Phasing power of cenrtic vs acentric reflections

[matte at skyfox.usask.ca]

Hello,
	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.

With thanks,

Allan Matte
Dept.of Biochemistry
University of Saskatchewan
CANADA

matte at sask.usask.ca