We report the results of studies of a series of water-soluble peptide adducts of iron mesoporphyrin IX (FeMPIX), In one group of compounds, the peptide-sandwiched mesohemes (PSMs; 1-5), two identical 13-residue peptides are connected to the propionate groups of FeMPIX via amide linkages with N epsilon nitrogens of lysine (Lys) residues. The corresponding monopeptide analogues of each PSM (1m-5m) have also been prepared. The imidazolyl side chain of a histidine (His) residue in each peptide coordinates to the mesohemin iron. The compounds differ from one another in the relative positions of the His and Lys residues in the peptide sequences. In 1, 2, 4, and 5 and the corresponding monopeptide systems, Fe-His coordination results in various extents of peptide helix induction in neutral aqueous solution. The peptides in 3 and 3m remain in random coil conformations. Helix content in 1-5 can be enhanced by addition of organic cosolvents, including 2,2,2-trifluoroethanol (TFE) and 1-propanol (PrOH). Data from EPR spectroscopy and from pH titrations suggest that in aqueous solution the folded forms of 3 and 5 (in which both Fe-His bonds are intact) are less stable than the folded forms of the other PSMs. Addition of TFE or PrOH increases the stability of the folded forms of 1, 2, and 4 by favoring helical conformations for the peptides whether or not the Fe-His bonds are intact, thus eliminating the helix unwinding that occurs upon bond breakage in aqueous solution. Increasing peptide helix content in 3 and 5 with TFE does little or nothing to increase the stability of their folded forms, consistent with the prediction that the His-Lys spacing in these compounds is not compatible with stable helical conformations for the peptides, UV/vis spectra of the Fe(II) complexes of 3 and 5 are also consistent with reduced structural stability of these PSMs, as a sizable population of pentacoordinate (high-spin) Fe(II) is in equilibrium with hexacoordinate (low-spin) Fe(II) at neutral pH, both in aqueous solution and in the presence of 30% (v/v) TFE at 8 degrees C. The Fe(II) complexes of 1, 2, and 4 remain predominantly hexacoordinate in both solvent systems.