G.A. Rodley

Observation of induced cotton effects for magnesium porphyrin-L-histidine species

Abstract Complexes of magnesium protoporphyrin and magnesium mesoporphyrin with L-histidine have been prepared in solution and studied by circular dichroism and optical rotatory dispersion spectroscopy. Large induced Cotton effects are observed, in particular for the Soret transitions. These are interpreted in terms of the formation of six-coordinate magnesium porphyrin-(L-histidine) 2 species where the asymmetry of the L-histidine groups is coupled to the magnesium porphyrin electronic transitions. The major, five-coordinate components of these solutions do not produce Cotton effects. These results, when compared with data for corresponding protein complexes, lend support for the coupled oscillator description of the origin of optical activity in hemoproteins. The observation of Cotton effects for porphyrin complexes of the type reported here may enable the existence of six-coordinate species to be more clearly established in instances where the major equilibrium product is a five-coordinate species.

Induced cotton effects for complexes of magnesium porphyrin with D- and L-proline, L-serine, L-threonine and L-tryptophan

Abstract Circular dichroism and optical rotatory dispersion results are reported for solutions of magnesium protoporphyrin and magnesium mesoporphyrin containing the amino acids, D and L-proline, L-serine, L-threonine and L-tryptophan. The observed induced Cotton effects are attributed to six-coordinate magnesium porphyrin (amino acid) 2 species. The Cotton effects are most pronounced for proline and the related pair of amino acids, serine and threonine. In the former case, the observed spectra are inverted with respect to those for other amino acids of the same chirality. It is suggested that this arises from the formation of an additional chiral centre at the ring nitrogen atom due to the coordination of proline to magnesium via this atom. Steric interactions produce an opposite chirality at nitrogen to that existing at the asymmetric carbon atom. For L-serine and L-threonine H-bonding of the -OH groups with the -COO − porphyrin side chains is proposed as the localising effect that produces the observed induced Cotton rotation. For L-tryptophan, where a weaker CD/ORD effect is observed, stacking of the aromatic rings of the indole and porphyrin groups is considered as a possible localising interaction.

Preparation and study of chiral magnesium porphyrin-amino acid complexes

Mixtures of magnesium protoporphyrin or magnesium mesoporphyrin with a variety of chiral amino acids (L-histidine, D- and L-proline, L-serine and L-threonine) produce prominent induced Cotton effects in the UV-visible region. By contrast magnesium deuteroporphyrin mixtures exhibit no optical rotatory dispersion (ORD)/circular dichroism (CD) spectra. It is proposed that the species producing the Cotton effects are six-coordinate species of the type Mg (porphyrin) (amino acid)2. For L-histidine and L-threonine CD spectra have shown that complexes of the opposite chirality can be obtained for different samples of magnesium protoporphyrin. For D- and L-proline such a change in sign of spectra was not found for the same magnesium porphyrin samples. Reasons for these observations are presented together with proposals regarding structural details of the six-coordinate complexes. It is also suggested that racemic samples of such amino acid-magnesium porphyrin mixtures could yield optical resolution of products on irradiation with circularly polarised light. Details of a study of this type are presented.

Spectral studies of magnesium porphyrin-apomyoglobin and apohemoglobin complexes

Complexes of magnesium protoporphyrin and magnesium mesoporphyrin with apomyoglobin and apohemoglobin have been prepared and studied by electronic, circular dichroism, and optical rotatory dispersion spectroscopy. The myoglobin complexes show prominent splittings and red shifts of the visible absorption bands, with respect to those for the hemoglobin analogs. Comparisons are made with other heme protein systems that display similar spectral features. Different reasons for the observations are considered, including multiple conformer formation, polarity effects of the protein environment, and the formation of six-coordinate magnesium species through H2O coordination for the Mb complexes (the latter explanation being favored).

Preparation and study of magnesium deuteroporphyrin myoglobin and hemoglobin species

Abstract The myoglobin and hemoglobin species containing magnesium deuteroporphyrin have been prepared and studied by electronic, circular dichroism and optical rotatory dispersion spectroscopy. The results are compared with those obtained for corresponding magnesium protoporphyrin and magnesium mesoporphyrin complexes. In all cases the magnesium-apomyoglobin species show additional band splittings. These may arise directly from differences in the protein environment or indirectly through water coordination to magnesium which is facilitated by features of the myoglobin heme pocket but inhibited in the hemoglobin complexes. The availability of results for three different porphyrins enables a red shift of spectral bands, observed in particular for MgPP-Mb ** , to be specifically associated with the presence of side-chain vinyl groups.

Magnesium porphyrin-globin and -amino acid complexes

Abstract Complexes of magnesium mesoporphyrin (MgMP) and magnesium protoporphyrin (MgPP) with apomyoglobin and apohemoglobin, and amino acid species of the type MgMP/PP (amino acid) 2 have been studied by electronic, circular dichroism (CD) and optical rotatory dispersion (ORD) spectroscopy [1−3]. In the case of the protein complexes, specific spectral differences were observed for the myoglobin (Mb) and hemoglobin (Hb) products. These have been interpreted in terms of the formation of six-coordinate magnesium aquo species, Mg(porphyrin)(-histidine)(H 2 O), for MgMPMb and MgPPMb and five-coordinate, Mg(porphyrin)(-histidine), species for MgMPHb and MgPPHb [1]. These results highlight the importance of relatively small differences in the protein environment of the heme group in myoglobin and hemoglobin, on binding at the sixth coordination site. It is possible that a favourable hydrogen bonding interaction with the distal imidazole group (of the type recently described for oxy Mb [4] and CO heme proteins [5]) may stabilize the binding of a water molecule to Mg in the Mb species. Aqueous solutions of the magnesium porphyrins containing the chiral amino acids, L-histidine, L-serine, L-threonine and L-proline, produce prominent induced Cotton effects [1, 2]. However the CD/ORD spectra of these species differ from those of the magnesium porphyrin Mb and Hb protein complexes. Electronic band positions indicate the complexes producing the Cotton effects to be six-coordinate Mg(porphyrin)(amino acid) 2 entities, containing two amino acids bound to the metal rather than only one amino acid residue as for the protein species. The electronic spectra also indicate the presence of five-coordinate (Mg(porphyrin)(amino acid) species in the solutions but these do not produce Cotton effects. Induced Cotton effects of the magnitude observed for the Mg(porphyrin)(amino acid) 2 complexes presumably require reasonable firm stereochemical location of the coordinated amino acids. For L-histidine we suggest the ligands are localized primarily by ππ bonding interactions between the ligand aromatic π system and the d π orbitals of Mg [2]. For the similar amino acids, L-serine and L-threonine, molecular models indicate that hydrogen bonding between the ligand −OH groups and the porphyrin COO − side chains fix these ligands in well-defined positions. L-proline is a special case in that binding through the ring nitrogen atom produces a new chiral centre at that atom. Interestingly, steric interactions between proline and the porphyrin ring determine the chirality of nitrogen to be opposite to that at the asymmetric carbon atom (as illustrated in Fig. 1). Inversion of this type occurs for a similar reason in the case of proline complexes Cu 2+ [6].

Denaturation of DNA in the presence of chromous (Cr2+) ions

The effect of Cr2+ ions on the Tm (melting temperature) of DNA has been investigated under appropriate conditions for the stabilization of DNA by Mg2+ ions. A significant lowering of Tm, analogous to that observed for Cu2+ under normal conditions, was found, for Cr2+ at pH = 4.2 and [Mg2+] = 5.3 mol per mole of DNA base pair. Cu2+ also lowers Tm under similar conditions. The similarity of the effects of Cr2+ and Cu2+ under comparable conditions may be related to similarities in their coordination properties. It is proposed that Cr2+ and Cu2+ ions facilitate denaturation by holding together groups on the DNA chains in such a manner that base pairing and base stacking are inhibited. Comparative results for Cr3+ and Co2+ are also given for these low pH/Mg2+ ion conditions.

Solid state spectral properties of related six-coordinate magnesium tetraphenylporphyrin complexes

The electronic spectra of in vivo chlorophyll molecules are generally shifted to the red region compared to those observed for corresponding species in organic solvents [I]. Two main mechanisms have been proposed for this red shift (1) chlorophyll-ligand-chlorophyll aggregation and (2) chlorophyll-protein interactions. However, the relevance of these mechanisms to the in vivo red shifts is still not well understood and further studies of in vitro systems and model magnesium complexes are required in order to resolve this issue [2] . One approach is to consider the effect of axial ligation to four-coordinated, planar magnesium entities. In vitro studies indicate that chlorophyll molecules tend to aggregate into dimers and oligomers [3]. In oligomeric chlorophyll species axial bonding produces both fiveand six-coordinated Mg centres. From equilibrium studies [4] it can be ascertained that the visible absorption maxima of six-coordinated chlorophyll molecules are red shifted with respect to those of five-coordinated entities. Consequently, Katz has suggested that six-coordinated chlorophyll-bifunctional ligand adducts are suitable model compounds for chlorophyll molecules in reaction centres of photosynthetic systems and that the red-shift of these adducts is related to the distance between the nucleophilic centres of the axial ligands [2] . Although many physical studies of six-coordinated chlorophyll molecules have been carried out the structural basis of the red-shift is unknown as no crystal structure of such entities has been reported. However a related six-coordinated structure, Mg(octaethylporphyrin)(pyridine)z, has been determined [5] and the relevance of magnesium porphyrin structures to chlorophyll has again been highlighted in another recent paper [6]. We have recently reported the crystal structures of three related six-coordinated MgTPP complexes, MgTPP( 1 -MeIm)z, MgTPP(4pic)Z and MgTPP(pip)* (TPP = tetraphenylporphyrin, I-MeIm = 1 -methylimidazole, 4-pit = 4picoline, pip = piperidine) [7], which, as shown earlier [5] , have the axial ligands weakly bound to Mg at comparatively large distances. We report here results from electronic spectral studies of Nujol