The isoelectric point of collagen can be found within this region at approximately pH of 7.4. It is within this region that is generally accepted as ‘full neutralisation’ in leather manufacture. There is a pH range between pH six and nine where predominantly carboxylate and ammonium groups are present.
Above a pH of six, all acid groups are deprotonated and above a pH of 12 all amine groups are deprotonated however, there are notable, important exceptions to this as will be discussed later. It is commonly stated that below a pH of three all acid groups are protonated and below a pH of nine all amine groups are protonated. Tyrosine (Tyr), an amino acid containing a phenolic ring, is also acidic, being in either its neutral phenolic form at low pH or as an anionic phenoxide at high pH. Histidine (His), lysine (Lys) and arginine (Arg) each contain side chains that may be in a protonated in positively charged form at lower pH or a deprotonated neutral form at higher pH. Aspartic acid (Asp) and glutamic acid (Glu) have side chains that have a carboxylic acid group present that, depending on the pH, may either be in its neutral protonated form at lower pH or its negative deprotonated carboxylate form at higher pH. The charge on collagen originates from the presence of acidic and basic amino acid residues that, depending on the amino acid and pH, may be positively charged, negatively charged or neutral. It is primarily responsible for swelling during liming, deswelling during deliming, determination of suitable pickling pH for the appropriate tannage and control of penetration and fixing of tanning, retanning, dyeing and fatliquoring agents. However, in the context of leather manufacture, the manipulation of charge on the collagen substrate is of equal importance as manipulation of charge is prevalent throughout almost all stages of wet processing. In this edited extract, AD Ballantyne and SJ Davis of the Institute of Creative Leather Technology (ICLT), University of Northampton, look at modelling the charge across pH and isoelectric point of bovine collagen during leather manufacture.Ĭollagen is of critical importance in leather due to its unique hierarchical structure based on the triple helix of tropocollagen.
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