replyto=maga-0711951111580001@130.60.120.10&subject=Re: PAGE: Run times and voltages

Nick Theodorakis nicholas_theodorakis at urmc.rochester.ed
Thu Feb 22 10:11:21 EST 2001


In <001c01c09cbc$d47fbda0$53a4ddc3 at cnrsmrs.fr>, zdenek berger wrote:
>
>This is a multi-part message in MIME format.

[note: not anymore; this response was de-MIMEd; please use plain text on usenet]

>  Why are you using volts and not amperes for your SDS-page?.

Because volts (or more accurately, V/cm) is the relevant unit for determining
electrophoretic mobility.

> In Current =
>Protocols,  the values of amperes for SDS-page are given, although it is =
>said that it is possible to use volts as well. 

Perhaps the author of that section should have consulted a basic
physical-biochemistry textbook first.  

>But they don't give any =
>idea about the conseauences (if there are any).

The consequences of using V/cm are better prediction of run times on different
electrophoresis units.

If you want to give the conditions of the run in amps, you will need to also
specify the cross-sectional area of the gel, and whether you have more than one
gel connected (in parallel) to the power supply. 


> I'm wondering what's =
>'better': whether apply the constant force (voltage) or current (and =
>thus have constant movement of ions) for SDS page and also for =
>transfering proteins in western blots. I don't also understand why is =
>the resistance changing during these two procedures. Do you have any =
>idea, please?

In general, resistance goes down when the temperature goes up. For transfers,
the buffer definitely heats up during the run, so the the resitance will go
down; consequently, for a constant voltage, the current will increase (V=IR).
This is why some people prefer to limit the the current during blotting (as by
running the transfer at constant current), so as not to blow out their power
supply.

For discontinuos gels, such as the standard Tris-Cl/Tris-glycine system, it's
more complicated because the ionic compostition of the gel changes during the
run.



-- 
Nick Theodorakis
nicholas_theodorakis at urmc.rochester.edu






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