Shouldn’t it be possible to look at the ratio of peak heights of O’s versus S 
or P to figure out which is more likely? The must be thousands of examples in 
the pdb with which to determine the ratio, even if one restricts the analysis 
to “high resolution” structures.

JPK

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From: CCP4 bulletin board <CCP4BB@JISCMAIL.AC.UK> On Behalf Of Yu Qiu
Sent: Sunday, February 17, 2019 9:11 AM
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] SO4 or PO4

Hi Shijun,

I had a similar issue a while ago that a acetylated lysine binding site was 
occupied by acetate from buffer. In this case, I would suggest trying 
crystallize in a conditions without SO4, e.g. using NH4Cl instead, if it is 
reproducible.

Best,
Yu

From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of ???
Sent: Sunday, February 17, 2019 8:57 AM
To: CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK>
Subject: [EXTERNAL] Re: [ccp4bb] SO4 or PO4


Dear all

Thanks for all your suggestions. PO4 is from my Ligand  PI3P, but I cannot see 
the electron density of the PI3P tail, and the binding site is exactly there. 
While my crystal buffer salt is 50mM (NH4)2SO4, so I am afraid the SO4 occupied 
the PO4 position.BTW, I can see the other SO4 electron density in the other 
places, but is small than this one, and the diffraction resolution is 1.6A.

Best Regards

Shijun


-----Original Messages-----
From:"Roger Rowlett" <rrowl...@colgate.edu<mailto:rrowl...@colgate.edu>>
Sent Time:2019-02-17 00:47:55 (Sunday)
To: CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK>
Cc:
Subject: Re: [ccp4bb] SO4 or PO4
Two things to look at that could provide a clue:

Examine the anomalous map for some density over the central atom. Sulfur will 
often, but not always have significant anomalous density depending on the 
wavelength and quality of data set.

Phosphate is normally HPO4= or H2PO4-. Look for phosphate donor to acceptor 
hydrogen bonding contacts. Sulfate rarely has donor to acceptor hydrogen 
bonding contacts, as it is SO4= at any reasonable pH.

Roger Rowlett

On Sat, Feb 16, 2019, 4:06 AM 张士军 
<21620150150...@stu.xmu.edu.cn<mailto:21620150150...@stu.xmu.edu.cn> wrote:

Dear all

I have got a crystal grown at the condition both have ion of SO4 and PO4, and 
the diffraction resolution is very well, but the problem is coming: how to tell 
which is which just from electron density? I think they are exactly same. 
Thanks a lot !!!

Beat Regards

Shijun

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