32 B. Dempsey, M. B. Lowe and J. N. Phillips 



step corresponding to a one-proton addition. It has not been possible fully 

 to interpret this behaviour theoretically, although it is certain that both the 

 ionic porphyrin species PH3+ and PH4++ are being stabilized by some type 

 of solubilization process. This leads to the observed pK values being a 

 function of the intrinsic aqueous basicity constant and of the ratio of the 

 solubilization constants of the species concerned in the equilibrium. Such a 

 ratio would tend to eliminate specific porphyrin solubilization effects and 

 hence one would expect the relative p^ values for a series of porphyrins in 

 anionic detergent solutions to parallel their values in water. In the case of 

 DMMP, TMCP and DMPP, the ipK^ values observed in 2-5% SDS were 

 5-94, 5-58 and 4-88, and the piQ values 2-06, 1-80 and 1-84 respectively. The 

 pATg values appear to reflect the relative electrophilic character of the side 

 chains, and this effect has been confirmed with a number of other porphyrin 

 derivatives. The pK^ values appear less sensitive to the nature of the side 

 chain. It is of interest to note that the pK^ of 1-84 for DMPP in 2-5 % SDS 

 is equal to the value for DMPP in water (pK^ = 1-8) as estimated indepen- 

 dently from solubility measurements (Dempsey and Phillips, unpublished). 

 This suggests that the solubilization constants for the species PH3+ and PH4++ 

 not only parallel each other but are in fact very similar in magnitude. 



Co-ordination Behaviour 



The interaction between porphyrin molecules and divalent metal ions can 

 be represented by an equilibrium of the type shown in equation (2), There are 

 little quantitative kinetic or thermodynamic data available about such 

 reactions, and it was therefore thought desirable to explore them using the 

 solubilization technique. 



It has been found that in cationic detergent solutions the reactions between 

 porphyrins and metal ions are markedly dependent on temperature and also 

 on the nature of the metal ion. At 20°C in 0-25 % CTAB no reaction has 

 been observed with Co++, Ni++, Cu++, Zn^, Cd++, Mg++, Mn++, Pb++, or 

 Fe++, over a period of weeks. On the other hand, at 100°C very rapid reac- 

 tions occurred with Cu++ and Zn++ under suitable pH conditions, though not 

 with any of the other metal ions studied. 



Accordingly the reactions involving Zn++ and Cu++ were investigated in 

 greater detail. These reactions were shown to conform to equation (2), the 

 apparent equilibrium constant (K'g) being given by: 



[MP][H+]^ 

 ' [M++][PH2]' ^ ^ 



Such constants were evaluated by determining spectroscopically the ratio 

 MP/PH2 at equilibrium, for a range of hydrogen and metal ion concentrations. 

 Typical formation curves are shown in Fig. 2 for the zinc-mesoporphyrin 



