Thermal shape fluctuations of grafted microtubules were studied using high resolution particle tracking
of attached fluorescent beads. First mode relaxation times were extracted from the mean square
displacement in the transverse coordinate. For microtubules shorter than
10
m, the relaxation times
were found to follow an L2 dependence instead of L4 as expected from the standard wormlike chain
model. This length dependence is shown to result from a complex length dependence of the bending
stiffness which can be understood as a result of the molecular architecture of microtubules. For microtubules
shorter than
5
m, high drag coefficients indicate contributions from internal friction to the
fluctuation dynamics.