Chitosans are made from chitin. Chitin is one of the most abundant biopolymers on Earth. It reinforces the cuticle of insects and spiders, the shells of crabs and shrimps, and the cell walls of fungi and moulds. It clearly is one of the success molecules of evolution! And it can easily be extracted from shrimp shell wastes of the fishery industries. Chemists found out that chitin is a long-chain molecule (a biopolymer) consisting of up to 5000 sugar molecules, each adorned with a molecule of acetic acid (that’s the molecule which makes vinegar acidic). If one removes some or all of the acetic acid residues, the polymers, which are now called partially or fully deacetylated chitosans, become water-soluble. That’s interesting, because now one can start playing with these molecules.
But what is more important: wherever an acetic acid molecule is removed, the “scar” that is left carries a positive charge. This is highly unusual! Most other biological molecules are negatively charged. Thus, they are attracted by the positive charges of chitosans: The molecules start interacting. But only those molecules will interact where the positive charges of chitosans exactly match the negative charges of the other biomolecules. Thus, the question of how many of the acetic acid residues of a chitin chain were removed, i.e. the degree of acetylation of the chitosan molecule, determines its positive charge density. And this in turn determines with which negatively charged biomolecules the chitosan will interact i.e.: its bioactivity!
In addition to the degree of acetylation, the degree of polymerisation, i.e. the chain length of the chitosan biopolymer, also influences its bioactivity. This is because small molecules, named oligomers, are more agile and may reach places which the large polymers may not reach. But large polymers may survive longer in the presence of enzymes that can degrade chitosans than oligomers which may quickly be degraded and, thus, inactivated completely. Such enzymes are present everywhere, in the soil, on and in the plant, even in animal and human tissues. These enyzmes may also slowly degrade chitosan polymers, liberating agile and active oligomers.
The team of Prof. Dr. Moerschbacher was among the first to show this dependence of bioactivities from both the degree of acetylation and the degree of polymerisation. In collaboration with chemists, they were able to show that some, but not all chitosans have antimicrobial properties. And some other chitosans have plant strengthening activities. Thus, if you use just any chitosan off the shelf without knowing its structure well, you cannot expect reliable performance in a given application. But if you know which chitosan you have to use, and if you know how to produce this chitosan with no batch-to-batch difference, you will see reliable performance, batch after batch!
Such chitosans which are well-defined in their degree of acetylation and in their degree of polymerisation are now called ‘second generation’ chitosans. They are available from some quality-concerned chitosan producers, but most still offer first generation chitosan with more or less unknown structure and often large batch-to-batch differences. First generation chitosan is less expensive and is sufficient for material applications of chitosans such as waste water purification. But products which are based on the bioactivities of chitosans require the rare, and more expensive second generation chitosans.