Structural motifs in the PDB

Question: Has anyone see a disulfide forming between the two cys of "CXC" in the middle of a loop, and create a sharp turn, where X is not a proline? I seems to me that geometrically this would be possible but I am not sure how stable it is, or how energetically unfavorable it might be.

This is the kind of task that can be carried out quickly and easily with PDBeMotif - and here's how to do it:

Surf to PDBeMotif at http://pdbe.org/motif
Click on the "Search" tab (or the "Search" link)
Select the option to search "Protein sequence, pattern" by clicking on the corresponding icon on the left of the page (fifth icon down)
In the sequence box enter: CXC
Select the radio button to search using "Pattern match on observed PDB sequences"
Click on "Add it to the search criteria"

If we were to carry out the current search, we would get all observed instances of Cys-X-Cys in the PDB. However, we want to specify that the two cysteines are covalently linked. This can be done as follows:

Select the option to search "Interactions" by clicking on the corresponding icon on the left of the page (sixth icon down)
In the panel, change the two drop-down menus labelled "Interacting residue" so that one says "1" and the other says "3" (from their default value "any"). Also set the "Bond type" drop-down to "Covalent bond".
Click on "Add it to the search criteria"

Now we have specified that we want to find instances of Cys-X-Cys, where there is a covalent bond between the two Cys residues (residues 1 and 3 in the pattern CXC).

Before you hit the "Search" button, you can limit the search with the "Normalise hits by" drop-down. If you select "-" you will get all instances in the PDB; if you select "UniProt", only one entry per UniProt ID will be retrieved, etc.
Hit the "Search" button
If you searched for all instances in the PDB, PDBeMotif will return a list of 147 hits in 55 PDB entries. If you normalised by PDB header, you will get 18 hits in 18 entries.
To visualise (some of) the hits, mark the check boxes of the instances you're interested in. Then select a visualisation program at the bottom of the page by clicking on its icon (e.g., Jmol). All the selected hits will be superimposed and shown in the selected viewer.

A screendump (you'll have to scroll to the right to see the superposition!):

(Note: if you're an advanced user, you may one day want to edit and submit your queries in XML - click on "Get Query XML" for more information.)