pH Titration Monitored by Quantum Cascade Laser-Based Vibrational Circular Dichroism

Vibrational circular dichroism (VCD) spectra of aqueous solutions of proline were recorded in the course of titrations from basic to acidic pH using a spectrometer equipped with a quantum cascade laser (QCL) as an infrared light source in the spectral range from 1320 to 1220 cm^–1. The pH-dependent spectra were analyzed by singular value decomposition and global fitting of a two-p<i>K</i> Henderson–Hasselbalch model. The analysis delivered relative fractions of the three different protonation species. Their agreement with the relative fractions obtained from performing the same analysis on pH-dependent Fourier transform infrared (FT-IR) and QCL-IR spectra validates the quantitative results from QCL-VCD. Global fitting of the pH-dependent VCD spectra of l-proline allowed for extraction of pure spectra corresponding to anionic, zwitterionic, and cationic l-proline. From a static experiment, only pure spectra of the zwitterion would be accessible in a straightforward way. A comparison to VCD spectra calculated for all three species led to assignment of vibrational modes that are characteristic for the respective protonation states. The study demonstrates the applicability of QCL-VCD both for quantitative evaluation and for qualitative interpretation of dynamic processes in aqueous solutions

Biomimetic Asymmetric Synthesis of (<i>R</i>)-GTRI-02 and (3<i>S</i>,4<i>R</i>)-3,4-Dihydroxy-3,4-dihydronaphthalen-1(2<i>H</i>)-ones

The NADPH-dependent tetrahydroxynaphthalene reductase (T₄HNR) from <i>Magnaporthe grisea</i> was used for the biomimetic synthesis of (<i>R</i>)-GTRI-02 by stereoselective reduction of 1-(3,6,8-trihydroxy-1-methylnaphthalen-2-yl)ethanone. This also led to the isolation of a (3<i>S</i>,4<i>R</i>)-<i>cis</i>-ketodiol formed by T₄HNR-catalyzed reduction of the corresponding hydroxynaphthoquinone. Flaviolin and lawsone also reduced to corresponding <i>cis</i>-ketodiols in good yields

Biomimetic Asymmetric Synthesis of (<i>R</i>)-GTRI-02 and (3<i>S</i>,4<i>R</i>)-3,4-Dihydroxy-3,4-dihydronaphthalen-1(2<i>H</i>)-ones

The NADPH-dependent tetrahydroxynaphthalene reductase (T₄HNR) from <i>Magnaporthe grisea</i> was used for the biomimetic synthesis of (<i>R</i>)-GTRI-02 by stereoselective reduction of 1-(3,6,8-trihydroxy-1-methylnaphthalen-2-yl)ethanone. This also led to the isolation of a (3<i>S</i>,4<i>R</i>)-<i>cis</i>-ketodiol formed by T₄HNR-catalyzed reduction of the corresponding hydroxynaphthoquinone. Flaviolin and lawsone also reduced to corresponding <i>cis</i>-ketodiols in good yields

Regio- and Stereoselective Intermolecular Oxidative Phenol Coupling in <i>Streptomyces</i>

Intermolecular oxidative phenol coupling is the main process in nature for the formation of atropo­selective biaryl compounds. Although well defined in plants and fungi, this type of dimerization reaction in bacteria is poorly understood. Therefore, the biosynthesis of juli­chromes, specto­mycins, and setomi­mycin was investigated. The monomeric subunits of these biarylic pre-anthra­quinones are derived from a common polyketidic precursor, yet the coupling reaction proceeds in a regio­selective manner, with the position of attachment of the two subunits depending on the specific streptomycete strain. By using genome analysis and deletion experiments, the biosynthetic gene clusters were identified. Furthermore, it was established that cytochrome P450 enzymes are fundamentally involved during dimerization of the polyketide monomers