Catalysis Science & Technology View Article Online PAPER View Journal | View Issue Morpholine-based buffers activate aerobic via Cite this: Catal. Sci. Technol.,2019, photobiocatalysis spin correlated ion pair 9,1365 formation† Leticia C. P. Gonçalves, *a Hamid R. Mansouri, a Erick L. Bastos, b Mohamed Abdellah,cd Bruna S. Fadiga,bc Jacinto Sá, ce Florian Rudroff a and Marko D. Mihovilovic a The use of enzymes for synthetic applications is a powerful and environmentally-benign approach to in- crease molecular complexity. Oxidoreductases selectively introduce oxygen and hydrogen atoms into myr- iad substrates, catalyzing the synthesis of chemical and pharmaceutical building blocks for chemical pro- duction. However, broader application of this class of enzymes is limited by the requirements of expensive cofactors and low operational stability. Herein, we show that morpholine-based buffers, especially 3-(N- morpholino)propanesulfonic acid (MOPS), promote photoinduced flavoenzyme-catalyzed asymmetric re- Creative Commons Attribution 3.0 Unported Licence. Received 14th December 2018, dox transformations by regenerating the flavin cofactor via sacrificial electron donation and by increasing Accepted 8th February 2019 the operational stability of flavin-dependent oxidoreductases. The stabilization of the active forms of flavin by MOPS via formation of the spin correlated ion pair 3ijflavin˙−–MOPS˙+] ensemble reduces the formation DOI: 10.1039/c8cy02524j of hydrogen peroxide, circumventing the oxygen dilemma under aerobic conditions detrimental to fragile rsc.li/catalysis enzymes. Introduction (TEOA), are well-known sacrificial EDs in photoredox catalysis and able to reduce photoexcited flavins.1,4 This article is licensed under a The toolbox of synthetic chemists includes photoredox and The limited operational stability of flavin-dependent oxi- biocatalytic transformations that benefit from the use of light doreductases compromises their broad application, especially energy.1,2 Flavin-dependent oxidoreductases are important under cell-free conditions.5 During the photoredox cycle, biocatalysts for various industrial applications3 but require electron and energy transfer reactions between excited flavin Open Access Article. Published on 11 February 2019. Downloaded 4/12/2019 6:53:31 AM. the reduction of their bound flavin cofactor, typically flavin and molecular oxygen produce deleterious reactive oxygen adenine dinucleotide (FAD) or flavin mononucleotide (FMN), to complete their catalytic cycle. Photoexcitation in the pres- ence of sacrificial electron donors (EDs, Fig. 1) is a convenient method to reduce and thereby regenerate the flavin cofactor. This approach circumvents the need for expensive additional cofactors such as dihydronicotinamide adenine dinucleotide phosphate (NADPH).1 Low cost amines, such as ethyl- enediaminetetraacetic acid (EDTA) and triethanolamine a Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9/163, 1060 Vienna, Austria. E-mail: [email protected] b Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, 03178-200 São Paulo, Brazil c Physical Chemistry Division, Department of Chemistry, Ångström Laboratory, Uppsala University, 75120 Uppsala, Sweden Fig. 1 Flavin-mediated photoredox and enzyme catalysis. Reduction d Department of Chemistry, Qena Faculty of Science, South Valley University, of flavins (Fl) occurs upon light irradiation in the presence of electron * 83523 Qena, Egypt donors (EDs). The excited flavin (Flox) oxidizes an ED resulting in a − e Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, semiquinone radical anion (Fl˙ ), which mediates the transfer of Poland reducing equivalents to the active sites of flavin-dependent enzymes. † Electronic supplementary information (ESI) available: Experimental condi- For simplicity, protonation equilibria are not shown. R = flavin adenine tions, supplementary figures and references. See DOI: 10.1039/c8cy02524j dinucleotide (FAD) or flavin mononucleotide (FMN). This journal is © The Royal Society of Chemistry 2019 Catal. Sci. Technol.,2019,9,1365–1371 | 1365 View Article Online Paper Catalysis Science & Technology – species (ROS) affecting the integrity of the enzyme.6 8 Photo- Control experiments were performed in the dark or in the ab- induced enzyme-catalyzed reactions, or photobiocatalysis, un- sence of the biocatalyst. der anaerobic conditions do not face this so-called ‘oxygen di- lemma’.1,9,10 However, this approach is not applicable to Nanosecond laser flash spectroscopy systems that employ enzymes that require oxygen for cataly- Nanosecond transient absorption kinetics were recorded sis such as flavin-containing monooxygenases (FMOs). At- using a frequency tripled Q-switched Nd:YAG laser coupled tempts to reduce the formation of ROS by replacing flavin with an OPO (Quantel Brilliant B) to obtain the desired wave- with less oxidizable deazaflavins were not successful.11 Stabi- length for the pump light (5 ns pulse at 10 Hz). A detection lization of the triplet state of the excited flavin (Flox) and the − system from Applied Photophysics (LKS80 spectrometer) was reactive semiquinone radical anion (Fl˙ ) is expected to im- used, equipped with a Xe arc lamp (pulsed or continuous prove the efficiency and applicability of light-driven enzy- wave), two monochromators and an R928-type PMT read matic reactions by avoiding the formation of ROS side prod- using a 600 MHz oscilloscope (Agilent Technologies Infinium ucts via energy or electron transfer in the presence of oxygen. − 10 GSa s 1) for kinetics. The kinetics were collected with the However, no additive has been shown to affect the reaction maximum resolution producing 20 000 point traces that were by this mechanism, so far. logarithmically oversampled starting from 50 points from the MOPS (3-(N-morpholino)propanesulfonic acid) and MES excitation about 10 ns after the pulse. The excitation light (3-(N-morpholino)ethanesulfonic acid) are Good's buffers12 wavelength was 445 nm. The excitation light power was con- used in biology because of their chemical inertness.13 The trolled by means of neutral density filters. Experiments were photoreduction of riboflavin and deazaflavin in the presence carried out in a quartz cuvette employing 100 μM FAD in 100 of a morpholine buffer, however, suggests that the buffer mM MOPS at pH 7.5. may act as a sacrificial ED.11,14 Herein, we show that MOPS not only acts as a sacrificial ED to regenerate the flavin cofac- Steady-state absorption measurements Creative Commons Attribution 3.0 Unported Licence. tor in flavin oxidoreductase photobiocatalysis, but also stabi- lizes the triplet state of the excited flavin and the reactive fla- The experiments were performed in a quartz cuvette using vin semiquinone radical anion preventing to some extent the the following conditions unless otherwise stated: 100 μM inactivation of fragile flavin-dependent oxidoreductases by FAD in 100 mM MOPS or Tris-HCl (pH 7.5) in the presence ROS under aerobic conditions, contributing to circumvention or absence of EDTA (25 mM). The cuvette cell was mounted of the oxygen dilemma in aerated photobiocatalysis and in a support with ports for the UV-vis fibre optic probe from expanding its applicability to enzymes that require oxygen for Ocean Optics capable of collecting a full UV-vis spectrum in catalysis. less than 10 s. The experiments consisted of sample illumina- tion for 60 min with a 445 nm CW laser, followed by a 30 This article is licensed under a Experimental min dark period. Standard photoredox enzyme-catalysed reactions Results and discussion Open Access Article. Published on 11 February 2019. Downloaded 4/12/2019 6:53:31 AM. Photoredox biotransformations were performed in a 96-well Proof-of-concept plate sealed with transparent sealing films, unless otherwise stated. Ketoisophorone (1a) reduction (1 mM) was performed In order to develop a simple and versatile light-induced re- in the presence of FMN (100 μM final concentration) generation system for both oxygen dependent and indepen- employing XenB (10 μM) as a reductase. The Baeyer–Villiger dent flavin-containing enzymes, we chose two oxidoreduc- oxidation of cyclohexanone (2a), 2-phenylcyclohexanone (3a) tases as model biocatalysts: first, XenB (an enoate reductase or bicycloij3.2.0]hept-2-en-6-one (4a) (1 mM final concentra- from Pseudomonas sp.)15 for the oxygen-independent enantio- tion) was performed in the presence of FAD (100 μM final selective reduction of ketoisophorone (1a) to levodione (1b), + concentration) and NADP (0.25 mM) employing CHMOAcineto and second, cyclohexanone monooxygenase from μ (10 M) as a biocatalyst. MOPS buffer (100 mM, pH 7.5) was Acinetobacter calcoaceticus NCIMB 9871 (CHMOAcineto), a well- employed as an electron donor and medium, unless other- known Baeyer–Villiger monooxygenase (BVMO) for the oxida- wise stated. Control reactions were performed in 100 mM tion of cyclohexanone (2a)toε-caprolactone (2b) under aer- Tris-HCl (pH 7.5) in the presence or absence of 25 mM EDTA ated conditions.16 Upon daylight irradiation, the effect of and in 100 mM sodium phosphate buffer (pH 7.5). The plate morpholine-based buffers on the photobiocatalytic process was placed over a cooling block in order to keep the reactions involving XenB and CHMOAcineto was evaluated under aerobic at 30 ± 3 °C. The reactants in the plate were irradiated for a conditions. EDTA in Tris-HCl buffer was used as a positive certain time using a 300 W daylight