Distributions of the hydrogen-based χ angles of Ser, Thr, Tyr, and Cys. (A) The distributions calculated directly from high-resolution (<1.2 Å) structures deposited in the PDB. (B) The distributions of the same structures where the hydrogen atoms have been manually placed to fit the electron-density of hydrogen-bonded hydrogen atoms. (C) The distributions from the structures determined by neutron diffraction. (D) The distributions from the hydrogen atoms that have been computationally-placed with Reduce from the top500 database of non-redundant structures from the Richardson lab.