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



likely that similar information on the asymmetric intermediate species 

 (PH3+) would further facilitate the understanding of this problem. 



Figure 3 compares the absorption spectra of the species, PHg, PH3+ and 

 PH4++ for DMPP solubilized in 2-5% SDS. Such curves are typical for 

 porphyrins having actio type spectra. It will be observed that in the Soret 

 region the absorption maximum for the monocation is displaced towards the 

 violet relative to both the other species. In the visible region the four-banded 

 spectrum associated with the neutral porphyrin (IV > III > II > I) changes 

 upon ionization to a three-banded type (II > III > I) and finally to the 

 typical two-banded dication spectrum (II > I). 



Measurements have been made also of the fluorescence excitation and 

 emission spectra of the various protoporphyrin species and of the zinc 

 protoporphyrin complex in 2-5% SDS. The fluorescence excitation and 

 emission maxima are shown in Table 1 along with the absorption maxima. 

 It will be observed that in each case the single fluorescent emission maximum 

 lies at wavelengths 10 to 100 A longer than the a absorption band. In general 

 the fluorescent excitation maxima correspond to the absorption bands. 



SUMMARY 

 This paper is concerned with physico-chemical studies of porphyrin esters 

 solubilized in aqueous detergent solutions. In particular, quantitative data 

 have been reported for: 



(i) the relative basicity of proto-, meso- and copro-porphyrins at 20°C ; 

 (ii) the chelation of zinc ions by mesoporphyrin at 60° and 80°C ; and 

 (iii) the absorption and fluorescence spectra of the monocationic species 

 of protoporphyrin at 20°C. 



Preliminary results have also been reported for the zinc-protoporphyrin 

 and copper-mesoporphyrin reactions at 80°C. The technique could readily 

 be adapted to other physico-chemical studies, e.g. the further co-ordination of 

 metalloporphyrins with other ligands and oxidation-reduction equilibria in 

 metalloporphyrin systems. 



Such studies, aimed at providing basic information on the physico- 

 chemical behaviour of these pigment prosthetic groups, seem an essential 

 prerequisite to a detailed understanding of the role of such molecules in 

 biological processes. 



A cknowledgem en t 



The authors are indebted to Miss I. Verners for her skilled experimental 

 assistance and to Dr. J. E. Falk for providing the purified porphyrin esters. 



REFERENCES 



Aronoff, S. (1958). J.phys. Chem. 62, 428. 



Aronoff, S. & Weast, C. A. (1941). J. org. Chem. 6, 550. 



Barnes, J. W. & Dorough, G. D. (1950). J. Amer. chem. Soc. 11, 4045. 



