All of the results can be related to the strong interactions found in the out-of-register stacking of Mms rings. The positively charged COH atom can interact with the negatively charged CD and CF atoms. Similarly, the interactions of the CO and OC atoms in the different rings also favor this Therefore, the stacking of rings will be a significant driving factor of the conformations adopted by the mimosine hexapeptide while for Phe and Tyr it has little or no influence. The most stabilizing interactions for these peptides are hydrogen-bonds between the amide groups in the backbones. These interactions give raise to b -strand-like conformations in which the rings are separated corresponding to the maxima centered at 0.84 nm. The greater freedom of movement of the rings in these conformations explains why these maxima are considerably broader than those corresponding to conformations with their rings tightly stacked. These b -strand-like conformations will also be observed for the mimosine hexapeptide, as the interactions between backbone groups compete with the stabilization of the MMS ring stacking. The differences between the results observed for the mimosine peptide in the two used force fields could be correlated with the MMS ring charges derived for ff03 being considerably lower than those obtained for ff99SB. Thus, the electrostatic interactions that favor the ring stacking are weaker for this force field.