Flow orientation of brain microtubules studied by linear dichroism

Flow orientation of bovine brain microtubules has been studied using phase-modulation detected linear dichroism, LD, in a Couette cell with radial light propagation. LD could be sensitively measured in a wide flow gradient interval: 10(-3)-10(3) s-1, without any apparent degradation of the microtubule structure. An extremely small flow gradient, 10(-3) s-1 is sufficient to give significant orientation, and 10 s-1 rapidly produced a very high degree of orientation. It is also shown that thermal convection effectively orients microtubules in vitro. The apparent linear dichroism is dominated by an anisotropic scattering from the aligned microtubules, superimposed on a weaker absorption dichroism due to intrinsic chromophores. The linear dichroism due to anisotropic turbidity, LD tau, is found to be an excellent tool for monitoring the formation of microtubules and in contrast to ordinary turbidity measurements, non-specific aggregates contribute to a far less extent. Time resolved LD tau was used to study the orientational relaxation of microtubules upon stopped shear. The relaxation towards random orientation can be described by a slow, multi-exponential decay. With increasing protein concentration the relaxation becomes slower and above approximately 1 mg/ml a fraction with a semipermanent orientation is formed. Finally, the development of orientation with time upon applying a small, constant gradient has been measured and the results are considered in terms of a model for flow orientation of rigid rods.