*** For details on how to be removed from this list visit the *** *** CCP4 home page http://www.ccp4.ac.uk ***
The expected dI/I should go up as a function of resolution (with a wiggle here and there depending on your heavy atom structure and macromolecule). the <dI/I>, or its amplitude related cousin, can be a bit deceptive. The presence of errors have a large influence on the values you get in your data. More informative (for your question) is the following expression: <delta I> \approx 2 f_light f" (Nlight Nheavy)^(1/2) (Parthasarathy & Srinivasan, 1964) note that as f_light does down as a function of resolution, so do your expected bijvoet differences, eventhough f" is constant. This effect coupled with a I/sigI that goes down (it usually does), makes the fraction of realiably measured Bijvoet intensities go down as a function of resolution. HTH Peter ----- Original Message ----- From: Peter Adrian Meyer <[EMAIL PROTECTED]> Date: Tuesday, November 1, 2005 6:31 am Subject: Re: [ccp4bb]: occcupancies > *** For details on how to be removed from this list visit the *** > *** CCP4 home page http://www.ccp4.ac.uk *** > > > > > > The problem with an anomalous signal-to-noise ratio of 1 is that you > > can't tell if you got anything just by looking at the data. > That is, if > > you have no signal at all you still expect your DANOs to have an > average> value that is equal to the average value of SIGDANO. > However, the > > signal does tend to be stronger for low-angle data. The "DelAnom > > correlation between half-sets" analysis done by SCALA does > appear to be > > a good way of detecting pretty weak anomalous signals. I highly > > recommend that you look at it. > > Why would the anomalous signal be stronger for low-angle data? > f'' (as I > understand it) is roughly constant with respect to scattering angle > (resolution), while f0 falls off rapidly with increasing > scattering angle. > delta f' (for anomalous) should behave similiarly. > > Am I missing something here? > > Pete > > Pete Meyer > Fu Lab > BMCB grad student > Cornell University > >
