Two identical 13 residue peptides (2) have been covalently linked to iron(III) mesoporphyrin IX via amide formation between the heme propionyl groups and the epsilon-amine of lysine side chains. The peptide was designed such that coordination of the imidazolyl side chain of the histidine residue in each peptide to the heme metal would lead to helix induction. The resulting peptide-sandwiched mesoheme (1) exists as a pair of interconvertible diastereomers due to the substitution pattern of the heme moiety. Bisimidazole coordination of the iron in 1 is demonstrated by ultraviolet/visible (UV/vis) spectroscopy. Circular dichroism (CD) experiments in the far UV (190-240 nm) reveal that in aqueous solution at 8 degrees C the peptides in 1 are approximately 52% alpha-helical and that helicity can be increased to a maximum measured value of 97% by the addition of 2,2,2-trifluoroethanol (TFE) and other cosolvents. Features arising from the heme moiety are observed in CD spectra, including the Soret and alpha and beta bands. We provide evidence that the heme contributes positive ellipticity to the far-UV CD spectrum, complicating quantitation of peptide helicity based on the mean residue ellipticity at 220 nm (theta(220)). The molar ellipticity at lambda(max) of the CD Soret band (theta(m403)) is a function of peptide conformation, changing markedly upon addition of cosolvents and with increases in temperature. The spectral changes likely result from a reorientation of the peptide backbone amides relative to the B-x and B-y electronic transitions of the heme. A chromophore-bearing analogue of the peptide used in the synthesis of 1 (6) exists in a random coil conformation in aqueous solution, exhibiting no innate helical tendencies. Helicity of 6 reaches 80% in 50 volume % TFE, but helicity can be increased even further by using the more acidic solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). A mixture of diastereomeric monopeptide adducts formed from 1 and 6 (3a,b) has also been isolated and is found to have strong concentration-dependent behavior. UV/vis suggests that these phenomena can be attributed to aggregation, which is a consequence of 3a,b having one face of the heme exposed to the solvent.