Conformation variations porphyrins

These calculations and previous results for model porphyrins and photosynthetic reaction centers suggest that conformational variations play a significant role in determining optical and redox properties of porphyrin derivatives and thus may offer attractively simple rationales, in conjunction with additional modulations imposed by protein residues or solvents, for their properties in vivo and vitro. 

The combination of theoretical and experimental results strongly suggests that conformational variations observed for porphyrin derivatives can provide a mechanism for altering optical and redox properties. Such effects, in combination with additional modulations induced by protein residues (or solvents), thus provide an attractive mechanism for fine-tuning the electronic properties of the chromophores in vitro and in vivo. 

We extend our considerations of the micro-environment imposed by the protein to the thesis that axial ligands, hydrogen bonds and neighboring residues of photo synthetic chromophores help to define a scaffolding that in turn controls the conformations of the molecules. Theoretical calculations indicate that conformational variations would shift the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of (B)Chls and thereby modulate their redox and light-absorption properties [17,32]. We consider here the evidence for and the consequences of such conformational differences in (B)Chls, and test our conclusions with a series of synthetic porphyrins that are significantly puckered. 

In the case of porphyrins different substitution patterns induce relevant electronic variations the energy level of the two HOMOs of the porphyrin n system (a/u and a2u) are in fact reversed in going from a meso-substituted porphyrin, such as tetraphenylporphyrin, to a -substituted one, such as octaethylporphyrin [44]. Electronic variations may also derive from conformational changes Smith and coworkers have reported hybrid compounds substituted both at meso and P positions, the Zn2+ complexes of 2,3,7,8,12,13,17,18-octaethyl- and 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin [46] (see Sect. 3.3). These complexes have a nonplanar conformation caused by the steric interactions of the substituents. Theoretical calculations indicate that such conformational changes cause a differential shift in the energies of the HOMO and LUMO and so modulate the electronic structure of the macrocycles. 

We have already noted that extensive conformational studies suggest that the metal substituted cytochromes c (eg Zncytc, porphcytc) are essentially isostructural with Fecytc. Thus we expect that for the Zn, Fe, and H. porphyrin cyto chromes metal-porphyrin dependent variations in structure will not greatly affect the general trend observed for the dependence of rate on AG. 

In any event, let us examine low-temperature fluorescence results from a magnesium-porphyrin myoglobin (Fig. If the reader actually consults the cited works, he/she may be confused by the term population distribution function (PDF) used by the authors, which is easily confused with the phonon wing (PW) discussed up to this point. Vanderkooi et al. use the term PDF as an acronym for the conformational distribution that Frauenfelder et al. have used to explain low-temperature reaction kinetic distributions, and they measure this distribution function by picking out the variation in a ZPL intensity with excitation wavelength. 

---