Rapid motions of protein molecules can be detected by optical techniques that exploit short light pulses. Nanosecond fluorescence polarization studies have shown that whole domains of proteins such as immunoglobin G and myosin can rotate over an appreciable angular range of times of nanoseconds. This type of motion, called segmental flexibility, may be characteristic of many large proteins and molecular assemblies. Time-resolved fluorescence polarization studies have also demonstrated that internal tryptophan residues in some proteins, such as azurin, are quite flexible in the subnanosecond time range. Vary rapid conformational transitions of chromophoric groups can also be viewed by resonance Raman spectroscopy, which displays vibrations that are coupled to electronic transitions. In these experiments, intense light pulses are used to trigger a change in a photolabile molecule. The resonance Raman spectrum of rhodopsin photolysed by a 30 ps pulse from a Nd:YAG laser exhibits lines that are characteristic of a distorted all-trans retinal chromophore. This finding suggest that much of the cis-trans isomerization of retinal is accomplished within a few picoseconds of the absorption of a photon by rhodopsin. The emerging picture is that proteins can be designed by nature to allow very rapid motions of selected regions.
