Elucidation of the mechanism of transitions between the cubic phase and the liquid-crystalline (L-alpha) phase andbetween different cubic phases is essential for an understanding of the dynamics of biomembranes and the development of new crystallization techniques. Recently, we found that electrostatic interactions due to the surface charges of lipid membranes induce transitions between the cubic phase and L-alpha. phase and between different cubic phases (Aota-Nakano, Y.; Li, S. J.; Yamazaki, M. Biochim. Biophys. Acta 1999, 1461, 96; Li, S. J.; Yamashita, Y.; Yamazaki, M. Biophys. J. 2001, 81, 983). In the present study, we used small-angle X-ray scattering to investigate the effects of a de novo designed peptide (WLFLLKKK, peptide-1, which has positive charges and a site that is partitioned in electrically neutral lipid-membrane interfaces) on the stability of the cubic phase of the monoolein (MO) membrane. As the peptide-1 concentration increased, a transition from the Q(224) to the Q(229) phase in the MO membrane at a 30 wt % lipid concentration occurred at R = 0.0090 (R is the molar ratio of peptide to MO), and at R greater than or equal to 0.040, the MO/peptide-1 membranes were in the L-alpha phase. As the NaCl concentration in the bulk phase increased, for the MO/peptide-1 membranes in the Q(229) phase in excess water, a Q(229) phase to Q(224) phase transition occurred at low concentrations of NaCl. Similarly, for the MO/peptide-1 membranes (R = 0.080) in excess water, at low NaCl concentrations, they were in the L-alpha. phase, but at greater than or equal to 0.40 M NaCl, they were in the Q(224) phase. These results indicate that peptide-1 was partitioned in the membrane interface of the MO membrane, electrostatic interactions due to peptide-1 in the membrane interface make the Q(229) phase more stable than the Q(224) phase, and with larger electrostatic interactions, the L-alpha. phase is more stable than these cubic phases. Increased peptide-1 concentration reduced the absolute value of spontaneous curvature of the MO/peptide-1 monolayer membrane. On the basis of these results, we discuss the mechanism of the effect of peptide-1 on the phase stability of the MO membranes.