ADSORPTION 71 



greater than the adhesion to water. The area of one molecule is 

 70 A- closely agreeing with the area for molecules, if vertically close- 

 packed (68 A-), while placed horizontally on the surface it would 

 require about 125 A'. Presence or absence of metal in complex com- 

 bination has little effect on the surface properties except that hematin 

 films are more homogeneous and collapse less easily. The side chains, 

 however, are of decisive importance. The surface activity increases 

 with increasing pH owing to the ionization of the carboxyl groups. 

 Hematoporphyrin, which instead of the vinyl groups has hydrophilic 

 hydroxyethyl groups, lies flat on the water surface and forms vapor- 

 expanded films compressible to liquid films (Alexander, 38). 



If alkaline solutions of protoporphyrin and hematin are injected under 

 monolayers of a variety of proteins, they penetrate rapidly into them (faster 

 at pH S.'i than at pH 7.2) and transform the protein films into rigid mono- 

 layers (tanning), increasing the surface pressure and decreasing the surface 

 potential. The less surface-active coproporphyrin is not bound to protein 

 monolayers. The porphyrins were also shown to penetrate into monolayers 

 of cholesterol and octadecylamine. While the penetration into cholesterol 

 is largely influenced by interaction with the hydrophobic part of the por- 

 phyrin molecule, the interaction with octadecylamine films resembles that 

 with protein. This makes it likely that the interaction is between the 

 carboxylic acid groups of porphyrin and the amino groups of protein. One 

 has to bear in mind that, in the "denatured protein" of the monolayer, 

 amino groups which in the native protein may not be available due to inter- 

 action with the protein's own carboxylic acid groups may be free and 

 immersed in the substrate. In the case of serum albumin, cataphoresis 

 experiments have shown that the linkage with protoporphyrin also occurs 

 in solution (Stenhagen and Rideal 2622). 



These experiments of Alexander and of Stenhagen and Rideal are 

 of importance for the problem of the combination of porphyrins and 

 porphyrin metal complexes with proteins, which will be discussed in 

 Chapter VI (Sections 3.3.3. and 3.3.4.) in connection with the prob- 

 lem of the linkage of hematin to proteins. They clearly demonstrate 

 the specific importance of the nature and of the position of the side 

 chains. The different behavior of coproporphyrin toward protein 

 monolayers may explain, e.g., why coproporphyrin is found in nature 

 only as iron-free porphyrin, while in vitro it forms hematin compounds 

 as readily as protoporphyrin. 



4.3. Light Absorption 



The absorption spectra of porphyrins are so characteristic that 

 they are of fundamental importance for the recognition of porphyrins. 



