THE PHYSICO-CHEMICAL BEHAVIOUR OF 



PORPHYRINS SOLUBILIZED IN AQUEOUS 



DETERGENT SOLUTIONS 



By B. Dempsey,* M. B. LowEf and J. N. PHiLLiPsf 



Department of Chemistry, Royal Military College, Duntroon, 

 and Division of Plant Industry, C.S.I.R.O., Canberra 



The Tetrapyrroles, and in particular the haem pigments, occur in a biologi- 

 cal environment which is essentially aqueous. It is therefore desirable to 

 determine their physico-chemical properties in an aqueous medium. Unfor- 

 tunately, the information available, particularly as regards their ionization 

 and co-ordination behaviour, is meagre (Phillips, 1960). The major experi- 

 mental obstacle to obtaining such data has been the very low solubility of 

 these compounds in water. Some measurements have been carried out in 

 non-aqueous and mixed solvent media (Conant, Chow and Dietz, 1934; 

 Aronoff and Weast, 1941; Aronoff, 1958; Barnes and Dorough, 1950; 

 Caughey and Corwin, 1955; Corwin and Melville, 1955). However, such 

 systems are unsuitable for electrochemical studies because of unknown ionic 

 activity effects. 



This situation led us to explore the use of aqueous detergent solutions as 

 solvent media (Pliillips, 1958). Studies have been carried out using fully 

 esterified porphyrin derivatives to avoid electrostatic effects arising from 

 the ionized carboxylic acid groups on the periphery of the nucleus. A wide 

 variety of porphyrin esters has been shown to disperse molecularly in a 

 number of detergent solutions, presumably by solubilization within the lipid 

 micelle. This phenomenon is analogous to a phase distribution equilibrium 

 in which one of the phases is of molecular dimensions. Macroscopically, 

 such a system would behave as a single phase and equilibration between the 

 phases would be expected to be extremely rapid. The distribution of the 

 porphyrin molecules between the aqueous and micellar phases can be repre- 

 sented by a simple equilibrium of the type : 



(PHg)^ ^ (PH2)(j 

 the solubilization constant K^ being defined by 



[«(PHjJM 



* Royal Military College. 

 t C.S.I.R.O. 



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