Electron Spin Resonance

Investigations of Macromolecular Porphyrin Structures

Multi-Porphyrin systems have important roles in energy harvesting, molecular electronics and structural motifs. ESR provides a number of powerful experiments to measure distances between spins and electron spin delocalization. Typical Zn-porphyrins may be synthetically-substituted at certain points with Cu(II) ions. As paramagnetic probe of otherwise diamagnetic Zn-prophyrins, Cu(II) provides an anisotropic g-tensor that allows orientation-selective measurements of dipolar interactions on the molecular frame. Furthermore the conjugated molecular framework leads to non-negligible exchange coupling between Cu(II) ions and the range of exchange coupling is probed with complete synthetic control and by the choice of substituent metal ions with varied ligand covalency.

Selective chemical or electrochemical oxidation or reduction of porphyrin systems leads to open-shell states, or radicals, which are readily measured for variations in g-value and linewidths, giving an indication of electron spin delocalization over the molecular frame. This is shown classically by the second moment of the ESR lineshape. As the optical properties of porphyrins are used in light-harvesting applications, the extent of electron spin delocalization in the photo-excited triplet state can be probed by Transient ESR techniques.

Transient ESR of Donor-Bridge-Acceptor molecules

As a part of cryptochrome studies of spin-correlated radical pairs, the Timmel group has worked on the ESR of DBA molecules such as Carotenoid-Porphyrin-Fullerene (CPF) Triads synthesized by Prof. Devons Gust (Univ. of Arizona, USA), that act as model systems for the mechanism of cryptochromes. Prof. Marilena DiValentin (Padova, Italy) has shown that these molecules provide a rich set of photophysical kinetic schemes that are well-represented in transient ESR methods. Dr. Kiminori Maeda and Prof. Timmel have pioneered new methods in separating the relaxation processes to probe these schemes.

Double Electron-Electron Resonance: Orientation-Selective methods development for ESR investigations.

Using site-directed mutagenesis, it is possible to engineer cysteine residues into specific sites in proteins of interest. The subsequent addition of a so-called spin label, a stable nitroxyl radical which binds to cysteine residues, enables us to probe normally ESR-silent biomolecules. Using the spin label as a molecular spy it is possible to gain information about the local environment and motion of the spin label. By exploiting the strong distance-dependence of the dipolar coupling interactions between two spin labels, it is also possible to perform distance measurements within a single protein or between two or more molecules.

Orientational selectivity
Orientationally-selective DEER spectra and the resulting structure prediction for the reductase-sulfur/ferredoxin protein complex in a P450 system
(Lovett et al., PCCP).

The ability to selectively label any given region of a system makes ESR the technique of choice for the study of large biological systems, which contain an overwhelming number of magnetic nuclei and so would be much more challenging to study using conventional NMR-based methods. Furthermore, ESR is able to probe systems containing paramagnetic transition metals which can cause the collapse of NMR spectra from paramagnetic relaxation.

ESR's particular strengths, namely its ability to characterize long-range interactions over several nanometers and on microsecond time scales, are the basis of our studies. The focus of our investigation lies in the application and development of state-of-the-art (pulsed, continuous wave and time-resolved) ESR methodologies to advance our understanding of vital biological phenomena by studying conformational changes such as those occurring during enzyme catalysis or in assemblies of large biological structures including proteins and viruses.

Our work combines the development of ESR techniques on model systems, and the application of the methodology to a variety of systems.

In particular, we currently focus on:

  1. The study of molecular wires (in collaboration with Professor Harry Anderson)
  2. MRI contrast reagents and their derivatives (in collaboration with Professor Stephen Faulkner )