J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

J-aggregates: from serendipitous discovery to supramolecular engineering of functional dye materials.

Energy transfer and trapping in photosynthesis

Redox reactions play important roles in almost all biological processes, including photosynthesis and respiration, which are two essential energy processes that sustain all life on earth. It is thus not surprising that biology employs redox-active metal ions in these processes. It is largely the redox activity that makes metal ions uniquely qualified as biological cofactors and makes bioinorganic enzymology both fun to explore and challenging to study.

Even though most metal ions are redox active, biology employs a surprisingly limited number of them for electron transfer (ET) processes. Prominent members of redox centers involved in ET processes include cytochromes, iron–sulfur clusters, and cupredoxins. Together these centers cover the whole range of reduction potentials in biology (Figure ​(Figure1).1). Because of their importance, general reviews about redox centers1−77 and specific reviews about cytochromes,8,24,78−90 iron–sulfur proteins,91−93 and cupredoxins94−104 have appeared in the literature. In this review, we provide both classification and description of each member of the above redox centers, including both native and designed proteins, as well as those proteins that contain a combination of these redox centers. Through this review, we examine structural features responsible for their redox properties, including knowledge gained from recent progress in fine-tuning the redox centers. Computational studies such as DFT calculations become more and more important in understanding the structure–function relationship and facilitating the fine-tuning of the ET properties and reduction potentials of metallocofactors in proteins. Since this aspect has been reviewed extensively before,105−110 and by other reviews in this thematic issue,2000,2001,2002 it will not be covered here.






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Figure 1


Reduction potential range of redox centers in electron transfer processes.

Metalloproteins containing cytochrome, iron-sulfur, or copper redox centers.

The green photosynthetic bacteria are characterized by the presence of chlorosomes appressed to the cytoplasmic side of the cytoplasmic membrane. The chlorosomes are filled with bacteriochlorophyll (BChl)$ c, d or e molecules in a highly aggregated state. The truly “green” bacteria contain mainly BChl c or d ; while the others look orange or brown because of a high content of carotenoid. From a phylogenetic point of view the green bacteria are really two separate “phyla” based on 16s rRNA (1-3), reaction center (RC) type and physiology. It is truly remarkable that such different types of bacteria (green filamentous bacteria and green sulfur bacteria) contain such similar light-harvesting entities as chlorosomes. Green filamentous bacteria (Chloroflexaceae) contain a quinone-type RC similar to those found in purple bacteria (Proteobacteria), whereas the green sulfur bacteria (Chlorobiaceae) contain an iron-sulfur-type RC similar to those found in heliobacteria and in photosystem I of cyanobacteria and chloroplasts. The filamentous bacteria live either as facultative photoautotrophs that grow in relatively bright light or as respiring chemoheterotrophs. They are found predominantly in hot springs, often in mixed population with cyanobacteria that provide organic carbon compounds for them. Most of our knowledge about the filamentous bacteria at the molecular level comes from one species, Chlorojlexus aurantiacus. A second species, Chlorojlexus aggregans, has recently been isolated and characterized by Hanada et al.

Chlorophyll Organization and Function in Green Photosynthetic Bacteria

— A comparison of the spectra of in vitro (3-hydroxymethyl-131-oxometallochlorin) and in vivo chlorosomal (bacterio-chlorophyll-c) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte-riochlorophyll-c aggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1 % (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Qy absorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm−1 (Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 31-hydroxy and a 131-keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five-coordinated metallochlorin macrocycle with an axial ligand (bands at 1500-1630 cm−1), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule, i.e. C=O…H-O…M (=Zn or Mg).

Synthetic Zinc and Magnesium Chlorin Aggregates as Models for Supramolecular Antenna Complexes in Chlorosomes of Green Photosynthetic Bacteria

Aggregates of bacteriochlorophylls c, d, and e and of zinc(II) methyl bacteriopheophorbides c and d, which exhibit large red shifts in hexane without the essential intervention of any carbonyl function on ring V are described. The aggregates appear to provide viable in vitro models for the antenna chlorophylls of green and brown photosynthetic bacteria and a rationale for the presence of the 2-(1-hydroxyethyl) group in bacteriochlorophylls c, d, and e.

Aggregation of the bacteriochlorophylls c, d, and e. Models for the antenna chlorophylls of green and brown photosynthetic bacteria

The proton nuclear magnetic resonance spectra of the soluble fragment of native bovine and genetically engineered wild-type rat ferricytochrome b{sub 5} reconstituted with a wide variety of hemes chemically modified at 2- and/or 4-positions have been recorded and analyzed. While all but one nonsymmetric heme yielded comparable amounts of the two heme orientations immediately after reconstitution, the relative proportion of the two orientations at equilibrium varied widely. The unpaired spin density distribution in the heme {pi} system leads to substituent hyperfine shift patterns in these paramagnetic complexes that are completely diagnostic of the heme orientation in the protein matrix. An empirical assignment strategy is outlined and applied which allows unequivocal assignment of the absolute orientation of a derivatized heme within the protein matrix. Using a series of hemes lacking 2-fold symmetry solely due to a single substitution, the preferences for localized site occupation of vinyls, methyls, and hydrogens are developed. The differences in this heme orientational preference among bovine, rat, and chicken ferricytochromes b{sub 5} could be correlated with the relative steric bulk of the residues at positions 23 and 25. Detailed thermodynamic analysis of the orientational preferences of native protoheme reveals that, while the same orientation as found inmore » X-ray crystal structures of bovine cytochrome b{sub 5} predominate at 25{degree}C in both proteins, the preference in the bovine protein is primarily for enthalpic reasons while in the rat protein the preference is due to entropic factors.« less

1H NMR Study of the Influence of Hydrophobic Contacts on Protein-Prosthetic Group Recognition in Bovine and Rat Ferricytochrome b5

Reversed-phase high-performance liquid chromatography and structural assignments of the bacteriochlorophylls-c☆

Mass spectrometry in structural and stereochemical problems. 266. Enhanced structural determination of substituted porphyrins by ammonia desorption chemical ionization mass spectrometry

Lisa A. Kehres

Papers

Aggregation of the bacteriochlorophylls c, d, and e. Models for the antenna chlorophylls of green and brown photosynthetic bacteria

1H NMR Study of the Influence of Hydrophobic Contacts on Protein-Prosthetic Group Recognition in Bovine and Rat Ferricytochrome b5

Reversed-phase high-performance liquid chromatography and structural assignments of the bacteriochlorophylls-c☆

Mass spectrometry in structural and stereochemical problems. 266. Enhanced structural determination of substituted porphyrins by ammonia desorption chemical ionization mass spectrometry

Structures of the bacteriochlorophyll c homologs: solution to a longstanding problem