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



Preliminary results suggest : 



(i) that Zn++ reacts faster and forms a more stable complex with DMMP 

 and TMCP than DMPP, as might be expected from the greater electro- 

 philic character of the unsaturated vinyl side-chain compared with its 

 saturated analogues; 



(ii) that Cu++ reacts faster and forms more stable complexes than Zn++ 

 with DMMP, in accord with the normal relative chelating ability of 

 the two ions (see Bjerrum, Schwarzenbach and Sillen, 1956-57); and 



(iii) that Co++ and Ni++ react infinitely more slowly than Cu++ or 

 Zn++, although there is evidence (Caughey and Corwin, 1955) to 

 indicate that in general Co++ and Ni++ form the more stable porphyrin 

 complexes. It is suggested that this reluctance on the part of Co++ 

 and Ni++ may be associated with their tendency to form hexaco- 

 ordinate compounds as compared with the tetraco-ordinating tendency 

 of Cu++ and Zn++. 



The overall kinetics conform to a simple bimolecular reaction involving 

 metal ions and the neutral porphyrin species (PHg). This, and the fact that 

 Zn++ reacts more readily with DMMP and TMCP than DMPP suggests 

 that the reaction mechanism is of the displacement rather than the dissociation 

 type (Basolo and Pearson, 1958). 



For purposes of comparison it is convenient to express KJ values in terms 

 of the more conventional stability or formation constants {Kf) defined by : 



_ [MP] 

 ^^"[M++][P=]' ^^ 



In the absence of acidic ^K data for mesoporphyrin the value for aetio- 

 porphyrin II at room temperature {"pK^ + pA'g '^ 32 (McEwen, 1936)) has 

 been used. This leads to extrapolated logio Kf values for zinc mesoporphyrin 

 at 20°C of '->-' +29. The high stability of porphyrin metal complexes is 

 illustrated by comparing this value with the corresponding figures in water for 

 the zinc complexes of, for example, 8-hydroxyquinoline-5-sulphonic acid 

 ('^16-0), ethylene diamine ('-^ll-O), and glycine ('-^9-5) (Bjerrum et al, 

 1956-57). 



Spectroscopic Behaviour 



Aqueous detergent solutions form useful solvent systems for studying the 

 spectroscopic properties of porphyrin molecules, their salts and metal com- 

 plexes. Anionic detergents are of particular interest in that they permit a 

 study of the spectral behaviour of the monocationic species (PH3+), a species 

 which has proved virtually impossible to obtain in normal solvent media. 

 Much theoretical argument (Piatt, 1956; Kuhn, 1959) concerning electron 

 distribution in the porphyrin nucleus has been based on the absorption spectra 

 of the symmetrical free porphyrin (PHg) and its dication (PH4++). It seems 



