Theoretical Studies of Molecular Magnetism and Molecular Recognition, and Experimental Studies of the Extracellular Domain of the Nicotinic Acetylcholine Receptor

Abstract

Investigations of molecular magnetism, molecular recognition, and the nicotinic acetylcholine receptor are reported. Chapter 2 Studies of the spin state preferences of organic biradicals provide information valuable for the design of molecular magnetic materials. Ab initio calculations on pyridine and pyridinium analogues of m-xylylene indicate that the neutral heterocycle is essentially equivalent to benzene as a ferromagnetic coupling unit, while the cationic pyridiniums behave much differently. Depending on the substitution pattern, a protonated pyridine can serve as a ferromagnetic coupling unit or an antiferromagnetic coupling unit. Similar calculations on the m-divinylbenzene dication lend further support to the polaronic ferromagnet design strategy. Both valence bond and molecular orbital analyses provide qualitative rationalizations of these results. Chapter 3 The quadrupole moments of aromatic compounds are increasingly being appreciated as important determinants in molecular recognition. The cation-π interaction between Na+ and a series of simple aromatic systems has been evaluated in a series of ab initio calculations. The total binding energies correlate very well with predictions based on a simple electrostatic model, which emphasizes the interaction of the cation with the quadrupole moment of the aromatic. The benzene···hexafluorobenzene stacking interaction is evaluated at several levels of theory. At the MP2/6-31G** level, it is estimated that the interaction is stabilizing by approximately 3.7 kcal/mol. This is a fairly strong non-covalent interaction, suggesting this motif may be a valuable supramolecular synthon. Chapter 4 To provide material suitable for structural studies of the nicotinic acetylcholine receptor, we have expressed and purified the N-terminal extracellular domain of the mouse muscle α subunit. A fusion protein (α210GPI) consisting of the 210 N-terminal amino acids of the a subunit and a glycosyl-phosphatidylinositol anchorage sequence was expressed in stably transfected mammalian cells. The α210GPI protein was cleaved from these cells and purified on an immunoaffinity column. Characterization of this protein demonstrates that it is processed as expected, exists as a monomer, and retains the two distinct, conformation-specific binding sites expected for the correctly folded a subunit. Circular dichroism studies of α210GPI suggest that this region of the receptor includes considerable β-sheet secondary structure, with a small proportion of α-helix.