[RASMB] Interference & membrane proteins

[Holger Strauss]

Hello everybody,

we're currently trying to examine a membrane-protein complex of the
ABC-transporter family. The complex needs ATP for stabilisation, so we
have to use interference optics - as we plan to measure drho/dc at
chemical equilibrium of solvent and solution, that didn't seem too much of
a problem. 
But: interference requires that ALL co-solvents in the solution sediment
EXACTLY the same way as they do in the solvent (which is why the reference
has to be in dialyis equilibrium, obviously) - meniscus matching granted. 
Now: we're beginning to doubt
that the detergent (decanoylsucrose) really sediments at the same rate(s)
in solvent and solution. 
First, we haven't been able up to now to produce
meaningful SV-fringe patterns. Most importantly, we got on several
occasions a negative fringe displacement at the bottom of the cell(s),
which could be interpreted as a selective accumulation of material in the
reference sector. Yet, there was mass transport at the meniscus. 
Secondly, looking at the detergent alone shows that
there are two limiting species, one at 30 kDa and one at 140 kDa, which
show a dependency on concentration (global weight average decreasing), but
aren't formed fully reversibly (local averages don't overlay), for what
thermodynamic/structural reasons ever.

Have others come across the same problems and what did they do about them? 
Are there references of studies on membrane proteins with interference
around (we're not aware of any)?
Are there truly monodisperse detergens?
General comments?

Sorry for being that lengthy.

Greetings to all, Holger

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Holger Strauss 

Forschungsinstitut fuer Molekulare Pharmakologie (FMP)
Robert-Roessle Strasse 10

13125 Berlin/Germany

Tel: +49 (0)30 94793 - 223 (office)
                     - 316 (lab)

Fax: +49 (0)30 94793 - 169 


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Science is spectrum analysis; art is photosynthesis.

                                                    Karl Kraus