172 V. HEMATIX COMPOUNDS 



static linkage, as well as to the pyrrole nitrogen atoms. Still more com- 

 plicated polymerizations probably occur on standing and may be accelerated 

 by heating, resulting in the elimination of water molecules, and formation 

 of /3-hematins [rf. Fig. i) and i/'-hematins. 



It is still a matter of controversy whether hematin in alkaline .solutions is 

 to be considered polymeric, dimeric, or monomeric. Dialysis {1172). diffu- 

 sion {S171)* and ultracentrifuge (102S) experiments indicate that the solu- 

 tions of heme and hematin are polydi.sper.se, with particle sizes varying 

 between 30,000 and 60,000. The potentiometric titrations of Conant and 

 Tongberg (Jf-Sl) in borate buffer of pH 9.15 fit the assumption of monomeric 

 heme and dimeric hematin, but are in experimental disagreement with later 

 results of Barron (180), which suggest that both are monomeric. From 

 potentiometric studies on nicotine iron porphyrin, Davies (-jSG) concluded 

 that uncombined hematin was dimeric in aqueous, but monomeric in 50% 

 aqueous alcoholic, phosphate buffer. In a careful .spectrophotometric study 

 of the equilibrium of hematin with dicyanide hematin, Hogness and collab- 

 orators (1307) found that hematin was a dimeric compound (cf. 2-'jSSa). 



In view of the fact that hematin dissolved in sodium carbonate reacts 

 rapidly with globin to form hemiglobin hydroxide, Holden (1313) suggests 

 that under these conditions a dynamic equilibrium exi.sts between polymeric 

 and monomeric hematin. The latter may well only exist in very low con- 

 centration. The sharpening of the visible and ultraviolet absorption bands 

 of hematin in alcoholic .solution accords with this idea. Holden (private 

 communication) has found that, except at great dilution, hematin in organic 

 solvents polymerizes, as is shown by the rapid diminution in the height of 

 its Soret band. 



There is evidence for the combination of hematin with borate (1S0JS8; 

 cf., however, 2538a), so that the presence of buffer anions may have some 

 effect on the state of aggregation (cf. Sect. o.). It was claimed (1015,3157) 

 that a solution of hemin in phosphate buffer of pH 6 differs in color and 

 catalytic properties from one of hematin in the same buffer; the former is 

 probably a colloidal .solution. 



The structure of "acid hematin" obtained by ether extraction of an alkaline 

 hematin solution after acidification with hydrochloric acid is also not clearly 

 understood. While it appears unlikely that the dissociated hyflroxyl group 

 would recombine under these conditions, there is no evidence that the sub- 

 stance in ether .solution is hemin, hemin cry.stals being insoluble in ether. ' 

 "Acid hematin" may perhaps be a coordination compound of hemin with 

 ether, which is not readily formed from cry.stalline hemin. A more likely 

 alternative is that it is hydroxyhemin as shown in Figure 3. 



Hematin forms solid, somewhat collapsible monomolecular films 

 on water-air interfaces, in which the molecules stand vertically, with 

 the two juxtaposed carboxylic acid groups buried in the water (cf. 

 Chapter III, Section 4.2.) (38,1363). 



* The porou.s di.sk diffusion method of Anson and Northrop (applied hy Zeile and 

 Reuter, -UTl) gave, however, too high values for the molecular weight of cytochrome c. 



