Hi Smith, You're possibly looking at a turn of 3_10 helix: http://en.wikipedia.org/wiki/310_helix Whether it's real or not will of course depend on your electron density. Shane Caldwell McGill University On Fri, May 29, 2015 at 11:35 AM, Smith Liu <[log in to unmask]> wrote: > Dear All, > > Even with the Coot secondary structure (for example helix) restraint > selected and by this way we keep the secondary structure, I find the > protein secondary structure formed in this way was not so typpical, for > example, not all CO ( i) and NH (i+4) forms H-bonds, and there are H-bonds > formed by CO ( i) and NH (i+3). I checks some RCSB PDBs, and find this kind > of untypical H-bonds for the Helix backbone were not rare. > > Do we accept this kind of untypical H-bonds containing PDB as normal, or > do we think we need further refine the structure based on the map? > > Smith > > > >