Optical control of ultrafast structural dynamics in a fluorescent protein

Area of Science:

• Biophysical Chemistry
• Structural Biology
• Ultrafast Spectroscopy

Background:

• Photoisomerization in fluorescent proteins occurs on ultrafast timescales.
• Femtosecond optical excitation triggers complex structural dynamics involving electronic and vibrational processes.
• X-ray crystallography probes ultrafast dynamics, but the origin of observed nuclear motions remains unclear.

Purpose of the Study:

• To investigate the origin of ultrafast structural dynamics observed in fluorescent proteins using high-resolution pump-probe X-ray crystallography.
• To differentiate between photoisomerization reaction dynamics and other vibrational processes.

Main Methods:

• High-resolution pump-probe X-ray crystallography was employed to probe structural dynamics.
• Coherent-control experiments using two-color, two-pulse optical excitation were performed.
• Analysis of X-ray crystallographic difference density and photoisomerization depletion.

Main Results:

• Observed sub-ångström, ultrafast motions and hydrogen-bonding rearrangements in the fluorescent protein active site.
• Demonstrated that these motions originate from impulsively driven coherent vibrational processes in the electronic ground state, not photoisomerization.
• Coherent-control experiments amplified X-ray difference density while fully depleting photoisomerization, confirming wave packet assignments.

Conclusions:

• The study clarifies the nature of ultrafast motions observed in fluorescent proteins via X-ray crystallography.
• Impulsively driven coherent vibrations, not the photoisomerization reaction, are responsible for the measured dynamics.
• Coherent-control mechanisms provide a powerful tool to assign wave packets and understand complex molecular dynamics.