Dong Heon Nam

Rational Design and Engineering of Quantum‐Dot‐Sensitized TiO2 Nanotube Arrays for Artificial Photosynthesis

Redox enzymes can catalyze complex synthesis reactions under mild conditions but conventional catalysts rarely accomplish this task. Despite the high potential of redox enzymes for the synthesis of valuable compounds (e.g., chiral alcohols and drug intermediates), [ 1–5 ] their application is hampered by the high cost of enzyme-specifi c cofactors that are required as a redox equivalent, such as nicotinamide adenine dinucleotide (NAD(P) H) and fl avin adenine dinucleotide (FADH). Thus, numerous efforts have been made over the past decades to accomplish in situ cofactor regeneration from their oxidized counterpart. [ 6–9 ]

Self-assembled, photoluminescent peptide hydrogel as a versatile platform for enzyme-based optical biosensors

A self-assembled peptide hydrogel consisting of Fmoc-diphenylalanine has been employed as a biosensing platform through the encapsulation of enzyme bioreceptors (e.g., glucose oxidase or horseradish peroxidase) and fluorescent reporters (e.g., CdTe and CdSe quantum dots). Enzymes and quantum dots (QDs) were physically immobilized within the hydrogel matrix in situ in a single step by simply mixing aqueous solution containing QDs and enzymes with monomeric peptide (Fmoc-diphenylalanine) solution. By using atomic force microscopy and scanning transmission electron microscopy, we observed that the self-assembled peptide hydrogel had a three-dimensional network of nanofibers (with a diameter of approximately 70-90 nm) that physically hybridized with QDs and encapsulated enzyme bioreceptors with a minimal leakage. We successfully applied the peptide hydrogel to the detection of analytes such as glucose and toxic phenolic compounds by using a photoluminescence quenching of the hybridized QDs. The Michaelis-Menten constant (K(M)) of the photoluminescent peptide hydrogel was found to be 3.12 mM (GOx for glucose) and 0.82 mM (HRP for hydroquinone), respectively, which were much lower than those of conventional gel materials. These results suggest that the peptide hydrogel is an alternative optical biosensing platform with practical advantages such as simple fabrication via self-assembly, efficient diffusion of target analytes, and high encapsulation efficiencies for fluorescent reporters and bioreceptors.

Eosin Y-sensitized artificial photosynthesis by highly efficient visible-light-driven regeneration of nicotinamide cofactor.

Dye‐sensitized photosynthesis: Eosin Y (EY), a dye photosensitizer, works efficiently as a molecular photoelectrode by catalyzing the visible‐light‐driven electron‐transfer reaction for efficient regeneration of NADH through a photosensitizer–electron relay dyad. Injection of the photosensitized electron resulted in highly accelerated regeneration of NADH, which can be used by glutamate dehydrogenase for the photosynthesis of L‐glutamate.

Nanobiocatalytic assemblies for artificial photosynthesis

Natural photosynthesis, a solar-to-chemical energy conversion process, occurs through a series of photo-induced electron transfer reactions in nanoscale architectures that contain light-harvesting complexes, protein-metal clusters, and many redox biocatalysts. Artificial photosynthesis in nanobiocatalytic assemblies aims to reconstruct man-made photosensitizers, electron mediators, electron donors, and redox enzymes for solar synthesis of valuable chemicals through visible light-driven cofactor regeneration. The key requirement in the design of biocatalyzed artificial photosynthetic process is an efficient and forward electron transfer between each photosynthetic component. This review describes basic principles in combining redox biocatalysis with photocatalysis, and highlights recent research outcomes in the development of nanobiocatalytic assemblies that can mimic natural photosystems I and II, respectively. Current issues in biocatalyzed artificial photosynthesis and future perspectives will be briefly discussed.

Cofactor-Free Light-Driven Whole-Cell Cytochrome P450 Catalysis†

Cytochromes P450 can catalyze various regioselective and stereospecific oxidation reactions of non-functionalized hydrocarbons. Here, we have designed a novel light-driven platform for cofactor-free, whole-cell P450 photo-biocatalysis using eosin Y (EY) as a photosensitizer. EY can easily enter into the cytoplasm of Escherichia coli and bind specifically to the heme domain of P450. The catalytic turnover of P450 was mediated through the direct transfer of photoinduced electrons from the photosensitized EY to the P450 heme domain under visible light illumination. The photoactivation of the P450 catalytic cycle in the absence of cofactors and redox partners is successfully conducted using many bacterial P450s (variants of P450 BM3) and human P450s (CYPs 1A1, 1A2, 1B1, 2A6, 2E1, and 3A4) for the bioconversion of different substrates, including marketed drugs (simvastatin, lovastatin, and omeprazole) and a steroid (17β-estradiol), to demonstrate the general applicability of the light-driven, cofactor-free system.

Visible Light‐Driven NADH Regeneration Sensitized by Proflavine for Biocatalysis

Harvest time: Proflavine drives the reduction of NAD(+) in the presence of a Rh-based electron mediator. Photoregenerated NADH was enzymatically active for oxidation by NADH-dependent L-glutamate dehydrogenase for the synthesis of L-glutamate. This work suggests that proflavine has the potential to become an efficient light-harvesting component in biocatalytic photosynthesis driven by solar energy.

Biocatalytic Photosynthesis with Water as an Electron Donor

Efficient harvesting of unlimited solar energy and its conversion into valuable chemicals is one of the ultimate goals of scientists. With the ever-increasing concerns about sustainable growth and environmental issues, numerous efforts have been made to develop artificial photosynthetic process for the production of fuels and fine chemicals, thus mimicking natural photosynthesis. Despite the research progress made over the decades, the technology is still in its infancy because of the difficulties in kinetic coupling of whole photocatalytic cycles. Herein, we report a new type of artificial photosynthesis system that can avoid such problems by integrally coupling biocatalytic redox reactions with photocatalytic water splitting. We found that photocatalytic water splitting can be efficiently coupled with biocatalytic redox reactions by using tetracobalt polyoxometalate and Rh-based organometallic compound as hole and electron scavengers, respectively, for photoexcited [Ru(bpy)3](2+). Based on these results, we could successfully photosynthesize a model chiral compound (L-glutamate) using a model redox enzyme (glutamate dehydrogenase) upon in situ photoregeneration of cofactors.

Photoelectrochemical Reduction of Carbon Dioxide to Methanol through a Highly Efficient Enzyme Cascade

Natural photosynthesis is an effective route for the clean and sustainable conversion of CO2 into high-energy chemicals. Inspired by the natural process, a tandem photoelectrochemical (PEC) cell with an integrated enzyme-cascade (TPIEC) system was designed, which transfers photogenerated electrons to a multienzyme cascade for the biocatalyzed reduction of CO2 to methanol. A hematite photoanode and a bismuth ferrite photocathode were applied to fabricate the iron oxide based tandem PEC cell for visible-light-assisted regeneration of the nicotinamide cofactor (NADH). The cell utilized water as an electron donor and spontaneously regenerated NADH. To complete the TPIEC system, a superior three-dehydrogenase cascade system was employed in the cathodic part of the PEC cell. Under applied bias, the TPIEC system achieved a high methanol conversion output of 220 μm h-1 , 1280 μmol g-1  h-1 using readily available solar energy and water.

Serum-stable quantum dot--protein hybrid nanocapsules for optical bio-imaging

We introduce shell cross-linked protein/quantum dot (QD) hybrid nanocapsules as a serum-stable systemic delivery nanocarrier for tumor-targeted in vivo bio-imaging applications. Highly luminescent, heavy-metal-free Cu0.3InS2/ZnS (CIS/ZnS) core-shell QDs are synthesized and mixed with amine-reactive six-armed poly(ethylene glycol) (PEG) in dichloromethane. Emulsification in an aqueous solution containing human serum albumin (HSA) results in shell cross-linked nanocapsules incorporating CIS/ZnS QDs, exhibiting high luminescence and excellent dispersion stability in a serum-containing medium. Folic acid is introduced as a tumor-targeting ligand. The feasibility of tumor-targeted in vivo bio-imaging is demonstrated by measuring the fluorescence intensity of several major organs and tumor tissue after an intravenous tail vein injection of the nanocapsules into nude mice. The cytotoxicity of the QD-loaded HSA-PEG nanocapsules is also examined in several types of cells. Our results show that the cellular uptake of the QDs is critical for cytotoxicity. Moreover, a significantly lower level of cell death is observed in the CIS/ZnS QDs compared to nanocapsules loaded with cadmium-based QDs. This study suggests that the systemic tumor targeting of heavy-metal-free QDs using shell cross-linked HSA-PEG hybrid nanocapsules is a promising route for in vivo tumor diagnosis with reduced non-specific toxicity.

New Platform for Cytochrome P450 Reaction Combining in Situ Immobilization on Biopolymer

We describe an efficienct chemical conversion platform with in situ immobilization of P450-BM3 on poly(3-hydroxybutyrate) granules. Through fusion with phasin, P450-BM3 is easily immobilized on poly(3-hydroxybutyrate) granules in Escherichia coli. In our work, the immobilized P450 exhibited higher stability and catalytic activity compared to free P450 against changes of pH, temperature, and concentrations of urea and ions. Through quick recovery of immobilized enzyme, the P450-P(3HB) complex successfully catalyzed an O-dealkylation reaction several times with maintained activity. Using the robust P450-P(3HB) complex, we performed a P450-catalyzed reaction on a preparative reactor scale (100 mL) and high-level production (12.3 μM) of 7-hydroxycoumarine from 7-ethoxycoumarin could be achieved.