Tohru Taniguchi

Exciton Chirality Method in Vibrational Circular Dichroism

The interaction of two IR chromophores yields a strong vibrational circular dichroism couplet whose sign reflects the absolute configuration of the molecule. We present a method to determine absolute configuration of a chiral molecule based on this couplet without need of theoretical calculation. Not only can this method analyze various molecules whose absolute configuration is difficult to determine by other spectroscopic methods, but also it can significantly enhance VCD signals.

Genome Mining for Sesterterpenes Using Bifunctional Terpene Synthases Reveals a Unified Intermediate of Di/Sesterterpenes.

Genome mining is a promising method to discover novel secondary metabolites in the postgenomic era. We applied the Aspergillus oryzae heterologous expression system to functionally characterize cryptic bifunctional terpene synthase genes found in fungal genomes and identified the sesterfisherol synthase gene (NfSS) from Neosartorya fischeri. Sesterfisherol contains a characteristic 5-6-8-5 tetracyclic ring system and is modified by cytochrome P450 monooxygenase (NfP450) to sesterfisheric acid. The cyclization mechanism was proposed on the basis of the analysis of in vivo and in vitro enzymatic reactions with isotopically labeled precursors. The mechanism involves C1 cation-olefin IV-olefin V cyclization followed by five hydride shifts, allowing us to propose a unified biogenesis for sesterterpenes branching from bicyclic (5-15), tricyclic (5-12-5), and tetracyclic (5-6-8-5) cation intermediates. Furthermore, the mechanism is distinct from that of a separate class of di/sesterterpenes including fusicoccins and ophiobolins. The difference between mechanisms is consistent with phylogenetic analysis of bifunctional terpene synthases, suggesting that the amino acid sequence reflects the initial cyclization mode, which is most likely related to the initial conformation of a linear prenyl diphosphate.

Structures of Spiroindicumides A and B, Unprecedented Carbon Skeletal Spirolactones, and Determination of the Absolute Configuration by Vibrational Circular Dichroism Exciton Approach

Spiroindicumides A (1) and B (2), novel spirolactone polyketides, were isolated from a filamentous fungus, Chaetomium indicum, cultivated in the presence of a histone deacetylase inhibitor. Their structures including relative configurations were determined by spectroscopic analyses. Their absolute configurations were unambiguously assigned by the vibrational circular dichroism (VCD) exciton chirality method using only ca. 0.3 mg of each sample. This study presents the first application of the VCD exciton approach to novel natural products. A possible biosynthetic pathway of the new compounds was also proposed.

Preparation of cruciferous phytoalexin related metabolites, (−)-dioxibrassinin and (−)-3-cyanomethyl-3-hydroxyoxindole, and determination of their absolute configurations by vibrational circular dichroism (VCD)

Abstract Cruciferous phytoalexin related metabolites, (−)-dioxibrassinin ( 1 ) and (−)-3-cyanomethyl-3-hydroxyoxindole ( 2 ) were prepared from isatin as racemates and were resolved by chiral HPLC. Their absolute configurations were determined by the new chiroptical technique, vibrational circular dichroism (VCD), as well as by the conventional electronic circular dichroism (ECD). It is concluded that the absolute configurations of the naturally occurring (−)- 1 and (−)- 2 are both S.

Short Synthesis of Berkeleyamide D and Determination of the Absolute Configuration by the Vibrational Circular Dichroism Exciton Chirality Method

The first synthesis of (±)-berkeleyamide D has been accomplished. The key features of this synthesis include the formation of an α,β-epoxy-γ-lactam via a Darzens reaction and the construction of a spirocyclic ring system by a C-acylation reaction followed by an intramolecular spirocyclization via an epoxide-opening reaction. Following optical resolution by chiral HPLC, the absolute configurations of both enantiomers of berkeleyamide D were determined by the vibrational circular dichroism exciton chirality method.

Structural Diversity of New C13-Polyketides Produced by Chaetomium mollipilium Cultivated in the Presence of a NAD+-Dependent Histone Deacetylase Inhibitor

Cultivation of Chaetomium mollipilium with nicotinamide, a NAD(+)-dependent HDAC inhibitor, stimulated its secondary metabolism, leading to the isolation of structurally diverse new C(13)-polyketides, mollipilin A-E (1-5) as well as two known compounds (6 and 7). Spectroscopic methods, X-ray single crystal diffraction analysis, and VCD elucidated the absolute configurations of structures 1-6, and plausible biosynthetic pathways for 1-7 were proposed based on structural relationships. Mollipilins A (1) and B (2) exhibited moderate growth inhibitory effects on HCT-116 cells.

Structurally Diverse Chaetophenol Productions Induced by Chemically Mediated Epigenetic Manipulation of Fungal Gene Expression

Epigenetic manipulation of gene expression in Chaetomium indicum using a HDAC inhibitor led to the isolation of structurally diverse chaetophenols, and 3, 4 and 5 bear unprecedented polycyclic skeletons. The expression of two silent genes (pksCH-1 and pksCH-2) for nonreducing PKSs involved in chaetophenol biosynthesis was associated with an increase of histone acetylation level. The heterologous gene expression study in Aspergillus oryzae revealed pksCH-2 to be the NR-PKS gene for 8.

Studying a cell division amidase using defined peptidoglycan substrates.

Three periplasmic N-acetylmuramoyl-l-alanine amidases are critical for hydrolysis of septal peptidoglycan, which enables cell separation. The amidases cleave the amide bond between the lactyl group of muramic acid and the amino group of l-alanine to release a peptide moiety. Cell division amidases remain largely uncharacterized because substrates suitable for studying them have not been available. Here we have used synthetic peptidoglycan fragments of defined composition to characterize the catalytic activity and substrate specificity of the important Escherichia coli cell division amidase AmiA. We show that AmiA is a zinc metalloprotease that requires at least a tetrasaccharide glycopeptide substrate for cleavage. The approach outlined here can be applied to many other cell wall hydrolases and should enable more detailed studies of accessory proteins proposed to regulate amidase activity in cells.

Stereochemical Analysis of Glycerophospholipids by Vibrational Circular Dichroism

The stereochemistry of glycerophospholipids (GPLs) has been of interest for its roles in the evolution of life and in their biological activity. However, because of their structural complexity, no convenient method to determine their configuration has been reported. In this work, through the first systematic application of vibrational circular dichroism (VCD) spectroscopy to various diacylated GPLs, we have revealed that their chirality can be assigned by the sign of a VCD exciton couplet generated by the interaction of two carbonyl groups. This paper also presents spectroscopic evidence for the stereochemistry of GPLs isolated from bacteria, eukaryotes, and mitochondria.

Chiral sulfinates studied by optical rotation, ECD and VCD: the absolute configuration of a cruciferous phytoalexin brassicanal C

The optical rotation, electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) of chiral sulfinates have been studied experimentally, and analysed by density functional theory calculation, aiming at establishing a reliable and convenient methodology to determine their absolute configuration. Through the study on a model chiral sulfinate with known absolute configuration, (R)-(+)-methyl p-toluenesulfinate ((R)-(+)-1), each technique was found to be reliable in assigning chirality of sulfinates. We then applied these methods to a synthetically prepared cruciferous phytoalexin, brassicanal C ((-)-2), and unambiguously determined its absolute configuration as S. The advantages and disadvantages of each spectroscopy on sulfinates are also discussed.