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Different relaxations in myoglobin after photolysis
Matteo Levantino1, Antonio Cupane, László Zimányi
1National Institute for the Physics of Matter and Department of Physical and Astronomical Sciences, University of Palermo, Via Archirafi 36, I-90123 Palermo, Italy.
Summary
Kinetic hole-burning (KHB) and protein relaxation in myoglobin were studied across a range of temperatures. Results reveal KHB dominates at lower temperatures, while protein relaxation and ligand migration become significant at higher temperatures.
Area of Science:
- Biophysics
- Protein Dynamics
- Spectroscopy
Background:
- Understanding protein dynamics, including structural relaxation and ligand migration, is crucial for myoglobin (Mb) function.
- Kinetic hole-burning (KHB) is a technique used to probe ultrafast dynamics in proteins.
Purpose of the Study:
- To elucidate the interplay between KHB, structural relaxation, and ligand migration in myoglobin (Mb).
- To analyze time-resolved absorption spectra in the Soret region of MbCO after photolysis.
Main Methods:
- Measured time-resolved absorption spectra of carbon monoxide myoglobin (MbCO) from 120-260 K and 350 ns to 200 ms.
- Analyzed spectral contributions of photolyzed (Mb*) and liganded (MbCO) states, considering homogeneous bandwidth, vibrational coupling, and conformational heterogeneity.
- Separated time-dependent spectral changes to identify ligand rebinding events.
Main Results:
- KHB is the dominant process at temperatures below 190 K for both Mb* and MbCO.
- At higher temperatures (230-260 K), MbCO shows minimal spectral shift, while Mb* exhibits an ~80 cm⁻¹ shift attributed to protein relaxation coupled to ligand migration.
- The time dependence of the Mb* spectral shift follows highly nonexponential relaxation kinetics, consistent with a glass-like behavior (Kohlrausch parameter 0.25).
Conclusions:
- This study successfully separates time-dependent spectral changes, providing insights into ligand rebinding events in myoglobin.
- Protein relaxation coupled to ligand migration significantly influences Mb* spectral shifts at elevated temperatures.
- The observed relaxation kinetics confirm the analogy between protein dynamics and glassy systems.