An appraisal of molecular force fields for the representation of polypeptides

A number of force fields of the molecular mechanics type have been tested for their ability to represent as an energy minimum, the observed crystal structure for three cyclic hexapeptides, cyclo ‐(‐Ala‐Ala‐Gly‐Gly‐Ala‐Gly‐), cyclo ‐(‐Ala‐Ala‐Gly‐Gly‐Ala‐Gly‐), and cyclo ‐(‐D‐Ala‐D‐Ala‐Gly‐Gly‐Gly‐Gly‐). The most effective force field tested was that recently proposed by Kollman and co‐workers, notwithstanding its use of “united” atoms for CH, CH 2 , and CH 3 groups. Fields proposed by Levitt, and adaptations of that of Scheraga and co‐workers, were also effective. Force fields in which hydrogens bonded to electronegative atoms were not specified explicitly were less accurate in representation.