Infrared spectroscopy is ideally suited for the investigation of
protein reactions at the atomic level. Many systems were investigated
successfully by applying Fourier transform infrared (FTIR) spectroscopy.
While rapid-scan FTIR spectroscopy is limited by time resolution (about
10 ms with 16 cm–1 resolution), step-scan FTIR spectroscopy
reaches a time resolution of about 10 ns but is limited to cyclic
reactions that can be repeated hundreds of times under identical conditions.
Consequently, FTIR with high time resolution was only possible with
photoactivable proteins that undergo a photocycle. The huge number
of nonrepetitive reactions, e.g., induced by caged compounds, were
limited to the millisecond time domain. The advent of dual-comb quantum
cascade laser now allows for a rapid reaction monitoring in the microsecond
time domain. Here, we investigate the potential to apply such an instrument
to the huge class of G-proteins. We compare caged-compound-induced
reactions monitored by FTIR and dual-comb spectroscopy by applying
the new technique to the α subunit of the inhibiting Gi protein and to the larger protein–protein complex of Gαi with its cognate regulator of G-protein signaling (RGS).
We observe good data quality with a 4 μs time resolution with
a wavelength resolution comparable to FTIR. This is more than three
orders of magnitude faster than any FTIR measurement on G-proteins
in the literature. This study paves the way for infrared spectroscopic
studies in the so far unresolvable microsecond time regime for nonrepetitive
biological systems including all GTPases and ATPases.
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