divalent metal ion mediated interaction of proteins with negatively charged membranes a model study employing molecular mechanics previous molecular mechanics calculations on the effect of caii and mgii ions on the conformation of the 1823 loop of bovine prothrombin maynard et al 1988 int j peptide protein res 31 137149 are extended to include the effect of a model phospholipid head group methyllseryl phosphate whereas the conformation of the gla21 pro22 amide bond remains decidedly trans in the absence of the model head group in its presence the cis caii ion induced but not mgii form is significantly lowered in relative energy the low energy caii structures establish a coordination sphere with more ligands than do the low energy mgii ion structures