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Transcript
Amino acids are often referred to as ‘the building blocks of life’. This is because they combine in different sequences to form proteins, which are fundamental to all living organisms. There are 21 amino acids, 9 of which are called ‘essential’ because they cannot be naturally found in the body. This worksheet will explain what makes an amino acid, along with some of their interesting features. Structure General formula: All amino acids have the same backbone, which contains exclusively COOHCHRNH2 carbon, oxygen, hydrogen and nitrogen. The R you can see in the general formula represents the different R group attached to each amino acid. These different R groups determine the chemical personality of an amino acid; they each have a distinct shape, size and charge distribution and can be large and bulky or small, and polar or non-polar. The R group is important because the properties of amino acids dictate how they interact with each other and the environment they’re in. Glycine (GLY) is the simplest amino acid as it’s R group is simply a hydrogen atom. There are aliphatic and aromatic R groups, as well as nitrogen, sulfur or oxygen containing groups. Zwitterions As you can see from the general formula and the glycine molecule above, an amino acid has a basic amine group and an acidic carboxylic acid group. However, the hydrogen from the carboxylic acid is able to migrate to the amine, leaving a zwitterion. Overall, a zwitterion has no electric charge, but separate parts of the ion are positively and negatively charged. Amino acids actually exist as zwitterions even in the solid state. They crystallise in a lattice like most salts with strong intermolecular ionic interactions. The nature of the ions can be controlled by adjusting the pH of a solution of amino acids, as demonstrated below with glycine. In alkali solution (increased pH), H+ is removed from the NH3+ group leaving NH2CHRCOO-. The amino acid in it’s zwitterionic form, NH3+CHRCOO-. In acidic solution (decreased pH), COO- picks up H+, giving NH3+CHRCOOH. At the isoelectric point, there is no overall charge on the molecule. Optical Isomers Most amino acids have four different groups attached to the central carbon atom (excluding glycine, which as you have seen has two hydrogen atoms). This means that they are chiral; the central carbon atom is a chiral centre. Because of this, each amino acid has two enantiomers whose mirror images cannot be superimposed onto one another – this is a type of stereoisomerism called optical isomerism. D-enantiomers rotate polarised light clockwise. Lenantiomers rotate polarised light anticlockwise. All natural amino acids have L-configuration. You don’t need to memorise all of the amino acids. Just remember that the R groups are all different, and it’s these that determine the chemical personality of an amino acid. R-groups Glycine (H) Gly, G Alanine (CH3) Ala, A Valine (CHCH3CH3) Val, V Isoleucine (CHCH3CH2CH3) Ile, I Serine (CH2OH) Ser, S Threonine (CHCH3OH) Thr, T Cysteine (CH2SH) Cys, C Methionine (CH2CH2SCH3) Met, M Phenylalanine (CH2Ph) Phe, F Tyrosine (CH2PhOH) Tyr, Y Tryptophan Trp, W Aspartic acid (CH2COOH) Asp, D Glutamic acid (CH2CH2COOH) Glu, E Asparagine (CH2CONH2) Asn, N Glutamine (CH2CH2CONH2) Gln, Q Histidine His, H Lysine (CH2CH2CH2CH2NH3+) Lys, K Arginine (CH2CH2CH2NHCNH2NH2+) Arg, R Produced by Lucy Jakubecz at Newcastle University as part of an MChem project. Leucine (CH2CHCH3CH3) Leu, L Proline Pro, P