acid with concomitant production of CO2 and ammonia (Larsen, 2008).
Free radicals generated by oxidative chemistry can wreak havoc on leather chemistry. These radicals can also be formed by UV radiation, oxidative enzymes (oxidases and superoxidases) and can give rise to a chain reaction, even in dry leathers which will result in degradation. Embrittlement of the leather substrate can also take place as oxidation and heat processes can: release plasticisers; decrease molecular volume; or could cause radical mediated step-growth polymerisations (making the collagen stiffer and brittle).
Autooxidation of fats (present as natural or added fat/oils) can also generate radicals, can produce aldehydics and ketones that can cause embrittlement, and can create fatty acids that in turn give rise to acid hydrolysis of the proteins.
At the end of the growth phase, the organisms need to move into full production of enzyme systems that can achieve what is known as secondary metabolism. The enzymes used have to breakdown complex chemistry, and it is the energy required and the ability of these enzymes that begins to slow down the organism life-cycle turnover. For a leather that is full of complex chemistry – or even chemistry that the organism is unable to handle – the result will be an apparent lowering of respiration (indicated by lower CO2 output). The stage where slower secondary metabolism is apparent is known as the stationary phase.
To end the growth story, it is sufficing to say that the organism after the stationary phase is plagued by metabolic product inhibitions, lacks nutrient (as it is generally in high competition with other organisms) and is in a death spiral. A natural system that is in this cycle is usually a closed system and deterioration will most likely rely on chemical rather than biological means moving forward.
References
Larsen, 1995. Fundamental aspects of the deterioration of vegetable tanned leathers. PhD thesis, The Royal Danish Academy of Fine Arts, Copenhagen.
Larsen, 2008. The chemical degradation of leather. Chimia 62: 899–902.
Florian, M-L. E. 2006. Ch. 5. The mechanism of deterioration in leather. In: Conservation of leather and related materials. (Ed. Kite, M. and Thompson, R.) Butterworth-Heinemann, Elsevier, Oxford. p. 36-57.