An analytic expression is derived for the distribution G(R) of the end-to-end distance R of semiflexible polymers in external potentials to elucidate the effect of confinement on the mechanical and statistical properties of biomolecules. For parabolic confinement the result is exact, whereas for realistic potentials a self-consistent ansatz is applied, so that G(R) is given explicitly even for hard-wall confinement. The theoretical result is in good quantitative agreement with fluorescence microscopy data for actin filaments confined in rectangularly shaped microchannels. This allows an unambiguous determination of persistence length LP and the dependence of statistical properties such as Odijk's deflection length λ on the channel width D. It is shown that neglecting the effect of confinement leads to a significant overestimation of bending rigidities for filaments.
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