On mutation of Arg55 in the CypA protein to alanine there appeared to be a large reduction in the projected field magnitude experienced at the substrate proline bond (Fig. ). The absolute magnitude of this reduction differed in the AMOEBA force field compared to the two additive force fields, but was similar in relative terms -fields were reduced by approximately 40-50 % upon CypA mutation with the cis-proline substrate, and by approximately 80-90 % upon mutation with the trans-proline substrate.