How about an oxidised cysteine? Sulfenic acid is a possibility
(http://en.wikipedia.org/wiki/Sulfenic_acid), although it will
generally oxidise further to sulfonic acid
(http://en.wikipedia.org/wiki/Sulfonic_acid).
I've seen them before in structures of old (4-5 year old) crystals
(see figure 2 of Biochemistry, 2005, 44 (42), pp 13820–13836. DOI:
10.1021/bi0512849).
Cheers,
Stephen
2009/4/21 James Stroud <[log in to unmask]>:
> Hello All,
>
> I have a couple of cysteines with some extra density about 1 covalent bond's
> length away from the sulfur center. It looks to be one atom's worth of extra
> density. Because I could fit it in an icon sized graphic and I anticipate
> that someone will suggest I post a picture, I'm attaching a picture of the
> positive fofc density.
>
> Does anyone have any idea of the usual culprits here? I see no negative
> density in the region.
>
> James
>
>
>
>
>
--
Dr Stephen Graham
Division of Structural Biology
Wellcome Trust Centre for Human Genetics
Roosevelt Drive
Oxford OX3 7BN
United Kingdom
Phone: +44 1865 287 549
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