Information for Questions 1 to 3:

Amino acids are the building blocks of proteins. The basic structure of an amino acid is:

where the R group is different for each of the 20 naturally occurring amino acids.

The pKa for the amine group –NH₃+ is: 9
The pKa for the carboxyl group –COOH is: 2

Some of the 20 amino acid side chains also have pKa values and others do not. Eg:

Amino acids are linked together by peptide bonds in which the carboxyl group of one amino acid reacts with the amine group of the next in a condensation reaction:

1. Many glycine molecules are placed in a solution with pH 9. In what proportion of these molecules will the NH₃+ group remain undissociated (or protonated)?

2. What will be the structure of the majority of molecules if histidine molecules are placed in a solution with pH 4?

3. A glycine-cysteine amino acid dimer is placed in a solution with pH 9. What is the approximate overall charge of the molecule?

ANSWERS

View the answers - PDF - 71KB