Molecular diffusion cannot account for spreading of isotope distribution peaks during rapid axoplasmic transport.

Diffusion coefficients calculated from spreading peaks of isotope distributions formed during rapid axoplasmic transport are two orders of magnitude larger than coefficients established for proteins in water. For slow transport, however, the coefficients approximate those of diffusion in water. We propose a near-equilibrium partitioning mass transfer process as the major source of peak spreading during rapid transport and suggest the axon can be modeled by a chromatographic column.