Chemistry_Grade_10-12 (1).pdf

Figure 1012 shows some examples of different amino

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next, and is sometimes simply represented by the letter ’R’ in a molecule’s structural formula. Figure 10.12 shows some examples of different amino acids. H 2 N C H H C O OH Carboxyl group Amino group glycine H 2 N C H CH 3 C O OH alanine Side chain (’R’) H 2 N C H CH 2 C O OH serine OH Figure 10.12: Three amino acids: glycine, alanine and serine Although each of these amino acids has the same basic structure, their side chains (’R’ groups) are different. In the amino acid glycine, the side chain consists only of a hydrogen atom, while alanine has a methyl side chain. The ’R’ group in serine is CH 2 - OH. Amongst other things, the side chains affect whether the amino acid is hydrophilic (attracted to water) or hydrophobic (repelled by water). If the side chain is polar , then the amino acid is hydrophilic, but if the side chain is non-polar then the amino acid is hydrophobic. Glycine and alanine both have non-polar side chains, while serine has a polar side chain. Extension: Charged regions in an amino acid In an amino acid, the amino group acts as a base because the nitrogen atom has a pair of unpaired electrons which it can use to bond to a hydrogen ion. The amino group therefore attracts the hydrogen ion from the carboxyl group, and ends up hav- ing a charge of +1. The carboxyl group from which the hydrogen ion has been taken then has a charge of -1. The amino acid glycine can therefore also be represented as shown in the figure below. 199
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10.6 CHAPTER 10. ORGANIC MACROMOLECULES - GRADE 12 H 3 N + C H H C O O glycine When two amino acid monomers are close together, they may be joined to each other by peptide bonds (figure 10.13) to form a polypeptide chain. . The reaction is a condensation reaction. Polypeptides can vary in length from a few amino acids to a thousand or more. The polpeptide chains are then joined to each other in different ways to form a protein . It is the sequence of the amino acids in the polymer that gives a protein its particular properties. The sequence of the amino acids in the chain is known as the protein’s primary structure . As the chain grows in size, it begins to twist, curl and fold upon itself. The different parts of the polypeptide are held together by hydrogen bonds, which form between hydrogen atoms in one part of the chain and oxygen or nitrogen atoms in another part of the chain. This is known as the secondary structure of the protein. Sometimes, in this coiled helical structure, bonds may form between the side chains (R groups) of the amino acids. This results in even more irregular contortions of the protein. This is called the tertiary structure of the protein. H 2 N C H H C O OH H 2 N C H CH 3 C O OH H 2 N C H H C O N H C H CH 3 C O OH + + H 2 O (a) (b) Peptide bond Figure 10.13: Two amino acids (glycine and alanine) combine to form part of a polypeptide chain. The amino acids are joined by a peptide bond between a carbon atom of one amino acid and a nitrogen atom of the other amino acid.
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