The Effect of pH on the Strength of Keratin (hair protein).

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The Effect of pH on the Strength of Keratin (hair protein)

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Keratin is a fibrous protein and has a structural function. Fibrous proteins contain polypeptides that bind together to form very long fibres running parallel to each other. These fibres are linked by cross-bridges that make the protein strong and insoluble.

The main constituent of hair is the protein keratin. Keratin can also be found in horns, nails, claws and feathers. These examples suggest keratin has a strong structure. Like most other proteins it is made of monomers called amino acids. All Amino acids contain both an acidic carboxylic group  (-COOH) and a basic amino group (-NH2); there is also an R group that changes with each different amino acid. They all consist of carbon, hydrogen, oxygen and nitrogen atoms. Keratin also contains a high concentration of the amino acid cysteine; this contains a sulphur atom. The sulphur atoms from two cysteines join together, forming a very strong disulphide bond. These bonds are covalent and form strong links making the tertiary structure of the protein very stable. The disulphide bonds occur down the length of the keratin fibre and the cross-linking between the keratin chains account for the strength of hair. Within each hair strand the keratin chains are also linked with ionic, salt and hydrogen bonding. Hydrogen bonding occurs from the attraction between the electronegative oxygen atoms on the CO groups and the electropositive H atoms on either the OH or NH groups. Although they are individually weaker than disulphide bonds, hydrogen bonds are in much higher proportions to the disulphide bonds making them important in maintaining the tertiary structure of the protein.

Hair is very resilient and has elastic properties. These elastic properties are also due to the flexibility of the hydrogen bonds. An untreated healthy hair can support a load of up to approximately 100g. The strong disulphide bonds in the cortex help to give strength to keratin. The long keratin molecules in the cortex are compressed to form a regular structure, which is not only strong but also flexible.

Protein structure and their properties are altered by the change in pH values. Keratin is therefore affected by the pH. The pH of a solution is the measure of how acidic or alkaline it is. An acidic solution (pH<7) has a greater abundance of H+ ions and a basic solution (pH>7) has more OH¯ ions present. Most proteins are stable over a limited range of pH values. Outside this range free hydrogen ions or hydroxyl ions affect the charges on amino acids residues, distorting the three-dimensional shape and can cause irreversible change in the protein’s tertiary structure. At very low pH all amino acids exist as ions with an overall positive charge, while at high pH they exist as ions with an overall negative charge.

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Protein structure can be easily disrupted by changes in pH value away from the optimum range. Unfolding of the formation due to pH is a result of ionisation of the R group inside the protein molecule. This damages and breaks the ionic and hydrogen bonds that help to hold the protein and give strength. Consequently weakening the hair strand and causing it to lose its elasticity.  

The hydroxyl ions can split the disulphide bonds, removing the cross-links between keratin chains, weakening the hair. This is why it is possible for strong alkali solutions to dissolve protein.

Most proteins have ...

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A good description of the plan including detailed, relevant background theory. The method could be improved by describing fully how all control variables are to kept constant so that only changes to the independent variable should affect the results.