Microsecond resolved infrared spectroscopy on non-repetitive protein reactions by applying caged-compounds and quantum cascade laser frequency combs

ABSTRACTInfrared 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 (about10 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 non-repetitive reactions, e.g. induced by caged compounds, were limited to the ms time domain. The advent of dual comb quantum cascade laser allows now for a rapid reaction monitoring in the μs 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, respectively, 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 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 μs time regime for non-repetitive biological systems including all GTPases and ATPases.