Aggregation of synthetic zinc chlorins with several esterified alkyl chains as models of bacteriochlorophyll-c homologs

In vitro demetalation of central magnesium in various chlorophyll derivatives using Mg-dechelatase homolog from the chloroflexi Anaerolineae

In the metabolic pathway of chlorophylls (Chls), an enzyme called STAY-GREEN or SGR catalyzes the removal of the central magnesium ion of Chls and their derivatives to their corresponding free bases, including pheophytins. The substrate specificity of SGR has been investigated through in vitro reactions using Chl-related molecules. However, information about the biochemical properties and reaction mechanisms of SGR and its substrate specificity remains elusive. In this study, we synthesized various Chl derivatives and investigated their in vitro dechelations using an SGR enzyme. Chl-a derivatives with the C3-vinyl group on the A-ring, which is commonly found as a substituent in natural substrates, and their analogs with ethyl, hydroxymethyl, formyl, and styryl groups at the C3-position were prepared as substrates. In vitro dechelatase reactions of these substrates were performed using an SGR enzyme derived from an Anaerolineae bacterium, allowing us to investigate their specificity. Reactivity was reduced for substrates with an electron-withdrawing formyl or sterically demanding styryl group at the C3-position. Furthermore, the Chl derivative with the C8-styryl group on the B-ring was less reactive for SGR dechelation than the C3-styryl substrate. These results indicate that the SGR enzyme recognizes substituents on the B-ring of substrates more than those on the A-ring.

Substituted Methylenation at the 132 -Position of a Chlorophyll-a Derivative via Mixed Aldol Condensation, Optical Properties of the Synthetic Bacteriochlorophyll-d Analogs, and Self-aggregation of Their Zinc Complexes

Chlorophyll-a derivatives possessing a substituted methylene group at the 13² -position were prepared by the mixed aldol condensation of methyl 3-hydroxymethyl-pyropheophorbide-a with aldehydes bearing a methyl, p-nitro/cyanophenyl, or pentafluorophenyl group. Their electronic absorption spectra were dependent on the substituents at the methylene terminal. The Soret bands were broadened with increasing the group electronegativity of the substituents, which was ascribable to the charge transfer from the core chlorin to the peripheral substituent in a molecule. Although their Qy absorption and fluorescence emission bands resembled each other, the emission intensities decreased with an increase in the electronegativity because of the intramolecular electron transfer quenching. Some of their zinc complexes self-aggregated in a less polar organic solvent to give red-shifted and broadened absorption bands with intense circular dichroism couplets, which were similar to those of bacteriochlorophyll-c/d aggregates in natural chlorosomes as the main light-harvesting antennas of green photosynthetic bacteria and their models. The J-aggregation was suppressed with an enhancement in the size of the 13² -substituents.

Synthesis of N-methylated unsymmetric porphyrinoids with restricted N-centered chirality from chlorophyll-a

Regioselectively N-methylated chlorophyll-a derivatives were prepared as their cationic and hydrophilic species. Both their epimerically pure samples at the chiral methylated nitrogen atom were obtained, and the stereochemistry was proposed by circular dichroism spectral analysis. This is the first example for restricted N-centered chirality in an unsymmetric chlorin core.

BciC-Catalyzed C132 -Demethoxycarbonylation of Metal Pheophorbide a Alkyl Esters

Bacteriochlorophyll c molecules self-aggregate to form large oligomers in the core part of chlorosomes, which are the main light-harvesting antenna systems of green photosynthetic bacteria. In the biosynthetic pathway of bacteriochlorophyll c, a BciC enzyme catalyzes the removal of the C13² -methoxycarbonyl group of chlorophyllide a, which possesses a free propionate residue at the C17-position and a magnesium ion as the central metal. The in vitro C13² -demethoxycarbonylations of chlorophyll a derivatives with various alkyl propionate residues and central metals were examined by using the BciC enzyme derived from one green sulfur bacteria species, Chlorobaculum tepidum. The BciC enzymatic reactions of zinc pheophorbide a alkyl esters were gradually suppressed with an increase of the alkyl chain length in the C17-propionate residue (from methyl to pentyl esters) and finally the hexyl ester became inactive for the BciC reaction. Although not only the zinc but also nickel and copper complexes were demethoxycarbonylated by the BciC enzyme, the reactions were largely dependent on the coordination ability of the central metals: Zn>Ni>Cu. The above substrate specificity indicates that the BciC enzyme would not bind directly to the carboxy group of chlorophyllide a, but would bind to its central magnesium to form the stereospecific complex of BciC with chlorophyllide a, giving pyrochlorophyllide a, which lacks the (13² R)-methoxycarbonyl group.

Sterically controlled and pH-dependent self-aggregation of synthetic zinc 3-(alkylamino)methylated chlorophyll-a derivatives in aqueous micellar solution

Zinc methyl 3-(methyl/ethylamino)methyl-pyropheophorbides-[Formula: see text] were prepared as models of photosynthetically active bacteriochlorophyll-[Formula: see text] possessing the 3[Formula: see text]-hydroxy group. The synthetic methylamino analog predominantly dimerized via two Zn[Formula: see text]N coordination bonds in aqueous Triton X-100 micelles, while the more sterically crowding and less coordinating ethylamino derivative self-aggregated to form large oligomers through single coordination and hydrogen bonds, Zn[Formula: see text]N–H[Formula: see text]O=C-13, similarly as in the natural self-aggregates of bacteriochlorophyll-[Formula: see text]. The artificial self-aggregates were dependent on the aqueous pH, but the dimer was less pH-sensitive and also more tolerant to the additional Triton X-100.

Rational Design of Photosynthesis-Inspired Nanomedicines

The sun is the most abundant source of energy on earth. Phototrophs have discovered clever strategies to harvest this light energy and convert it to chemical energy for biomass production. This is achieved in light-harvesting complexes, or antennas, that funnel the exciton energy into the reaction centers. Antennas contain an array of chlorophylls, linear tetrapyrroles, and carotenoid pigments spatially controlled by neighboring proteins. This fine-tuned regulation of protein-pigment arrangements is crucial for survival in the conditions of both excess and extreme light deficit. Photomedicine and photodiagnosis have long been utilizing naturally derived and synthetic monomer dyes for imaging, photodynamic and photothermal therapy; however, the precise regulation of damage inflicted by these therapies requires more complex architectures. In this Account, we discuss how two mechanisms found in photosynthetic systems, photoprotection and light harvesting, have inspired scientists to create nanomedicines for more effective and precise phototherapies. Researchers have been recapitulating natural photoprotection mechanisms by utilizing carotenoids and other quencher molecules toward the design of photodynamic molecular beacons (PDT beacons) for disease-specific photoactivation. We highlight the seminal studies describing peptide-linked porphyrin-carotenoid PDT beacons, which are locally activated by a disease-specific enzyme. Examples of more advanced constructs include tumor-specific mRNA-activatable and polyionic cell-penetrating PDT beacons. An alternative approach toward harnessing photosynthetic processes for biomedical applications includes the design of various nanostructures. This Account will primarily focus on organic lipid-based micro- and nanoparticles. The phenomenon of nonphotochemical quenching, or excess energy release in the form of heat, has been widely explored in the context of porphyrin-containing nanomedicines. These quenched nanostructures can be implemented toward photoacoustic imaging and photothermal therapy. Upon nanostructure disruption, as a result of tissue accumulation and subsequent cell uptake, activatable fluorescence imaging and photodynamic therapy can be achieved. Alternatively, processes found in nature for light harvesting under dim conditions, such as in the deep sea, can be harnessed to maximize light absorption within the tissue. Specifically, high-ordered dye aggregation that results in a bathochromic shift and increased absorption has been exploited for the collection of more light with longer wavelengths, characterized by maximum tissue penetration. Overall, the profound understanding of photosynthetic systems combined with rapid development of nanotechnology has yielded a unique field of nature-inspired photomedicine, which holds promise toward more precise and effective phototherapies.

Spectral signatures of five hydroxymethyl chlorophyll a derivatives chemically derived from chlorophyll b or chlorophyll f

Chlorophylls (Chls) are pigments involved in light capture and light reactions in photosynthesis. Chl a, Chl b, Chl d, and Chl f are characterized by unique absorbance maxima in the blue (Soret) and red (Q_y) regions with Chl b, Chl d, and Chl f each possessing a single formyl group at a unique position. Relative to Chl a the Q_y absorbance maximum of Chl b is blue-shifted while Chl d and Chl f are red-shifted with the shifts attributable to the relative positions of the formyl substitutions. Reduction of a formyl group of Chl b to form 7-hydroxymethyl Chl a, or oxidation of the vinyl group of Chl a into a formyl group to form Chl d was achieved using sodium borohydride (NaBH₄) or β-mercaptoethanol (BME/O₂), respectively. During the consecutive reactions of Chl b and Chl f using a three-step procedure (1. NaBH₄, 2. BME/O₂, and 3. NaBH₄) two new 7-hydroxymethyl Chl a species were prepared possessing the 3-formyl or 3-hydroxymethyl groups and three new 2-hydroxymethyl Chl a species possessing the 3-vinyl, 3-formyl, or 3-hydroxymethyl groups, respectively. Identification of the spectral properties of 2-hydroxymethyl Chl a may be biologically significant for deducing the latter stages of Chl f biosynthesis if the mechanism parallels Chl b biosynthesis. The spectral features and chromatographic properties of these modified Chls are important for identifying potential intermediates in the biosynthesis of Chls such as Chl f and Chl d and for identification of any new Chls in nature.

Push-pull pyropheophorbides for nonlinear optical imaging

Pyropheophorbide-a methyl ester (PPa-OMe) has been modified by attaching electron-donor and -acceptor groups to alter its linear and nonlinear optical properties. Regioselective bromination of the terminal vinyl position and Suzuki coupling were used to attach a 4-(N,N-diethylaminophenyl) electron-donor group. The electron-acceptor dicyanomethylene was attached at the cyclic ketone position through a Knoevenagel condensation. Four different derivatives of PPa-OMe, containing either electron-donor or electron-acceptor groups, or both, were converted to hydrophilic bis-TEG amides to generate a series of amphiphilic dyes. The absorption and emission properties of all the dyes were compared to a previously reported push-pull type porphyrin-based dye and a commercial push-pull styryl dye, FM4-64. Electrochemical measurements reveal that the electron donor group causes a greater decrease in HOMO-LUMO gap than the electron-acceptor. TD-DFT calculations on optimized geometries (DFT) of all four dyes show that the HOMO is mostly localized on the donor, 4-(N,N-diethylaminophenyl), while the LUMO is distributed around the chlorin ring and the electron-acceptor. Hyper-Rayleigh scattering experiments show that the first-order hyperpolarizabilities of the dyes increase on attaching either electron-donor or -acceptor groups, having the highest value when both the donor and acceptor groups are attached. Two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) images of the bis-TEG amide attached dyes in lipid monolayer-coated droplets of water-in-oil reveal that the TPEF and SHG involve transition dipole moments in different orientations.

Regioselective addition of amines to the trifluoromethyl-β-diketonate moiety of a chlorophyll derivative

The terminal trifluoroacetyl moiety in a β-diketonate group at the 3 position of a synthetic chlorophyll derivative was reacted with butylamine in dichloromethane to give the hemiaminal-type adduct. Electronic absorption spectra showed the keto carbonyl group at the 3 position of the adduct was converted into the enol type under the basic conditions.

Control of higher-order structures of zinc chlorophyll coordination polymers

The higher-order structures of the zinc chlorophyll coordination polymers were controlled by the regiochemistry of the coordination sites.

Synthesis of long-wavelength chlorins by chemical modification for methyl pyropheophorbide-a and their in vitro cell viabilities

A series of novel chlorophyll-a homologs with long wavelength absorption were synthesized via modification of methyl pyropheophorbide-a used as starting material. For introducing electron-withdrawing group methylenemalononitrile moiety was established on the periphery of modified chlorin by Knoevenagel reaction of malononitrile with formyl group at 3-, 15-position and 131-carbonyl group on the exocyclic ring. All of chlorins containing the methylenemalononitrile structure show Qy-absorption at more than 700 nm. Moreover, we have examined a preliminary in vitro photodynamic anticancer effect of these new derivatives on mouse sarcoma S-180 cell line.

Facile Synthesis of 131-Oxo-porphyrins Possessing Reactive 3-Vinyl or 3-Formyl Group, Protochlorophyll-a/dDerivatives by 17,18-Dehydrogenation of Chlorins

Methyl protopyropheophorbides-a/d were prepared by modification of readily available chlorophyll-a. The key step was oxidation of 131-oxo-chlorin to the corresponding porphyrin, and 2,3-dichloro-5,6-dicyano-p-benzoquinone was effective for 17,18-dehydrogenation. A reduction in oxidation potential of the chlorin was necessary for the oxidation. Facile oxidation was achieved by insertion of zinc as the central metal, reduction of the 13-carbonyl group and/or aggregation by high concentration. The synthetic 131-oxo-porphyrins had vinyl and formyl groups at the 3-position. These groups were very reactive in the molecule and selectively converted to other functional groups.

Synthesis and Self-aggregation of Zinc 20-Halogenochlorins as a Model for Bacteriochlorophylls c/d

Zinc 20-halogenochlorins 2(20- F ), 3(20- Cl ) and 4(20- Br ) were synthesized by halogenation of a chlorophyll a derivative at the 20-position as a model for bacteriochlorophyll ( BChl )c, which possesses a methyl group at the 20-position and 20-unsubstituted BChl d. Visible spectra in a polar tetrahydrofuran ( THF ) solution showed that 2-4 were monomeric and the planarity of the chlorin ring, was distorted with increasing bulkiness of the 20-substituent. Visible, circular dichroism and IR spectra revealed that 2-4 self-aggregated to form oligomers similarly with 20-unsubstituted 1 and BChls c/d in the heterogeneous thin film as well as in homogeneous non-polar solvents (1% (v/v) THF-hexane). Therefore, the in vitro self-aggregates of 2-4 are good structural models for in vivo BChls c/d self-aggregates, the main antenna components of photosynthetic green bacteria. Fluorescence spectra showed that monomeric 3 and 4 were less emissive than 1 and 2 due to the heavy atom effect which could not be observed in the oligomeric species, indicating that the in vitro aggregates should be promising as functional (light-harvesting) models.

Synthesis and Self-aggregation of Zinc 20-Halogenochlorins as a Model for Bacteriochlorophylls c/d

Zinc 20-halogenochlorins 2(20- F ), 3(20- Cl ) and 4(20- Br ) were synthesized by halogenation of a chlorophyll a derivative at the 20-position as a model for bacteriochlorophyll ( BChl )c, which possesses a methyl group at the 20-position and 20-unsubstituted BChl d. Visible spectra in a polar tetrahydrofuran ( THF ) solution showed that 2-4 were monomeric and the planarity of the chlorin ring, was distorted with increasing bulkiness of the 20-substituent. Visible, circular dichroism and IR spectra revealed that 2-4 self-aggregated to form oligomers similarly with 20-unsubstituted 1 and BChls c/d in the heterogeneous thin film as well as in homogeneous non-polar solvents (1% (v/v) THF-hexane). Therefore, the in vitro self-aggregates of 2-4 are good structural models for in vivo BChls c/d self-aggregates, the main antenna components of photosynthetic green bacteria. Fluorescence spectra showed that monomeric 3 and 4 were less emissive than 1 and 2 due to the heavy atom effect which could not be observed in the oligomeric species, indicating that the in vitro aggregates should be promising as functional (light-harvesting) models.

Synthesis of methyl pyropheophorbide-a pyrazole derivatives and their in vitro cell viabilities on A549 cells

Porphyrins and chlorins containing acetylacetone residues on the peripheral substituents are known to readily react with hydrazine derivatives to form porphyrin and chlorin derivatives possessing a pyrazole moiety. In this study, we have carried out the synthesis of methyl pyropheophorbide-a derivatives containing a pyrazole moiety on its peripheral position. Moreover, we have examined a preliminary in vitro effect of these new derivatives on A549 cancer cell in photodynamic therapy.

Chlorophyll-a derivatives with various hydrocarbon ester groups for efficient dye-sensitized solar cells: static and ultrafast evaluations on electron injection and charge collection processes

Five chlorophyll-a derivatives, chlorins-1-5 possessing C3(2)-carboxy and O17(4)-esterified hydrocarbon groups including methyl, hexyl, dodecyl, 2-butyloctyl, and cholesteryl were synthesized. Their performance as sensitizers in dye-sensitized solar cells (DSSCs) was compared. These sensitizers have similar surface coverage on the unit surface of TiO(2) film and their absorption spectra on transparent TiO(2) films were identical. On the basis of DFT and TD-DFT calculations of these sensitizers in ethanol, a major difference between them was the geometry of the hydrocarbon ester group, to affect their electron injection and charge recombination with the TiO(2) electrode rather than the energy level of their molecular orbitals. DSSC based on chlorin-3 with a dodecyl ester group gave a solar energy-to-electricity conversion efficiency of 8%, which was the highest among all the chlorophyllous sensitizers. The large photocurrent in the chlorin-3 sensitized solar cell can be explained by the least impedance in the electrolyte-dye-TiO(2) interface in electrical impedance spectroscopy measurements. Subpicosecond time-resolved absorption spectroscopic studies have also been carried out to evaluate the electron injection and charge recombination dynamics in the dye-TiO(2) interface. For the electron injection and charge recombination processes, a charge separated state of the dye-TiO(2) complex has been found to be free from the type and concentration of dye sensitizer, reflecting the same type of electron transfer process for all the five chlorin sensitizers. A new quenching pathway of the dye excitation, which is probably from the exciton annihilation, in addition of the charge recombination has been observed for chlorin-1 and chlorin-5, but not for chlorin-3. The higher open-circuit photocurrent observed in the present dyes with larger ester groups can be attributed to the reduced leaking of charges in the TiO(2)-electrolyte interface, which was supported by the longer electron lifetimes.

Covalently linked zinc chlorophyll dimers as a model of a chlorophyllous pair in photosynthetic reaction centers

A heterodimer, where zinc pyropheophorbide-a was linked with zinc pyropheophorbide-d through ethylene glycol diester, was prepared, as well as the corresponding homodimers. The synthetic dimers were complexed with methanol in benzene to give folded dimers by mutual Zn...O(Me)-H...O=C13(1) bonding. Such complexes had furthest red (Qy) absorption bands at longer wavelengths than the monomeric species. These red-shifts were ascribable to excitonic coupling of the Qy transition states in the chlorin pi-pi stacking conformer. In the heterodimeric system, a minor band was observed at the shorter wavelength side of the main Qy band. This observation can be explained by an additional contribution of Qy vibronic state to the exciton-coupled states. Based on the experimental results, a pair of chlorophyll(Chl)-d with Chl-a as well as a Chl-d homopair were proposed as dimers in reaction centers of Chl-d dominating cyanobacteria.