Distance determination
with pulsed EPR has become an
important
technique for the structural investigation of biomacromolecules, with
double electron–electron resonance spectroscopy (DEER) as the
most important method. GdIII-based spin labels are one
of the most frequently used spin labels for DEER owing to their stability
against reduction, high magnetic moment, and absence of orientation
selection. A disadvantage of GdIII–GdIII DEER is the low modulation depth due to the broad EPR spectrum of
GdIII. Here, we introduce laser-induced magnetic dipole
spectroscopy (LaserIMD) with a spin pair consisting of GdIII(PymiMTA) and a photoexcited porphyrin as an alternative technique.
We show that the excited state of the porphyrin is not disturbed by
the presence of the GdIII complex and that herewith modulation
depths of almost 40% are possible. This is significantly higher than
the value of 7.2% that was achieved with GdIII–GdIII DEER.
Abstract. EPR distance determination in the nanometre region has become an
important tool for studying the structure and interaction of macromolecules.
Arbitrary waveform generators (AWGs), which have recently become
commercially available for EPR spectrometers, have the potential to increase
the sensitivity of the most common technique, double electron–electron
resonance (DEER, also called PELDOR), as they allow the generation of
broadband pulses. There are several families of broadband pulses, which are
different in general pulse shape and the parameters that define them. Here,
we compare the most common broadband pulses. When broadband pulses lead to a
larger modulation depth, they also increase the background decay of the DEER
trace. Depending on the dipolar evolution time, this can significantly
increase the noise level towards the end of the form factor and limit the
potential increase in the modulation-to-noise ratio (MNR). We found
asymmetric hyperbolic secant (HS{1,6}) pulses
to perform best for short DEER traces, leading to a MNR improvement of up to
86 % compared to rectangular pulses. For longer traces we found symmetric
hyperbolic secant (HS{1,1}) pulses to perform
best; however, the increase compared to rectangular pulses goes down to 43 %.
Abstract. Laser-induced magnetic dipole (LaserIMD) spectroscopy and
light-induced double electron–electron resonance (LiDEER) spectroscopy are
important techniques in the emerging field of light-induced pulsed dipolar
electron paramagnetic resonance (EPR) spectroscopy (light-induced PDS). These techniques use the
photoexcitation of a chromophore to the triplet state and measure its
dipolar coupling to a neighboring electron spin, which allows the
determination of distance restraints. To date, LaserIMD and LiDEER have been
analyzed with software tools that were developed for a pair of two S=1/2
spins and that neglected the zero-field splitting (ZFS) of the excited triplet.
Here, we explore the limits of this assumption and show that the ZFS can
have a significant effect on the shape of the dipolar trace. For a detailed
understanding of the effect of the ZFS, a theoretical description for
LaserIMD and LiDEER is derived, taking into account the non-secular terms of
the ZFS. Simulations based on this model show that the effect of the ZFS is
not that pronounced in LiDEER for experimentally relevant conditions. However,
the ZFS leads to an additional decay in the dipolar trace in LaserIMD. This
decay is not that pronounced in Q-band but can be quite noticeable for lower
magnetic field strengths in X-band. Experimentally recorded LiDEER and
LaserIMD data confirm these findings. It is shown that ignoring the ZFS in
the data analysis of LaserIMD traces can lead to errors in the obtained
modulation depths and background decays. In X-band, it is additionally
possible that the obtained distance distribution is plagued by long distance
artifacts.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.