Chemical studies of some life processes (1984)
AuthorsOng, Chin Choonshow all
This thesis is primarily concerned with model complexes of some living systems. The two areas of biological interest are photosynthetic systems and chemical interaction between living cells. In both cases the chemical properties are very complex and generally not well understood. Mg porphyrin was used as a model for chlorophyll. The chemical properties of Mg porphyrin were determined by physical techniques using (1) different axial ligands and (2) protein environment models. Equilibrium studies indicate that the Mg centre is preferentially five-coordinated but under high ligand concentration and low temperature, six-coordination is observed. The spectrum of the six-coordinate Mg porphyrin species is red-shifted in the same manner as also observed for chlorophyll molecules in vivo. X-ray studies show that the six-coordinate Mg porphyrin species have long axial bonds with the Mg atom situated in the plane of the porphyrin ring. The shorter the axial bond lengths, the larger the red-shifts. The interactions of Mg porphyrin with protein environments have been studied by electronic circular dichroism and optical rotatory dispersion spectroscopy. In all cases, the spectra of Mg porphyrin-apomyoglobin are more complex and more red-shifted than those of Mg porphyrin-apohemoglobin analogues. These effects may arise directly from differences in protein interactions with porphyrin side chains or indirectly through water coordination to the Mg atom. The latter appears to be facilitated by features of the apomyoglobin heme pocket but inhibited in apohemoglobin. X-ray studies suggest that the key feature may be hydrogen bonding of the water molecule to the distal histidine group of apomyoglobin. This hydrogen bonding also enhances the displacement of the Mg atom out of the plane of the porphyrin ring. This could be crucial for the charge separation required for the first step in photosynthesis. The induced Cotton effects for the Mg porphyrin-protein complexes are different from those observed for Mg porphyrin in chiral amino acid solutions. For Mg porphyrin-protein complexes, these effects are interpreted in terms of the coupled oscillator mechanism between the transitions of Mg porphyrin and protein, while for Mg porphyrin in chiralamino acids, these effects are explained in terms of the formation of six-coordinate (Mg porphyrin) (amino acid)₂ complexes. Mg porphyrin-protein complexes are more stable to light than Mg porphyrin in amino acid solutions. This stability is explained in terms of the smaller accessibility of the heme pocket and the effects of the coordinated imidazole group, which protects the porphyrin ring from decomposition. The observations of different photodecomposition patterns for Mg porphyrin-apomyoglobin and Mg porphyrin-apohemoglobin are then correlated with the different protein conformations. The coupling of oscillating chemical reactions in a two reactor cell system was used as a model for the interaction between living cells. The degree of interaction was measured by a coupling constant. Generally, two coupling constants were observed depending upon the time of coupling. The coupling constant for starting oscillating reactor cells is smaller than that for organised oscillating reactor cells. This shows that starting oscillating reactor cells are more likely to be entrained or influenced by external factors. The small differences in the coupling constants for different kinds of apparatus are interpreted in terms of the effect of apparatus design. Finally, the medical and technological significance of all these results are highlighted.