Apocytochrome c, the heme-free precursor of cytochrome c, has been used extensively as a model to study molecular aspects of posttranslational translocation of proteins across membranes. In this report, we have used Fourier-transform infrared spectroscopy to gain further insight into the mechanism of apocytochrome c interaction with membrane phospholipids. Association of apocytochrome c with model membranes containing the acidic lipid dimyristoylphosphatidylglycerol (DMPG) as a single component results in a drastic perturbation of phospholipid structure, at the level of both the acyl chains and the interfacial carbonyl groups. However, in a binary mixture of DMPG with acyl chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54), the perturbing effect of the protein on the acidic phospholipid is greatly attenuated. In such a membrane with mixed lipids, the physical properties of the DMPG and DMPC components are affected in a similar fashion, indicating that apocytochrome c does not induce any significant segregation or lateral-phase separation of acidic and zwitterionic lipids. Analysis of the apocytochrome c spectrum in the amide I region reveals that binding to phospholipids causes considerable changes in the secondary structure of the protein, the final conformation of which depends on the lipid to protein ratio. In the presence of a large excess of DMPG, apocytochrome c undergoes a transition from an essentially unordered conformation in solution to an alpha-helical structure. However, in complexes of lower lipid to protein ratios (less than or equal to approximately 40:1), infrared spectra are indicative of an extended, intermolecularly hydrogen-bonded beta-sheet structure. The latter is suggestive of an extensive aggregation of the membrane-associated protein.