Diffractive optics-based heterodyne detected four-wave mixing studies of protein dynamics: insights into ligand escape and cooperativity in heme proteins

Summary form only given. The relationship between molecular structure and function is of fundamental importance for understanding biological systems. The heme proteins hemoglobin and myoglobin provide ideal model systems for investigating this relationship because their structure and function are well characterized. In addition, they are amenable to optical probes, allowing their functional processes to be initiated by photodissociation. Previous studies on the femtosecond timescale have characterized the dynamics of myoglobin from femtoseconds to nanoseconds. The current work extends these studies to the millisecond regime to capture the full range of functionally relevant motions. These motions are often small and require a highly sensitive spectroscopy for their study. Diffractive optics-based four-wave mixing provides the sensitivity needed to observe changes in radius of <0.001 /spl Aring/. The use of diffractive optics facilitates the separation of Real and Imaginary parts of the /spl chi//sup 3/ signal by providing the required beam geometry for mixing the signal with a reference beam. In addition it offers passive phase-stabilization. A novel detection method that exploits the symmetry of the four-wave mixing experiment has been implemented to provide automatic isolation of the Real part of the signal. This simplifies the interpretation of the data by obviating the need to identify the Imaginary part of the signal. Further improvement in the signal-to-noise is an added benefit of this method.