Proteins. Some reactions of the R groups

Proteins

2. The a-amino acid building blocks

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2.4 Chemical properties of a-amino acids
c. Some reactions of the R groups

The side chain group influences the chemical properties of an a-amino acid . A functional group in the side chain will add to the reactions that the a-amino acid can take part in.

Questions (Group 5):
Using the three letter abbreviations, list those a-amino acids with side chain groups that:
a. can react with an alcohol to form an ester
b. are unsaturated
c. would give a violet complex if a neutral solution of iron(III) chloride was added
d. would be hydrolysed in aqueous acid to give a -COOH group and ammonium ions
e. can react with an acid chloride to give an ester.

alternative

The ninhydrin reaction
Ninhydrin (or 1,2,3-indantrione monohydrate) is an important analytical reagent. It reacts with a-amino acids to produce a purple coloured compound. We can use this colour-forming reaction to locate the colourless a-amino acid spots on a chromatogram (see Part 2, Basic experiment 4b) or the intensity of the purple colour can be used as a way of measuring the amount of a-amino acid in a sample.
An outline scheme for the ninhydrin reaction is shown on the left and below.

ninhydrin

Reactions that can link R groups
In most proteins some a-amino acid units are covalently linked to other units at a distance along the same chain or in a different chain altogether. This happens when side chain R groups react together.

The most common type of R group link is the disulphide bridge (-S-S-) that forms from two cysteine R groups joining together. This type of cross-link is important in maintaining the three dimensional structure of protein molecules (see Figure 5 in chapter 2).

The making and breaking of a disulphide bridge are redox reactions .

Monosodium glutamate, MSG - a flavour enhancer
MSG is probably the most common additive that we use to increase and improve the flavour of food. It is the sodium salt of glutamic acid and has the `E number’ E621.

The Chinese have been using it for a very long time as it is one of the components of soy sauce. The Chinese make soy sauce by the hydrolysis and fermentation of soya beans. Hydrolysis breaks down the soya proteins into amino acids and one of these is glutamic acid.

On its own MSG has only a faint meaty taste, but together with processed food products and other additives it has a powerful flavour enhancing effect. Food technologists use MSG in a variety of foods, mainly processed meats and dehydrated products.

Some people seem to react badly to MSG and experience the symptoms of what is called `Chinese restaurant syndrome’. These can include flushing, dizziness and palpitations. Although scientists do not think MSG is harmful it is no longer added to food prepared specially for babies and infants.

using msg Fermented products containing soya.
Right - a `hot’ red pepper paste.
Left - a savoury soy sauce.
Both are sources of monosodium glutamate an a-amino acid flavour enhancer.

	Unilever Education Advanced Series: Proteins