[RASMB] back to reality

[Ariel Lustig]

Dear colleauges, lets come back to reality
At  least for new-comers , but not only for them, the subject of predicting or better , calculating the time to achieve equilibrium makes no / or only very little sense , since  as I belive that every  one of us use the
"overspeed procedure" discribed  by Hexner, Radford and Beams PNAS (1961), 47,1848 that reduces time to achieve equilibrium to  50% or more. If so, I ask myself what means time to  equilibrium ? from the time  rotor speed is achieved and not yet a concentration gradient is established, or  a steep graient at both solution  column ends? all non -sense ! I agree that for polydispersed or selfassociating  systems 
using an overspeed procedure is difficult (and dangrous especially it some parts are not reversible), but not less difficult as to calculate time to equilibrium as you suggested of a non monodispersed systeme.
In  FRACTION N° 1 1967 issued  by  Beckman Instruments ( may be based on the same  equation
that Walter  Stafford mentioned recently) K.E Van Holde discribed  a formula where the time  depends on Diffusion ! and here again to new-comers. If at all making the exercise to calculate time to  equilibrium.
It is to use Dobs and not D20w that is correected  for Viscosity  of  water. There may be a huge  difference
between the two Diffusion  values if  per example you use sucrose in the  buffer as I do measuring 
membrane  proteins. The density has no influence for  Diffusion  only the  viscosity of the solvent.
yours....ariel
ariel.lustig at bluewin.ch
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