1. Titration of haem with pyridine in alkaline media of low ionic strength yields a true pyridine haemochromogen, compound III, at very low concentrations of pyridine. 2. Graphical analysis of this titration gives the first spectrophotometric evidence for a dimeric haem. 3. Compound III is unstable and tends to aggregate to a second compound, compound II, whose formation is enhanced under those conditions favourable to hydrophobic bonding. 4. At higher concentrations ofpyridine, compound II is dispersed to yield the classical pyridine haemochromogen, compound I, whose spectral properties are essentially those ofpyridine haemochromogen in a non-aqueous medium.Since the elucidation of the nature of haemochromogen by Anson & Mirsky (1925) and the discovery by Keilin (1925) (v/v) RESULTSThe three compounds noted by Smith (1958Smith ( , 1959 in the titration of alkaline haem with pyridine are referred to below as compounds III, II and I in order of their appearance in the titration. Compound I is the final product of the titration, the classical pyridine haemochromogen. Table 1 lists the spectral properties of these compounds. The extinction coefficients given are the highest 187

No abstract

1. Pyridine reacts with alkaline haematin to form a dipyridine dihydroxy dimeric haematin in which there is no competition between pyridine and OH(-) for coordination positions on the iron of haematin. 2. Imidazole competes directly with OH(-) to form the monomeric imidazole parahaematin in alkaline media. 3. The monomeric imidazole parahaematin aggregates readily to form a species that is precipitated on standing. 4. A structure is proposed for the haematin-pyridine compound in alkaline media.

1. Caffeine reacts with haematin to form a caffeine-haematin compound that has a characteristic absorption spectrum. 2. Graphical analysis of the titration of haematin with caffeine shows that 2mol.prop. of caffeine split the dimeric haematin. 3. Thermodynamic parameters suggest that the reaction involves the making and breaking of hydrophobic bonds. 4. Graphical analysis shows dimeric haem to be split by 2mol.prop. of caffeine to yield a compound with an unusual multibanded absorption in the Soret region. 5. It is postulated that the linkage between the haem groups of dimeric haem and the haematin groups of dimeric haematin is essentially hydrophobic in nature.