Investigating dynamics in biomolecular solids by solid-state NMR

Abstract

Solid-state nuclear magnetic resonance (ssNMR) is a powerful non-destructive tool in the analysis of structural and dynamic properties of a variety of complex biomolecules. The vast majority of ssNMR measurements are conducted on spin-½ nuclei, such as ¹³C and ¹⁵N, with the aid of isotopic enrichment. In this thesis we have used ¹⁴N and ¹³C at natural abundance levels to study the structure and dynamics of small molecules with the view to developing these as tools with which to investigate pharmaceuticals and other biomolecular systems. Nitrogen-14 is an element ubiquitous in a vast number of APIs (active pharmaceutical ingredients), though due to the large quadrupole coupling, detection of the naturally abundant (>99%) NMR active isotope ¹⁴N poses a multitude of challenges. However, the presence of this large anisotropic interaction can be advantageous as it offers a wealth of information on the conformation and dynamics in molecular systems. This thesis focuses on exploiting the quadrupolar interaction to glean valuable insight on (1) the dynamics in a family of quaternary ammonium salts (acetylcholine salts) and (2) the influence of the membrane protein Fk-1 on the phospholipid (POPC) bilayer. Through the study of the ¹⁴N lineshape and relaxation as function of temperature, we have been able to probe the dynamics revealing that such measurements can provide valuable insights into how crystal packing and polymorphism can influence the properties of these pharmacologically important sites. We have complemented these ¹⁴N studies with natural abundance CP-MAS ¹³C experiments, studying the influence of temperature range on the lineshape of quaternary ammonium groups. By applying ab initio quantum mechanical calculations to newly derived crystal structures, we have been able to model the chemical exchange processes that lead to complex ¹³C lineshapes to further characterise the dynamics of this important pharmacophore. In an expansion of these studies, we have used ¹⁴N MAS-NMR to study the interaction of phosphatidylcholine headgroups with integral membrane proteins. Employing variable temperature studies, we have been able to use ¹⁴N NMR and complementary ²H NMR of deuterated lipids chains to investigate how integral membrane proteins interact and perturb the structure and phase behaviour of the lipid bilayer.

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ORCID for Philip Williamson: orcid.org/0000-0002-0231-8640

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