Femtosecond dynamics of intracellular water probed with nonlinear optical Kerr effect microspectroscopy

A nonlinear optical Kerr effect (OKE) microscope was developed and used to elucidate the ultra-fast diffusive motions of intracellular water molecules . In the OKE microscope, a pump-induced birefringence is sensed by a delaye d probe pulse within a spatially confined volume that measures 0.5 mum in t he lateral direction and 4.0 mum along the axial coordinate. This microscop e allows the recording of time-resolved Kerr signals, which reflect the ult ra-fast structural relaxation of the liquid, exclusively from intracellular aqueous domains. Because relaxation occurs on a picosecond time scale, onl y local diffusive motions are probed. The microscopic OKE signal is therefo re insensitive to long-time-scale hindered translational motions enforced b y intracellular mechanical barriers but probes the intrinsic orientational mobility of water molecules in cells instead. The Kerr response as determin ed from single intact mammalian cells under physiological conditions shows a structural relaxation time of 1.35 ps, which is 1.7 times slower than the Kerr decay observed in pure water. The data indicate that the mobility of water molecules in cellular domains is moderately restricted due to the hig h intracellular content of proteins and solutes.