Hi all,
I'm trying to calculate the theoretical time for a vesicle to sediment during centrifugation. I have (I think) all the information, but the numbers I'm getting out don't seem to make sense.
Sedimentation time (in hours): t = k/s
Where:
k is the centrifuge clearing factor (761.84 in my case)
s is the sedimentation coefficient (in Svedbergs)
s = vesicle mass/(6π * viscosity * particle radius)
In my case:
vesicle mass = 0.24pg
viscosity = 0.001kg/meter/second
vesicle radius = 500nm
Can anyone help? I'm expecting a value in the area of 5-20 minutes, but I'm getting very large answers.
Thanks!
Hefin
Hi Hefin,
did you took into account that you have to transfer your sedimentation coefficient s into Svedbergs (wich is 1 Svedberg = 100fs) ?
Hi Hefin,
I ran into some problems with your definition of the sedimentation coefficient. When I confirmed my memory by a tectbook on Physical Chemistry, I found that the sedimentation coefficien is defined as
s = (vesicle mass - mass of the solvent volume displaced by the vesicle)
/ (6π * viscosity * particle radius)
Is your "vesicle mass" such a difference or just the calculated mass. I ask because we had the problem of vesicle separation about twenty five years ago. The specific mass of the vesicle membrane was lower than that the electrolyte, and the vesicles would float up under centrifugation due to the buoyant force. Therefore, we had to prepare the vesicles in CsCl solution and spun them down in NaCl solution to get a sedimentation at all. Under this condition we found centrifugation times in the order of 300 min.
Thanks both for your answers! So my vesicle mass is just the absolute mass in pg of the vesicle. I know that the vesicles do indeed pellet in my solvent, I would just like a theoretical estimation of pelleting times (in fluids with different viscosities and centrifuges with different K factors) and the the moment, the values I'm getting are completely nonsensical. How might I estimate or find out the mass of solvent volume displaced by the vesicle?
Thanks!
Hefin
Mass of the displaced solution:
e.g. 0.17 M NaCl, 20 °C -> density rho = 1.005 g/mol
radius of vesicle r = 500 nm;
-> volume V = 4/3*pi*r^3 = 5.2*10^-13 ml
mass of solvent volume = volume * rho = 5.26*10^-13 g = 0.526 pg
This is a problem: your vesicle mass = 0.24 pg < 0.526 ng !!
Those vesicle cannot sediment.
There must be something wrong with the given vesicle mass.
I do not know how reliable the "centrifuge clearing factor" is.
What I know, however, is the formula to calculate the centrifugation time
tmax = 9/2*ln(re/ra)/omega^2 / s
re = end distance from the rotation axis
(= bottom of the centrifugation tube)
ra = minimum distance from the rotation axis
(= surface of the aqueous phase in the centrifugation tube)
omega = angular velocity ( omega/(2*pi) = number of turns per time)
s = sedimentation coefficient
according to the given definition of tmax, your k must be = 9/2*ln(re/ra)/omega^2,
and it must have physical units either 1/min^2 or 1/s^2.
Which one is correct?
There is a difference of a factor of 3600 between both!
I hope that will help to get some progress.
Best wishes,
Hans-Jürgen
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