Aspartic acid , ionisation

The pH has a great influence on the ionisation of polar functional groups in the protein-bound amino acids, and thus on the abihty of proteins to interact with water (bind water). Ionised functional groups of proteins interact with water in a similar way to salt ions. The ionised basic side chains of lysine and histidine bind about four molecules of water by hydrogen bonds, the acidic side chains of glutamate and aspartate bind about six water molecules, while the neutral carboxyl groups of amino acids (interaction of dipole-dipole type) bind two water molecules of as well as the polar non-ionised side chains of serine and other amino acids. 

Ionic bonds are formed between ionised acidic side-chains and ionised basic side-chains. Figure 28.14 shows the ionic bonds that form between the negatively charged aspartate side-chain and the positively charged lysyl side-chain. 

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