Lysine ionisation

Proton dissociation produces a shift in the resonance positions of nearby nuclei. The pK values of ionising groups are readily determined from a graph of chemical shift versus pH. A classic example is ribonuclease. The C-2 H histidine peaks lie at low field, outside the main spectral envelope, and the pK values of the four histidines range from 5.8 to 6.7 [108]. Methylation of lysozyme lysines gives 6 new peaks at 270 MHz [109] and the e-NH2 pK values can be determined. 

Q1.3 Describe the ionisation or ionisations that occur when fully protonated lysine (Figure 1.10) is subjected to increasing pH. What is the dominant structure present at the isoelectric point ... 

Changes in the solution pH are typically used to elicit a change in the conformation of a peptide. As the formation of secondary stractures in peptides strongly depends on the charge of ionisable amino acid residues (e.g., lysine and glutamic acid), a change in... 

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. 

A wellknown example is given by the ionisation of poly L-glutamic acid. Inversion of the size of the rotatory power in the visible region [24], the change from complex ORD to quasi simple ORD [25] and the change from three main dichroic bands to one only [26] result from the conformational transition from the helix-a unionized form to the random coil ionized form. A similar behaviour has been observed for a stereoregular polybase poly L-lysine [26]. 

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