Hello all,

I have a 1.8A structure (Rfree/Rwork = 20.5/17.4) with 1420 water molecules modelled. There are approximately a dozen waters, all well structured with hydrogen bonds to protein atoms, with positive difference density (>4sigma, sometimes >5) at their centre. The only ions present in my buffer were Cs, Cl and a small amount of Na. I thought Cl ions might be a possibility, but many of them are in close proximity to acidic residues and/or one-another. It's probably worth noting that the same structure solved using data to 2.25A from the same crystal at a different wavelength doesn't contain these peaks (the offending waters look normal).

Has any come across this before? Thoughts?

Thanks,