Challenges in pulsed dipolar EPR – metal ions, exchange coupling and multiple spins

Abstract

The function of biomacromolecules is inextricably connected to their structure and structural changes. Distance measurements by pulsed dipolar electron paramagnetic resonance (EPR) have advanced the structure investigation of biologically important systems in recent years. Towards the study of challenging biomolecules, the use of model systems can aid in confirming EPR theory and benchmarking applications. This thesis is an interdisciplinary work comprising synthesis of chemical models and their EPR spectroscopy aiming at assessing the precision and accuracy of the EPR experiments on custom-made systems for future translation of the gained knowledge to more challenging biological systems. Multi-spin systems, paramagnetic metal ions, incomplete dimerisation processes and electron spin exchange interactions can all independently complicate data interpretation and are subjects of study in this thesis. Firstly, multiply labelled systems are studied in silico to understand multi-spin effects on the extracted distance distributions. A new method for recovering distance distributions in these cases is proposed. Secondly, rigid nitroxide biradicals featuring variable degrees of conjugation through the linking groups are subjected to continuous-wave (CW) and pulsed electron-electron double resonance (PELDOR or synonymously double electron-electron resonance DEER) studies for extracting the exchange coupling constant and a potential distribution in exchange couplings. Thirdly, metal-ligand binding studies on templated dimer model systems allows to monitor and quantify dimerisation processes and identify the cooperativity of binding from the PELDOR modulation depths. This is achieved by coordination of a spin-labelled terpyridine ligand to Zn(II), Fe(II), Co(II) and Cu(II) ions acting as the template in different metal-to-ligand ratios followed by nitroxide-nitroxide distance measurements. In the case of Cu(II)-ligand model, where Cu(II) and nitroxide spectra overlap, the modulation depth in the nitroxide-nitroxide distance measurement is derived from relaxation filtered nitroxide-nitroxide distance measurements and with a post-processing correction method. Lastly, relaxation induced dipolar modulation enhancement (RIDME) spectroscopy which is currently gaining increased attention is further scrutinised here. The RIDME modulation depths of nitroxide ligands with a varying degree of Cu(II) ions bound demonstrate for the first time that the technique can be quantitative with respect to its modulation depth in close similarity to PELDOR. This thesis showcases the use of model systems for benchmarking EPR studies tackling the current challenges of EPR applications. It is envisioned that the insights gained here will be of great value once transferred to challenging biological samples.

Date of Award	21 Jun 2017
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Bela Ernest Bode (Supervisor)