3 research outputs found
Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B<sub>12</sub>
Ultrafast, polarization-selective
time-resolved X-ray absorption
near-edge structure (XANES) was used to characterize the photochemistry
of vitamin B<sub>12</sub>, cyanocobalamin (CNCbl), in solution. Cobalamins
are important biological cofactors involved in methyl transfer, radical
rearrangement, and light-activated gene regulation, while also holding
promise as light-activated agents for spatiotemporal controlled delivery
of therapeutics. We introduce polarized femtosecond XANES, combined
with UV–visible spectroscopy, to reveal sequential structural
evolution of CNCbl in the excited electronic state. Femtosecond polarized
XANES provides the crucial structural dynamics link between computed
potential energy surfaces and optical transient absorption spectroscopy.
Polarization selectivity can be used to uniquely identify electronic
contributions and structural changes, even in isotropic samples when
well-defined electronic transitions are excited. Our XANES measurements
reveal that the structural changes upon photoexcitation occur mainly
in the axial direction, where elongation of the axial Co–CN
bond and Co–N<sub>Im</sub> bond on a 110 fs time scale is followed
by corrin ring relaxation on a 260 fs time scale. These observations
expose features of the potential energy surfaces controlling cobalamin
reactivity and deactivation
Ultrafast X‑ray Absorption Near Edge Structure Reveals Ballistic Excited State Structural Dynamics
Polarized ultrafast
time-resolved X-ray absorption near edge structure
(XANES) allows characterization of excited state dynamics following
excitation. Excitation of vitamin B<sub>12</sub>, cyanocobalamin (CNCbl),
in the αβ-band at 550 nm and the γ-band at 365 nm
was used to uniquely resolve axial and equatorial contributions to
the excited state dynamics. The structural evolution of the excited
molecule is best described by a coherent ballistic trajectory on the
excited state potential energy surface. Prompt expansion of the Co
cavity by ca. 0.03 Ã… is followed by significant elongation of
the axial bonds (>0.25 Ã…) over the first 190 fs. Subsequent
contraction
of the Co cavity in both axial and equatorial directions results
in the relaxed S<sub>1</sub> excited state structure within 500 fs
of excitation
Megahertz pulse trains enable multi-hit serial femtosecond crystallography experiments at X-ray free electron lasers
The European X-ray Free Electron Laser (XFEL) and Linac Coherent Light Source (LCLS) II are extremely intense sources of X-rays capable of generating Serial Femtosecond Crystallography (SFX) data at megahertz (MHz) repetition rates. Previous work has shown that it is possible to use consecutive X-ray pulses to collect diffraction patterns from individual crystals. Here, we exploit the MHz pulse structure of the European XFEL to obtain two complete datasets from the same lysozyme crystal, first hit and the second hit, before it exits the beam. The two datasets, separated by <1 µs, yield up to 2.1 Å resolution structures. Comparisons between the two structures reveal no indications of radiation damage or significant changes within the active site, consistent with the calculated dose estimates. This demonstrates MHz SFX can be used as a tool for tracking sub-microsecond structural changes in individual single crystals, a technique we refer to as multi-hit SFX