Applications of light induced Electron Paramagnetic Resonance to biological systems - from structure determination towards biological quantum gates

Electron Paramagnetic Resonance (EPR) is a fast-growing technique with applications in diverse fields including structural biology and quantum information processing. It is used to study systems containing unpaired electrons, analogous to NMR.

Understanding the structure of a biological system, or changes under different perturbations, is important for determining functionality. Pulsed dipolar EPR can be used to quantify nanometre distances between spin-centres, chemical moieties with unpaired electrons, by measuring the dipolar interaction between them. Usually spin-centres used in EPR have unpaired electrons in their ground states, e.g. nitroxides, radicals or some metal clusters/ions, and are often added to the system via structure modification (mutagenesis) and tagging.

Recently we have shown that optically-excited triplet-states can be used to measure distances via Light Induced Triplet-Triplet Electron Resonance (LITTER). Two lasers are coupled into the EPR spectrometer, independently forming two triplet-states, and microwave pulses are used to manipulate the electron spins and detect the dipolar interaction between the triplet spin-centres. LITTER has great promise to be applied in biological systems, particularly those with native cofactors, such as heme groups, that can be used to form the optically-generated triplet states. This is advantageous as it does not require modification or tagging of the biological system, which may cause structural changes.

This project will explore and develop the use of LITTER and other light-induced EPR techniques to investigate the structure of different protein systems; for example, dimeric Myoglobin, Hemoglobin, Neuroglobin, Protochlorophyllide Reductase and Morphinioine Reductase using the triplet-states of native cofactors or chromophore tags. As cofactors are tightly bound within a protein structure the distribution of distances and dipolar interactions between centers will be well-defined, making their use potentially advantageous over chromophore tags which are attached at surface accessible sites and have some flexibility.

Initally we aim to investigate and characterize the properties of different native chromophores that make them suitable or otherwise for study using the novel LITTER method, using both EPR and other biophysical techniques. We aim to compare the results of LITTER in these systems to more established biophysical methods for measuring distances in biological systems such as pulsed dipolar EPR using stable spin centers and Förster Resonance Energy Transfer (FRET). The development of the novel light induced EPR spectroscopy techniques places this project within the remit of the EPSRC.

Furthermore, polarization of the triplet spin-state population formed on optical excitation can lead to an increase in signal relative to conventional EPR methods. The combination of well-defined dipolar interactions that can be present between the cofactors in a biological system and the polarized populations make such systems exciting potential targets for encoding quantum information processing algorithms using EPR methods. As such a second aim is to implement a EPR based quantum information processing pulse sequence that can be used to detect the presence of entanglement between the triplet spin-centers.