70 III. PORPHYBIN CHEMISTRY 



four volumes of its ether solution ; this value has so far not been used 

 extensively. 



The carboxyl groups of porphyrins are easily esterified. Refluxing 

 for 30 minutes in 1% methyl alcoholic hydrochloric acid or keeping 

 at room temperature in methyl alcohol saturated with hydrochloride 

 gas transforms them into the methyl esters, which can be taken up 

 in chloroform, washed with sodium carbonate solution, and recrystal- 

 lized from a chloroform-methyl alcohol mixture. Both free por- 

 phyrins and esters are slightly soluble in alcohols. The crystals of 

 porphyrins and their esters have a violet metallic surface color. 

 The esters can be saponified by the action of methyl alcoholic potas- 

 sium hydroxide at room temperature. 



4.2. Adsorption and Surface Properties 



The porphin ring forms a large planar plate (c/. Section 6) of pre- 

 dominantly hydrophobic character. In the porphyrins with two 

 propionic acid side chains of type IX (e.g., protoporphyrin) the 

 adjacent carboxylic acid groups confer hydrophilic character only 

 to one edge of the essentially planar molecule (Fig. 12). 



water 



Fig. 12. Surface activity of protoporphyrin and hematin. 



On a water surface protoporphyrin (and also hematin) thus forms 

 monolayers of somewhat unstable condensed solid films with mole- 

 cules vertically orientated and closely packed, in which the two 

 carboxyl groups are turned toward the water, the two vinyl groups 

 toward the air. The cohesion between the molecules themselves is 



