58 J. E. Falk and D. D. Perrin 



may well create a specific micro-environment about the haem molecule. 

 The evidence v/hich Dr. Wang has found (Wang, Nakahara and Fleischer, 

 1958) that the dielectric properties of the medium affect some co-ordination 

 properties of haem may be an example of such an effect. Nevertheless, we 

 believe that it is important to see how much or how little can be learned 

 from the study of model systems. The haemochromes which have been 

 studied in any detail that bear any likely relationship to natural haemo- 

 proteins are, practically without exception, complexes between haems and 

 ligands containing unsaturated nitrogen atoms (=N — ). But complexes of 

 protohaem with such ligands do not have the range of absorption maxima of 

 the cytochromes b (which are protohaem complexes), nor do they have 

 comparable redox potentials. 



We wish to outline here some theoretical aspects of porphyrins and 

 metalloporphyrins and to draw some inferences from existing data. We 

 discuss below (p. 74) some studies we are making of complexes formed by 

 several different haems with ligands of a variety of chemical types. 



SOME THEORETICAL ASPECTS OF PORPHYRINS AND 

 METALLOPORPHYRINS 



It is convenient to picture the porphyrin molecule as a framework of atoms 

 held together by ordinary, two-electron, single {g) bonds, while the remainder 

 of the valence electrons occupy molecular orbitals which extend over the 

 whole of this framework. The strong delocalization of these mobile (tt) 

 electrons confers considerable stability and 'aromatic' character on the 

 porphyrins. Electron-withdrawing substituents on the peripheral carbon 

 atoms reduce the 77-electron density on the pyrrole nitrogens, so that it 

 becomes easier for the protons to dissociate from the two pyrrole N — H 

 groups which make the porphyrin molecule a weak dibasic acid. Though little 

 precise data exist, this clearly increases the acid strength of the porphyrin 

 (lowers its pi^„) and, as discussed later, raises the oxidation-reduction potential 

 of metalloporphyrins. 



The Absorption Spectra of Porphyrins 



Another consequence of the extensive 7r-electron delocalization is that the 

 highest of the occupied molecular orbitals and the lowest of the vacant 

 orbitals differ in energy by an amount small enough for transitions between 

 them to give rise to absorption bands in the visible and near ultra-violet. 

 In the porphyrins themselves in neutral solvents there are four bands in the 

 visible in addition to the Soret band at about 400 m^. There appears to be 

 good reason to believe (Piatt, 1956) that the four visible bands are really two 

 pairs of bands which would be superimposed if the porphyrin nucleus were 

 strictly square and uniformly substituted. X-ray analysis of the closely 

 similar phthalocyanine molecule has shown that its structure is slightly 



