Wool fibres are composed of a polymer called keratin which is constructed from monomer units of amino acids. This makes wool a protein. Keratin has a helical configuration in which the degree of polymerisation is unknown since there are quite a large number of possible amino acid combinations. Keratin is composed of twenty amino acids of variable sequence. In general, the amino acids arginine, cystine and glutamic acid constitute at least one third of the wool polymer. The helical keratin structure is referred to as alpha-keratin. If the wool fibre is stretched, the alpha-keratin can unfold giving rise to a new configuration called beta-keratin. A beta-keratin arrangement will always try and revert back to the 'relaxed' helical alpha-keratin configuration.

The peptide group of keratin can be hydrolysed by acids leaving the disulphide bonds which cross link the wool polymer intact. Whist this would weaken the fibre, it would not result in complete decomposition. On the other hand, wool dissolves readily in alkali conditions. This is because the disulphide bond holding the fibre together are hydrolysed by alkali.

Ultra violet wavelengths of sunlight break the peptide and disulphide bonds at the fabric surface causing yellowing and dulling of colour.

Wool is quite difficult to ignite. It will burn but not readily. More information on this can be found here.