In reference to the low and high B atoms in a map and the ripples, one can
actually calculate something of educational value using my web applets (with
all the caveats imposed by simplicity of the 1-d case). Here is how:
Go to
http://www.ruppweb.org/new_comp/structure_factors.htm
and set the B-value of the C atom to 100 (yes it takes integer 100 in
contrast to instructions) and the B-value of the O atom to 2. Leave the
default for the rest and execute (you can pick a name for the SF file if you
like).
then goto
http://www.ruppweb.org/new_comp/fourier_maps.htm
set the grid to 100 and optionally enter the file name you used before
and execute
Look at the resulting map and the peak shapes. The relative scale,
broadening, and ripples all show up as discussed.
Merry Christmas, BR
-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Ian
Tickle
Sent: Friday, December 24, 2010 4:26 AM
To: [log in to unmask]
Subject: Re: [ccp4bb] Resolution and distance accuracies
I have a program which computes the atomic electron density profile
(attached) as you would see it in a map, using accurate scattering factors
and taking the resolution limit into account. I wouldn't call the profile
for a C atom with B=100 at 2.5 Ang resolution 'flat', maybe 'flatter'.
'Flat' would imply that it's lost in the noise of other atoms with B=100.
My point is that it's relative. Since my average B is 85 Ang.^2, an
individual B of 100 or even 120 doesn't seem out of the ordinary at all. If
the average B were 10 then I would agree that anything over say 50 would
appear flat and insignificant.
The reason I think is simply that atoms with low B factor have series
termination ripples around them which can swamp the density of other atoms
with high B factor (for example the ripples from a B=10 C atom are half the
height of the peak of a B=150 atom). So the net 'noise'
level in a map with low average B is much higher than in one with high
average B, so that any atoms with high individual B just get lost in the
noise.
Cheers
-- Ian
On Thu, Dec 23, 2010 at 8:05 PM, Ronald E Stenkamp
<[log in to unmask]> wrote:
> Something related to the results in the 1984 paper, but never
> published, is that the calculated electron density for an atom with a
> B of 100
> Angstroms**2 is so flat that you wonder how those atoms can be seen in
> electron density maps.
>
> Ron
>
> On Thu, 23 Dec 2010, Bernhard Rupp (Hofkristallrat a.D.) wrote:
>
>>> can anyone point me to a more exact theory of distance accuracy
>>> compared
>>
>> to
>>>
>>> optical resolution, preferably one that would apply to microscopy as
>>> well.
>>
>> Stenkamp RE, & Jensen LH (1984) Resolution revisited: limit of detail
>> in electron density maps. Acta Crystallogr. A40(3), 251-254.
>>
>> MX, BR
>>
>
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