Thermodynamics of Pyridine and 1-Alkyl-Pyridinium Complexes of Protohaemin in Aqueous Solution

Abstract

1. In alkaline aqueous solution (pH > 10) protohaemin forms a green coloured pyridine complex containing two ligands per haemin dimer. The thermodynamics of this reaction is characterized by a considerable entropy changc due to hydrophobic interactions between both partners.2. Similar haemin complexes are formed by N-alkyl-pyridinium halides. The absorption spectra are nearly identical to those of the corresponding pyridine complcxes although there is no possibility of interaction with the positively chargcd haemm iron. Pyridinium cations dcvelop a higher affinity towards haemin than the neutral pyridines. This bchaviour is attributed to electrostatic attractions between the propionylic groups of porphyrin and the positively charged pyridinium salts.3. The introduction of methyl groups intensifies the haemin complex stability both of the pyridines and the pyridinium salts. In order of increasing methylation of the ligands the reaction entropy incrcascs. 4.Following the substitution of pyridine in position 4 by a cyano group or following the transition from N-alkyl-pyridinium halides to N-alkyl-quinolinium halides the affinity towards haemin increases and the thermodynamics show a decrease of the reaction enthalpy whilst the entropy of thc system changes only little. For this behaviour n-donor-acceptor interactions should be rcsponsible.5. The results demonstrate the contribution of several types of chemical bond in the reaction of protohaemin with ligands. Apart from the well investigated coordinative linkage to the iron atom, n-donor-acceptor and hydrophobic interactions between ligands and the porphyrin system as well as ionic bonds between the propionylic groups of the porphyrin and positively charged ligands contribute to their haemin complex formation. 6 . The significance of the results is discussed with regard to the haemin-protein interaction in haemoproteins.

Coordination of N-Donor Ligands by the Monomeric Ferric Porphyrin N-Acetylmicroperoxidase-8

“…In the case of the pyridines this could involve binding by donor−acceptor π−π interactions with the porphyrin system. The formation of such donor−acceptor complexes between aromatic systems and iron porphyrins in general and hemepeptides in particular 1i, is well-established. These processes all occur without significant shifts in band positions.…”

σ- and π-Bonding Modes of Pyridine and Imidazole Type Ligands in the Transition States of Their Reactions with [CoIII(protoporphyrin IX dimethyl ester)(MeO)(MeOH)] in Methanol

“…pyridines. The idea of π−π interactions in porphyrin systems was first put forward by Mohr and Scheler . They observed that in alkaline aqueous solution (pH >10) protohemin forms a green pyridine complex containing two ligands per hemin dimer.…”

“…The idea of π-π interactions in porphyrin systems was first put forward by Mohr and Scheler. 20 They observed that in alkaline aqueous solution (pH >10) protohemin forms a green pyridine complex containing two ligands per hemin dimer. They also found that similar hemin complexes are formed by N-alkylpyridinium halides, the absorption spectra being nearly identical to those of the corresponding pyridine complexes, although there is no possibility of interaction with the positively charged hemin iron.…”

Unusual coordination and metal-ligand geometry of a vanadyl porphyrin in aqueous solution

“…as has been observed for other porphyrins. 8,12,22 In contrast, in earlier work on meso-substituted vanadyl porphyrins large red shifts on binding piperidine were observed. 23 Binding is again weak (K = 0.4).…”