(Hydr)oxo-bridged heme complexes: From structure to reactivity

Iron–porphyrins ([Formula: see text] hemes) are present throughout the biosphere and perform a wide range of functions, particularly those that involve complex multiple-electron redox processes. Some common heme enzymes involved in these processes include cytochrome P450, heme/copper oxidase or heme/non-heme diiron nitric oxide reductase. Consequently, the (hydr)oxo-bridged heme species have been studied for the important roles that they play in many life processes or for their application for catalysis and preparation of new functional materials. This review encompasses important synthetic, structural and reactivity aspects of the (hydr)oxo-bridged heme constructs that govern their function and application. The properties and reactivity of the bridging (hydr)oxo moieties are directly dictated by the coordination environment of the heme core, the nature and ligation of the second metal center attached to the (hydr)oxo group, the presence or absence of a linker, and the degree of flexibility around that linker within the scaffold. Here, we summarize the structural features of all known (hydr)oxo-bridged heme constructs and use those to categorize and thus, provide a more comprehensive picture of structure–function relationships.

Porphyrin-DNA conjugates: porphyrin induced adenine-guanine homoduplex stabilization and interduplex assemblies

DNA has found widespread uses as a nanosized scaffold for assembly of patterned multichomophoric nanostructures. Herein we report the synthesis, self-assembly, stability, and spectroscopic studies of short alternating non-self-complementary DNA sequences 5'-(dGdA)(4) and 5'-(dAdG)(4) with non-charged tetraarylporphyrins covalently linked to the 5' position of deoxyadenosine or deoxyguanosine via a phosphate or amide linker. The linker, the metal in the porphyrin coordination center, and the neighboring nucleobase have very distinct effects on the duplex formation of porphyrin-deoxyguanosine-deoxyadenosine oligodeoxynucleotides. At ionic strength between 5 mM and 40 mM, free base trispyridylphenylporphyrin appended to the 5' termini of 5'-(dAdG)(4) oligonucleotide via short non-polar amide linker served as a hydrophobic molecular cap inducing deoxyadenosine-deoxyguanosine antiparallel homoduplex. At ionic strength of ≥60 mM, the free base porphyrin functioned as a molecular 'glue' and induced the formation of porphyrin-DNA inter-homoduplex assemblies with characteristic tetrasignate CD Cotton effects in the porphyrin Soret band region. When the porphyrin cap was covalently attached to 5' position of deoxyguanosine or deoxyadenosine via charged phosphate linker, no significant deoxyadenosine-deoxyguanosine hybridization was observed even at elevated ionic strengths.

Syntheses of boronic-acid-appended metalloporphyrins as potential colorimetric sensors for sugars and carbohydrates

Boronic-acid-based dyes have been widely studied in the past decade as potential sensors for sugars and carbohydrates, owing to the strong interaction of boronic acid and diols. In this work, two boronic-acid-appended metalloporphyrins were synthesized by new routes and tested as potential sugar and carbohydrate sensors. Boronic acid substituents were attached either to a β-pyrrolic position ((2-dihydroxylboryl-5,10,15,20-tetraphenylporphyrinato)zinc(II), 1) or to the meso-position ((10-dihydroxylboryl-5,15-diphenylporphyrinato)zinc(II), 2) of the porphyrin core. These complexes were tested as potential carbohydrate sensors; unfortunately, no UV-vis or fluorescence spectra changes were observed with the addition of glucose or fructose. Molecular orbital calculations confirm little electronic conjugation between the porphyrin π-system and the boronic acid substituents in either pyrrolic or meso-positions.

Role of environmental factors on the structure and spectroscopic response of 5'-DNA-porphyrin conjugates caused by changes in the porphyrin-porphyrin interactions

We have explored the utility, strength, and limitation of through-space exciton-coupled circular dichroism in determination of the secondary structure of optically active chromophoric nanoarrays using the example of end-capped porphyrin- and metalloporphyrin-oligodeoxynucleotide conjugates. We put special emphasis on the explanation of the origin and significance of the distinctive multiple bands in the CD spectra (trisignate and tetrasignate CD bands). Such CD profiles are often observed in chiral aggregates or multichromophoric arrays but have never before been studied in detail. We found that variation of temperature and ionic strength has a profound effect on the geometry of the porphyrin-DNA conjugates and thus the nature of electronic interactions. At lower temperatures and in the absence of NaCl all three 5'-DNA-porphyrin conjugates display negative bisignate CD exciton couplets of variable intensity in the Soret region resulting from through-space interaction between the electric transition dipole moments of the two end-capped porphyrins. As the temperature is raised these exciton couplets are transformed into single positive bands originating from the porphyrin-single-strand DNA interactions. At higher ionic strengths and low temperatures, multisignate CD bands are observed in the porphyrin Soret region. These CD signature bands originate from a combination of intermolecular, end-to-end porphyrin-porphyrin stacking between duplexes and porphyrin-DNA interactions. The intermolecular aggregation was confirmed by fluorescence and absorption spectroscopy and resonance light scattering. DeVoe theoretical CD calculations, in conjunction with molecular dynamics simulations and Monte Carlo conformational searches, were used to mimic the observed bisignate exciton-coupled CD spectra as well as multiple CD bands. Calculations correctly predicted the sign and shape of the experimentally observed CD spectra. These studies reveal that the exciton-coupled circular dichroism is a very useful technique for the determination of the structure of optically active arrays.

Regioselective Borylation of Porphyrins by CH Bond Activation under Iridium Catalysis to Afford Useful Building Blocks for Porphyrin Assemblies

Highly regioselective and efficient borylation of a variety of porphyrins has been achieved by reaction with bis(pinacolato)diboron through C-H bond activation under iridium catalysis on the basis of the synthetic protocol developed by Miyaura, Hartwig, and Smith. A boryl group can be selectively introduced at sterically uncongested positions in the peripheral aryl groups of porphyrin substrates whose peripheral beta-positions are sterically hindered. Curiously, beta substituents adjacent to the aryl group to be borylated have unexpectedly large effects on the regioselectivity, because the iridium catalyst can discriminate between subtle steric differences. Chemoselective borylation was also achieved for several functionalized porphyrins. This borylation protocol can be applied to various monomeric and oligomeric functional porphyrins, hence offering an efficient route to elaborate multiporphyrin-based molecular constructs.

Highly Sensitive Chiral Shift Reagent Bearing Two Zinc Porphyrins

A new type of chiral receptor (R,R)- or (S,S)-1b with C(2) symmetry was synthesized. An induced-fit type of binding behavior of 1b for diamines was revealed by CD spectroscopy. NMR studies demonstrated that 1b can function as a highly sensitive chiral shift reagent for the determination of the enantiomeric purity of chiral diamines, aziridine, and isoxazoline at the microgram level. [structure: see text]

Remarkable stability and enhanced optical activity of a chiral supramolecular bis-porphyrin tweezer in both solution and solid state

Interaction of the achiral syn (face-to-face) conformer of the ethane-bridged bis(zinc octaethylporphyrin) with the enantiopure 1,2-diaminocyclohexane results in the exclusive formation of a supramolecular chiral tweezer. This 1:1 host-guest complex exhibits remarkable stability in both solution (even upon photoexcitation) and solid-state phases, with a high degree of optical activity arising from the two-point interaction mode and optimal spatial geometry.

A hydrogen-bonding receptor that binds cationic monosaccharides with high affinity in methanol

A dicarboxylate host (1) binds cationic monosaccharides such as D-glucosamine HCl (2), D-galactosamine-HCl (3), and D-mannosamine-HCl (4) with high affinity (K1 = 8.0 x 10(4)-2.0 x 10(5) M(-1)) in methanol. In circular dichroism (CD) spectroscopy a positive exciton-coupling band was observed near 290 nm; this indicates that the saccharides are recognized by multiple point interactions. Since the corresponding neutral monosaccharides are not significantly bound, one may conclude that complex formation is primarily due to the electrostatic interaction between NH3+ in the guest and one carboxylate in the host and secondarily due to hydrogen-bonding interactions of OH groups with the other carboxylate and/or nitrogen bases. Molar ratio plots and Job plots indicate that host 1 and cationic monosaccharide guests form CD-active, pseudo-cyclic 1:1 complexes at low guest concentration followed by the formation of CD-silent, acyclic 1:2 1-saccharide complexes at high guest concentration. The possible binding modes are discussed in detail on the basis of molecular mechanics calculations and chemical shift changes in 1H NMR spectra. The results of competition experiments with several cationic reference compounds bearing fewer OH groups than 2-4 are consistent with the proposed binding model. Thus, the present study is a rare example of saccharide recognition in a protic solvent, where in general, hydrogen-bonding interactions are rarely useful because of strong solvation energy. These are apparently the strongest saccharide complexes involving noncovalent interactions between host and guest. We believe that the findings are significant as a milestone toward development of new saccharide recognition systems ultimately useful in aqueous solution.

Molecular design of artificial molecular and ion recognition systems with allosteric guest responses

Positive or negative allosterisms are ubiquitously seen in nature where the biological events must be efficiently regulated in response to chemical or physical signals from the outside world. The biomimetic design of such allosteric systems is of great significance in order to regulate the complexation ability or the catalytic activity of artificial receptors according to an allosteric manner. Furthermore, the methodology is very useful to amplify and convert weak chemical or physical signals into other signals which are convenient for us to read out and record. Allosteric systems are classified into four different categories: positive heterotropic, negative heterotropic, positive homotropic, and negative homotropic. In this Account, we account for our artificial allosteric systems and discuss the basic concept for molecular design of such allosteric systems and what kinds of new functions come out of such dynamic systems.

Supramolecular chirality induction in bis(zinc porphyrin) by amino acid derivatives: Rationalization and applications of the ligand bulkiness effect

The achiral syn conformer (face-to-face) of the ethane-bridged bis(zinc porphyrin) (syn-ZnD) transforms into the corresponding chiral extended anti bis-ligated species (anti-ZnD.L2) in the presence of enantiopure ligands (L: amino acid derivatives). The mechanism of the supramolecular chirality induction is based on chiral ligand binding to zinc porphyrins and subsequent formation of either right- or left-handed screw structures in anti-ZnD.L2. The screw structure formation arises from steric interactions between the bulkiest substituent at the asymmetric carbon of the ligand and the peripheral ethyl groups of the neighboring porphyrin ring directed towards the covalent bridge. The sign and amplitude of the induced circular dichroism (CD) are dependent on the steric bulk of the substituents at the chiral center. The greater difference in size between the chiral center's substituents gives the stronger induced CD signal. Rationalization of the ligand bulkiness effect on chirality induction by amino acid derivatives, application of this supramolecular system for the determination of ligand absolute configuration, and relative bulkiness of the substituents at the asymmetric carbon are discussed.