In contrast to porphyrins and chlorins, the direct metalation of bacteriochlorins is difficult. Nevertheless, Cu2+ and Zn2+ can be introduced into bacteriopheophytin in acetic acid, whereas Cd2+ can be inserted in dimethylformamide. The former reactions depend on the substituents of the isocyclic ring:  they are facilitated if enolization of the β-ketoester system is inhibited. Starting with [Cd]-bacteriochlorophyll-a or its 132-hydroxy derivative, a series of metallo-bacteriochlorins with central divalent ions Pd2+, Co2+, Ni2+, Cu2+, Zn2+, and Mn2+ have been obtained by transmetalation. Like in the parent Mg complex, the four principal optical transitions are well-separated in these complexes, and their responses to changes in the central metal and its coordination state can be followed in detail. The energies of the Q y and B x transitions are almost independent of the central metal, whereas the Q x and B y transition energies change significantly, depending on the central metal as well as the presence of additional axial ligands. If the complexes are grouped by their coordination number, empirical linear correlations exist between these shifts and the ratio / , where is Pauling's electronegativity value and is the ionic radius of the metal. A similar correlation was found for those 1H NMR signals influenced mainly by the ring current and for the redox potentials. This observation was in contrast with the linear relationships with alone, found for metal-substituted porphyrins. The spectral variations influenced by the central metal and its state of ligation are attributed, within the framework of the four-orbital model, to the electrostatic interaction of the electron densities in the four orbitals with the effective charge of the central metal ions, which is most pronounced for the a2u orbital (HOMO-1). Ligation studies have revealed that addition of the first axial ligand decreases the effective charge of the central metal by approximately 50% and addition of the second axial ligand by another 20% with respect to the absence of axial ligands. The singlet−triplet splitting deduced from fluorescence and phosphorescence measurements is similar for [Pd]-, [Cu]-, [Zn]-, and [Mg]-BChl (4550 ± 100 cm-1).

Chromopeptides were prepared by proteolytic digestion of phytochrome (far-red absorbing form, Pfr) and of phycocyanin. The phycocyanobilin peptide, the chromophore of which is Z,Z,Z-configurated, was modified to the Z,ZE isomeric chromophore. It has been demonstrated earlier that the Pfr chromopeptide and the Z,Z,E-configurated phycocyanin chromopeptide behave similarly with regard to spectral and chromatographic properties and reactivity. We present evidence here, obtained by high-resolution 'H NMR spectroscopy, that both the modified phycocyanobilin chromophore and the phytochrome chromophore obtained directly from Pfr are 15E-configurated.Plant development is influenced by light in many ways. An important photoreceptor of higher plants is the chromoprotein phytochrome (1-3), which can mediate light-dependent irreversible differentiation (e.g., seed germination, flowering, and stem and leaf growth) and reversible modulations (e.g., leaflet or chloroplast movement, root tip adhesion, and transmembrane potentials).A characteristic property of phytochrome in vivo (in the plant cell) and in vitro is its photoreversibility. The physiologically inactive (red-absorbing) Pr form (Ama. = 660 nm) is transformed by red light to the physiologically active Pfr form (Amak = 730 nm), which in turn is reconverted by far-red light to the Pr form. nm Pr = Pfr 730 nmThe chemical structure of the Pr chromophore (structure la), including its linkage to the protein, was elucidated by combination of oxidative degradation and UV/visible spectroscopy (4-6), by comparison of the cleaved chromophore with the product obtained by total synthesis (7,8), and by high-resolution NMR spectroscopy of a chromopeptide (9). It is closely related to the structure of the phycocyanin chromophore

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