Comment on "Revisiting the Ramachandran plot from a new angle".

that the ‘‘forbidden [bridge] region is well-populated in folded proteins, which can provide longer-range intramolecular hydrogen-bond partners.’’ However, using only a hard sphere model, Ramachandran and colleagues predicted the f and c combinations as a function of s that are observed in proteins of known structure. In particular, they showed that f and c combinations in the bridge region are not forbidden at large s. It is certainly possible that hydrogen-bonding may influence the adopted value of s (which in turn may then influence the allowed values of f and c), but the existence of f and c combinations within the bridge region is not on its own prime facia evidence of the role of hydrogen bonding. ALICE QINHUA ZHOU COREY S. O’HERN LYNNE REGAN* Department of Molecular Biophysics & Biochemistry Yale University, New Haven, CT Department of Mechanical Engineering & Materials Science, Yale University, New Haven, CT Department of Physics, Yale University, New Haven, CT Department of Chemistry, Yale University Yale University, New Haven, CT (Received 16 August 2011)