Phasing power of cenrtic vs acentric reflections
ccp4 at yorvic.york.ac.uk
Fri Feb 2 07:13:17 EST 1996
In article <1FEB96.17403227 at skyfox.usask.ca> matte at skyfox.usask.ca writes:
>From: matte at skyfox.usask.ca
>Date: 1 FEB 96 17:40:32 GMT
>Organization: University of Saskatchewan
> In MIRAS calculations with MLPHARE from the CCP4 suite, I've
>noticed that the phasing power (FH/<E>) for acentric reflections
>is always greater than for the centric reflections for a given derivative.
>Its not intuitively obvious to me why this should be the case, as either
>the r.m.s. isomorphous difference would have to be lower, or the closure
>error larger, on average, for the centric than for acentric reflections.
>This seems to make no sense, since the phase of the centric reflections
>should in principle be uniquely pre-determined, and so the closure error
>very small. This is certainly seen in the higher figures-of-merit
>for centric vs acentric reflections, as one would expect. A recent paper
>by J.P. Abrahams and A.G.W. Leslie (Acta Cryst. D52, 30-42, 1996) on the
>structure determination of F1 ATPase indicated similar results with MLPHARE,
>also in P212121 (compare PhPa and PhPc of Table 1). I must be missing
>something. Please enlighten me if you have some ideas about this.
>University of Saskatchewan
>matte at sask.usask.ca
Actually MLPHARE does not quote (FH/<E>) as the phasing power. It
give (FH/(lack of closure)) in a resolution range. <FH> is much the
same for centric and acentric reflections, but <lack of closure> for
centrics should be proportional to <FH> + errors, while for acentrics
it should be proportional to <FH>/sqrt(2) + errors - that is related
to the fact that <Diso> = <FH cos(random angle)>.. If your data was
free then you would see an exact sqrt ratio between the two - in fact
it isnt so the ratio will in general be < sqrt(2).
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