teins cause complications in devising a general theory of The change in surface pressure ( P) during adsorption of hemin, adsorption. A feasible procedure would be to study the microperoxidase-8 (MP-8), microperoxidase-11 (MP-11), and cyinterfacial behavior of protein segments as a function of tochrome c at the air-water interface was studied using the Wilincreasing molecular weight, and in this way progressively helmy plate method. The surface activity decreases in the order approach the subject of investigation -the protein molehemin ú cytochrome c ú MP-8 ú MP-11. It is suggested that cule. This idea can be partly realized for cytochrome c as the higher surface activity of MP-8, when compared to MP-11, is three products of enzymatic digestion of this protein, determined by the more pronounced anchoring of its heme group heme-containing peptides microperoxidase-8, -9, and -11 at the air-water interface. The measurements indicated instability , are commercially available. The adof the surface films of microperoxidase-8 and -11. It is suggested that this effect is due to susceptibility of the substances to aggrega-sorption of microperoxidases has been shown to be tion into the subphase. The adsorption rates ( d G/dt) of hemin strongly affected by pH as well as by the interfacial propand cytochrome c measured at constant area were in good erties of substrates like silver ( 1, 2 ) , hydrophobic silicon agreement with those measured at constant P. This enabled appli-( 3 ) , and platinum and gold ( 4 ) . However, it is likely cation of the theory of an interfacial pressure barrier in the case that these interfaces possess fixed adsorption sites, which of hemin and protein adsorption. The calculated area change per complicates the determination of the influence of the admolecule on adsorption (DA) obtained for hemin indicated that sorbate's structural differences on the adsorption process. the entire molecule needs to penetrate the surface film before ad-The air -water interface does not have that disadvantage. sorption in a perpendicular (or tilted) direction to the air-water In the present study, we investigated the interfacial behavinterface. The DA value of 1.5 nm 2 for cytochrome c supports the ior of hemin chloride (MW 652, Fig. 1), the heme peptides conclusion that only a small portion of the protein molecule needs MP-8 (MW for free acid 1526) and MP-11 (MW for free to enter the interface in order for adsorption to then continue spontaneously. Thus, the surface pressure studies revealed that acid 1881), and cytochrome c (MW 12384) at air-water systematic change in chemical composition of the complex protein surfaces. The heme-containing peptides MP-8 and -11 are molecule does not necessarily lead to systematic changes in interfaformed by enzymatic hydrolysis of equine heart cytochrome cial properties.