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Re: WSN: Dielectric Constant for weak electrolyte (fwd)
Sender: bruce_bush@merck.com (Bruce Bush)
Subject: Re: WSN: Dielectric Constant for weak electrolyte
Tony Vargese writes:
> I am trying to look at the effect of large voltage changes on cell
>membrane currents. To do this I need to know what the dielectric
>constant is for solutions like those found in the body:
Dear Dr. Varghese (Dear Tony):
This is a very interesting problem. Though I have no expertise in the
area, I wonder whether aqueous dielectric constant(s) have much effect on
membrane currents? Perhaps it is good enough to assume infinite (or very
large) static dielectric constant, and some fast (or infinite) diffusion rate
for each ion species within the aqueous media -- i.e. assume instantaneous
equilibration of electrolyte at constant potential throughout the cell?
My simplistic reasoning is that the membrane has a much smaller
static dielectric constant than the aqueous compartments inside and outside
the cell. Furthermore, the time scale for ion current across the membrane
is likely to be limited by the channel itself, rather
than by diffusion of the ion in question within the aqueous media.
The membrane potential will influence this ion current, and it depends
in a time-dependent way on all the other ion concentrations. However,
the flux of the other ions will depend on the conductances of their own
channels more than on their diffusion constants in the aqueous phases.
A more precise model would use measured ion mobilities in a physiological
electrolyte, under an applied uniform field as a function of frequency.
The mobilities or conductances constitute an (imaginary) dielectric response
function for each ion current, that takes into account the viscous effects
of the water on the ion in question as well as the time-dependent dielectric
response of the water and all the other ions. Probably the experimental
response function could be plugged directly into a geometric model of the
membrane-channel system. Again I imagine that the results of solving
such a model would depend little on the details of this response function:
equilibration would be very fast, with hardly any concentration gradients.
Regards --
Bruce Bush Merck Research Laboratories Rahway NJ 07065 USA (908) 594-6758
bruce_bush@merck.com