Bridging the gap between Molecular Dynamics and EPR spectroscopy: Application to Liquid Crystal systems

Lead Research Organisation: University of East Anglia
Department Name: Chemistry

Abstract

Liquid crystals (LCs) are an intermediate state of matter that combines both long range order, as found in crystals, and disorder, a characteristic of fluids. This combination of properties gives rise to unique optical and electrical properties that allow LCs to be switched rapidly by electric fields. These properties underlay their widespread application, for example, in displays as seen in flat TV screens and digital watches. The ordering of molecules in LC phases can be, for example, as parallel rods (termed nematic) or as stacks of discs (termed columnar discotics). The latter can conduct charge up and down the columns. This offers potential applications as organic electronic charge transport materials in devices such as light emitting diodes (LEDs), one-dimensional conductors, to provide new types of photoconductors, and photovoltaic solar cells. The underlying molecular organisation must be engineered to produce electron-hole recombination for luminescence or separation for solar cells. Molecular organisation can be by vacuum deposition or the formation of films. The growing demand for LCs with new properties is leading to the development of new mixtures of liquid crystals, new ways to align LC layers for holographic video projection, and the production of new polymeric liquid crystals. The exploitation of self-organising LCs is an important avenue of current research. In order to design novel materials with desired properties it is necessary to describe and predict their behaviour from the molecular level. Recently, major advances in both theoretical and experimental areas have emerged which promise progress in the studies of complex partially ordered molecular systems such as LCs. Firstly, spin labels, specially designed chemical agents that carry a stable unpaired electron, can be introduced within complex molecular systems in order to report on the order and dynamics of surrounding molecules. Because an electron has a magnetic moment it can interact with an external magnetic field. Electron Paramagnetic Resonance (EPR) measures this interaction in the form of spectral line shape. The orientation of the spin label to the magnetic field has a dramatic effect on this line shape and therefore molecular mobility, dynamics and distribution can be studied. EPR is a technique that acts as a snapshot of very fast molecular motions and can resolve molecular re-orientational dynamics of the introduced spin probe over times shorter than a billionth of a second. Recent advances in EPR instrumentation, using different frequencies with spin labels and probes has become an important method for studying structure and dynamics of complex phases such as LCs, of proteins and their complexes, DNA/RNA, polymers, lipids and nanostructures. Secondly, the huge increase in computer power over the last decade has led to an increase in the use of molecular dynamics (MD) simulations as a tool to understand complex chemical systems with the potential to predict various properties of complex self-organising systems such as LCs, LC mixtures and composite systems. Yet there is no general methodology and user-friendly computational tools which are able to link directly state-of-the art MD simulations of complex molecular systems with the simulation and analysis of EPR spectra.The aim of this proposal is to bridge the gap between MD simulations and EPR spectroscopy and to develop methodology, which would enable one to obtain a detailed description and reach unambiguous conclusions about molecular arrangement and interactions within complex molecular systems. In the proposed work, this methodology will be applied to different types of LCs, mixtures and hybrid systems. The output of this proposal will be available to the international scientific community in the form of user-friendly software.

Planned Impact

Our proposal aims to develop a general advanced methodology, which will bring together EPR spectroscopy and state-of-the-art computer modelling techniques. Our methodology will contribute to the understanding of the order and dynamics of complex systems at molecular level and will be applied to LC systems. The methods developed could eventually be employed in both research and industry laboratories around the world as a research tool to pursue the design of new materials with useful properties. LC materials are widely used in numerous devices with wide application in many branches of science and technology: biology, science, medicine and also everyday life. A novel tool with enhanced possibility for a detailed analysis of molecular behaviour within LCs will foster the development of new materials with useful properties. Therefore, in the longer term, our proposed research will have impact in enhancing the quality of life. In the shorter term, beneficiaries from our research will be various industrial laboratories who use or are planning to use EPR spectroscopic tools and computer modelling for characterisation and design of novel LC materials. Since, spin labelling EPR approaches are widely used to characterise bio-molecular systems other beneficiaries would be biomedical laboratories and pharmaceutical laboratories who use or will use in the future site-directed mutagenesis with spin labelled EPR. Non-academic laboratories will benefit from our research through the application of novel methodology and user friendly software for MD-EPR simulations and analysis of data. In order to successfully achieve this we will focus on the following: a) making methodology convincing to non-academic users, b) making it easy to use and c) disseminating it as widely as possible by seeking efficient roots to deliver new knowledge to experimental and industrial laboratories in the UK and worldwide. Additional benefits would include: Design and synthesis of novel spin probes for discotic/columnar LCs as a result of collaboration with synthetic chemists and research and professional skills in different disciplines, which a PDRA will gain through working on the project and obtaining research training. We will develop a website, from which users will be able to download software and other supporting files and detailed instructions on how to use them. We plan to disseminate our new methodology as widely as possible. This will involve various knowledge transfer activities for engagement and communication with non-academic communities. This will include publication in non-specialist media, arranging day training workshop and developing links with industrial laboratories. VSO has experience in communication with media and dissemination research to people outside research community through his previous engagement with media and knowledge transfer activities. We also intend to take full advantage of our dedicated website in order to present successful examples of application using non-specialised language and video images. Collaborations with other research partners will play an important role in efficient dissemination and exploitation of our results. In order to disseminate our results as widely as possible and to achieve maximal impact we will integrate our codes with highly popular and widely used EPR simulation suite developed by Prof. Hanson's group from the Centre for Magnetic Resonance, University of Queenland, Australia. This EPR simulation suite has been distributed by Bruker Biospin, a world leading commercial manufacturer of EPR spectrometers. Our research will involve collaboration with the synthetic group at UEA led by Dr Andrew Cammidge. One of the outcomes of the proposed MD-EPR work will be the design and synthesis of novel spin probes. Those probes which will be most suitable for a combined MD-EPR approach, in terms of providing information about mobility and order in discotic/columnar systems, would be patented.

Publications

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Gopee H (2013) Probing columnar discotic liquid crystals by EPR spectroscopy with a rigid-core nitroxide spin probe. in Angewandte Chemie (International ed. in English)

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Tyrrell S (2013) Simulation of electron paramagnetic resonance spectra of spin-labeled molecules from replica-exchange molecular dynamics. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Lawrence EJ (2014) An electrochemical study of frustrated Lewis pairs: a metal-free route to hydrogen oxidation. in Journal of the American Chemical Society

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Oganesyan V (2014) The 28th British Liquid Crystal Society Annual Meeting 2014 in Durham in Liquid Crystals Today

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Jayasooriya UA (2016) Rate of Molecular Transfer of Allyl Alcohol across an AOT Surfactant Layer Using Muon Spin Spectroscopy. in Langmuir : the ACS journal of surfaces and colloids

 
Description In order to design novel materials with desired properties it is necessary to describe and predict their behaviour from the molecular level. Recently, major advances in both theoretical and experimental areas have emerged which promise progress in the studies of complex partially ordered molecular systems such as liquid crystals (LC). Firstly, spin labels, specially designed chemical "agents" that carry a stable unpaired electron, can be introduced within complex molecular systems in order to report on the order and dynamics of surrounding molecules. Because an electron has a magnetic moment it can interact with an external magnetic field. Electron Paramagnetic Resonance (EPR) measures this interaction in the form of spectral line shape. The orientation of the spin label to the magnetic field has a dramatic effect on this line shape and therefore molecular mobility, dynamics and distribution can be studied. EPR is a technique that acts as a "snapshot" of very fast molecular motions and can resolve molecular re-orientational dynamics of the introduced spin probe over times shorter than a billionth of a second. Secondly, the huge increase in computer power over the last decade has led to an increase in the use of molecular dynamics (MD) simulations as a tool to understand complex chemical systems with the potential to predict various properties of complex self-organising systems such as LCs and composite systems.

In this project we have developed a novel general theoretical and computational approach for calculation of EPR spectra from MD trajectories. This approach has bridged the gap between theoretical modelling and EPR allowing prediction of motional EPR spectra completely from MD simulations of bulk systems. Such an approach not only greatly simplifies the interpretation and analysis of experimental results, providing unambiguous conclusions about molecular order and motions, but also serves as a rigorous test bed for molecular modelling. Software for prediction of EPR spectra from MD has been generated using MATLAB.

We have applied our new MD-EPR simulation methodology combined with the variable-temperature EPR to both nematic and discotic LCs. In particular, we have carried out the first successful simulation of motional EPR spectra of nematic LCs doped with nitroxide spin probes directly and completely from fully atomistic MD. Predicted changes in molecular order, dynamics and EPR line shapes across the nematic-isotropic (N-I) phase transition show excellent agreement with experiment. A unique combination of state-of-the-art molecular modelling and EPR spectroscopy has revealed a nanoseconds exchange dynamics between partially ordered and disordered meta-stable states at the critical point of the N-I phase transition.

We carried out the first application of EPR spectroscopy to discotic columnar liquid crystal using specially designed and synthesised planar rigid core spin probe. For discotic LCs MD-EPR simulations have been achieved by a combination of the atomistic modelling of solute probe molecule with coarse-grained solvent.

We have calculated various parameters from MD trajectories to complement the interpretation of EPR data and demonstrated the advantages of using our combined MD-EPR approach for providing a new level of detail in molecular motions and order. In particular, we showed how an accurate estimate of molecular rotational correlation times in liquid crystals can be achieved and correlated with the motions of the spin probe. Our method also allows an accurate description of the director distribution in LCs.
Most recently we have reported for the first time the application of our MD-EPR methodology to lyotropic liquid crystals to study the molecular order and dynamics in different aggregation states, namely, pre-micellar, micellar, rod and lamellar lyotropic aggregates.
We also have extended our MD-EPR methodology to spin labelled proteins and DNA structures.
Exploitation Route The developed general methodology can be transferred by others to the study of a wide range of liquid crystals.
New knowledge on the molecular motions and self-organization in both nematic and discotic liquid crystals can guide the synthesis of novel systems with desired functionalities.
Novel spin probes can be used by others to probe different liquid crystals.
Sectors Chemicals,Other

 
Description Our novel MD-EPR simulation methodology has attracted strong international interest and have been expanded to soft matter systems beyond liquid crystals (e.g. biological membranes). Requests have been made for the use of our software by several research groups. During and after the completion of the project the PI has been approached by different research labs (e.g. Kent State University, Cambridge Engineering Department, Centre of Molecular Materials for Photonics and Electronics) who develop new technological devices based on liquid crystal applications. The requests were concerned with the application of the developed methodology to the modelling of novel materials as well as measurement and analysis of their properties. This collaborative work is currently in progress. Also, in 2013 the PI was approached by Acal Energy Ltd., a UK based industrial company developing polyoxomoetalates for PEM fuel cell technology, who were interested in applying our methods. We have provided consultancy to them and performed variable temperature EPR experiments and interpretation of results for their systems. A recent review published by the PI in 'SPR: Electron Paramagnetic Resonance' of the RSC with the title: 'Computational approaches for simulating motional EPR spectra' is partly based on the methodological developments and applications carried out during the project. It has attracted strong international interest and serves as a teaching tool for different researches.
First Year Of Impact 2012
Sector Chemicals,Other
Impact Types Economic,Policy & public services

 
Description Faculty PGR Studentship
Amount £65,000 (GBP)
Organisation University of East Anglia 
Sector Academic/University
Country United Kingdom
Start 09/2013 
End 10/2016
 
Description Novel nitroxide spin probes for liquid crystal studies 
Organisation University of East Anglia
Country United Kingdom 
Sector Academic/University 
PI Contribution Design, synthesis and application of novel spin probes to discotic columnar liquid crystals
Collaborator Contribution Synthesis of two novel spin probes
Impact As a result, the first application of EPR spectroscopy to columnar discotic liquid crystal using spin probes has been reported in high impact journal Angewandte Chemie Int. Ed.
Start Year 2010
 
Title Advanced software for prediction and analysis of EPR spectroscopic data from the results of Molecular dynamics simulations 
Description The software is developed in two versions, one based on MATLAB code for just spectra generation and a more advanced one based on C programming combined with the Qt GUI application development framework for building a state-of-the-art modern GUI for cross-platform applications. It provides simulation of CW EPR spectra directly and completely form single dynamical trajectories of spin probe's re-orientational diffusional motions. Applicable to both Brownian Dynamics (BD) and Molecular Dynamics (MD) trajectories. 
Type Of Technology Software 
Year Produced 2011 
Impact So far the software has been used by the group members at UEA for the simulation and analysis of the EPR spectra of various systems with doped spin probes leading to the publication in high impact journals. 
 
Description Committee member of British Liquid Crystal Society (BLCS). 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact From 2012 is serving as Committee member of British Liquid Crystal Society.

As a Committee member I have been invited to write a Conference report paper about the Liquid Crystals meeting in Durham in 2014
Year(s) Of Engagement Activity 2012,2014