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Optical-Pump THz-Probe Spectroscopy of Myoglobin in Solution

Mr. Adrian Buchmann1, Dr. Sebastian Jung2, Mr. Lion-Luca Stiewe1, Prof. Martina Havenith1
1Ruhr-Universit├Ąt Bochum, Bochum, Germany. 2ZEMOS, RUB, Bochum, Germany

Abstract

Proteins are highly flexible structures with modes stretching over all timescales accessible to modern time-resolved spectroscopy. One of these, the picosecond timescale, has recently gained interest, revealing preferred modes within proteins. These preferred modes are suspected to connect allosteric sites to active sites and facilitate catalytic activity. Proteins are perfectly adapted to serve in aqueous environments, controlling the water around them for optimal function. THz spectroscopy of protein solutions has the potential to observe the interaction of the protein with its solvation shell but has so far lacked time resolution. Time-resolved spectroscopy on myoglobin has long focused on its chromophore, a haem group, revealing its relaxation time and electronic structure. However, resolving its energy transfer from the protein side has proven more challenging. Optical-Pump THz-probe spectroscopy is uniquely suited to observe this energy flow out of the chromophore after the haem has been excited and observe the dynamics of the protein and its solvation environment in the THz spectrum with sub-picosecond time resolution. Applying this to an aqueous solution of myoglobin enabled us to see the vibrational coupling of the excited chromophore to its protein environment and further to the hydrogen-bond network around it. This revealed a several picosecond long timescale for energy dissipation into the solvent hydrogen-bond network Utilizing the low-frequency modes as a reporter for vibrational relaxation allows a unique perspective, revealing a new view of the energy transfer within proteins and their connection to the surrounding hydrogen-bond network, potentially opening up the door to time-resolved calorimetric techniques.