pKa Determination of Alpha Conotoxin MII and Analogs
Dr. Owen McDougal
The pKa for histidine and aspartate residues is important in understanding the mechanism of Alpha Conotoxin MII (α-CTx MII) binding to Nicotinic Acetylcholine Receptors. Nuclear Magnetic Resonance (NMR) spectroscopy was used to characterize the pKa of the imidizole ring of histidine and the carboxylate of aspartate at differing pH (3.3, 5.5, 7.2, 8.9). NMR was used to observe the chemical shift change due to protonation/deprotonation of the side chain functional groups when the peptide was placed under conditions of variable pH. A protocol reliant on a control sample of imidazole as an internal pH reference for NMR studies has been established. The chemical shift of the ring protons in imidazole serves as an internal pH reference that allows the pKa value of histidine and aspartate residues to be determined by observed change of chemical shift or disappearance of exchangeable proton. An understanding of pKa values for select amino acid side chains will allow a mechanism of action to be proposed for analogs of α-CTx MII in an attempt to design peptides effective as treatments for neurodegenerative diseases such as Parkinson’s Disease.