Diffractive optics-based heterodyne-detected four-wave mixing signals of protein motion: From “protein quakes” to ligand escape for myoglobin

Abstract

Ligand transport through myoglobin (Mb) has been observed by using optically heterodyne-detected transient grating spectroscopy. Experimental implementation using diffractive optics has provided unprecedented sensitivity for the study of protein motions by enabling the passive phase locking of the four beams that constitute the experiment, and an unambiguous separation of the Real and Imaginary parts of the signal. Ligand photodissociation of carboxymyoglobin (MbCO) induces a sequence of events involving the relaxation of the protein structure to accommodate ligand escape. These motions show up in the Real part of the signal. The ligand (CO) transport process involves an initial, small amplitude, change in volume, reflecting the transit time of the ligand through the protein, followed by a significantly larger volume change with ligand escape to the surrounding water. The latter process is well described by a single exponential process of 725 ؎ 15 ns at room temperature. The overall dynamics provide a distinctive signature that can be understood in the context of segmental protein fluctuations that aid ligand escape via a few specific cavities, and they suggest the existence of discrete escape pathways.

Protein structural dynamics in solution unveiled via 100-ps time-resolved x-ray scattering

“…4C around 1 μs corresponds to departure of CO from the protein into the surrounding solvent. The timescale for this departure is similar to that reported using transient grating techniques (26,27). The integrated SAXS feature also appears to be sensitive to ligand migration to other internal cavities in the protein.…”

Myoglobin: The hydrogen atom of biology and a paradigm of complexity

“…E-mail: frauenfelder@lanl.gov. structure in a quake-like motion (10,11). The changes are not large but are important for the function.…”

Two-color Transient Grating Spectroscopy of a Two-level System

“…where (8) With this short time expansion approximation, the two auxiliary functions, and , can be approximately written as (9) where . We further assume that and to find .…”

“…In the present paper, we will consider two-color transient grating spectroscopy (2C-TG), where the pump and the probe frequencies are different from each other. 7,8 In this case, the initial wavepacket or doorway state, created by the interactions with the pump pulses is located at a different region of phase space than that of the window specified by the width and frequency of the probe pulse. On the other hand, in the case of conventional 1-C TG, the phase space region of the initial wavepacket is identical with the window state.…”