Physico-Chemical Behaviour of Porphyrins in Aqueous Detergent Solutions 31 



were either 2-5% (w/v) sodium dodecyl sulphate (SDS) or 0-25% (w/v) 

 cetyltrimethyl ammonium bromide (CTAB). 



Ionization Behaviour 



The ionization of porphyrins solubilized in aqueous detergent solutions 

 can readily be studied spectroscopically. Two general types of behaviour 

 occur, the one with cationic and non-ionic and the other with anionic 

 detergent solutions. 



In the former case two species only, the neutral porphyrin (PHo) and the 

 dication (PH4++) are observed upon spectroscopic titration within the pH 

 range 0-12. The variation in optical density {E) with pH at a given wave- 

 length is such as to indicate that the reaction involves two protons : 



PH2 + 2H+ ^ PH4++ (3) 



the overall dissociation constant (A'obs) of the conjugate acid PH4++ being 

 given by 



_[PHJ[H+P 

 ^^^'^ ~ [PH,++] • ^^^ 



This does not necessarily imply the simultaneous addition of two protons in 

 the kinetic sense. A more likely explanation is that under these conditions 

 the monocation behaves as a stronger base than the free porphyrin. 



The equihbria postulated to account for this behaviour are illustrated 

 below. 



^, 



K,= 



[(H+)J[(PH,)J 

 (PH,).;.'(PH,), "" [(PH3+)J 



+°" ^^ ^^ ^ [(H+),][(PH3+)J 



(PH3+), A4 = -——_——, 



(PH4++), ^ _ [(H+)J^[(PH,), + (PH,),] 



[(PH,++)J 



^obs = 



In terms of the scheme outUned the observed constant (ATobs) would corres- 

 pond to K^K^{\ + K,). 



The pXobs (= - logio/sTobs) values for DMMP and TMCP in 0-25% 

 CTAB were found to be 2-08 ± 0-05 and 2-24 ± 0-05 respectively. Unfor- 

 tunately the corresponding value for DMPP was too low (< -f-O-S) to be 

 determined with any certainty. It is clear, however, that the observed values 

 will depend on the nature and concentration of the detergent, and that com- 

 parisons of pAT values for different porphyrins in the same detergent solution 

 will reflect differences both in their aqueous basicities and in their solubiliza- 

 tion constants. 



In anionic detergent solutions three species, corresponding to PH,, PH3+ 

 and PH4++, may be observed upon spectroscopic titration, each ionization 



