Systematic biotechnological production of isoprenoid analogs with bespoke carbon skeletons

Natural products are widely used as pharmaceuticals, flavors, fragrances, and cosmetic ingredients. Synthesizing and evaluating analogs of natural products can considerably expand their applications. However, the chemical synthesis of analogs of natural products is severely hampered by their highly complex structures. This is particularly evident in isoprenoids, the largest class of natural products. Here, we develop a yeast cell-based biocatalytic method that enables the systematic biotechnological production of analogs of different classes of isoprenoids (including monoterpenoids, sesquiterpenoids, triterpenoids, and cannabinoids) with additional carbons in their skeletons. We demonstrate the applicability of this approach through two proof-of-concept studies: the biosynthesis of the highly valued aroma ingredient ethyllinalool, and the production of cannabinoid analogs with improved cannabinoid receptor agonism. This method is simple, readily adaptable to any cell factory, and enables the tailored expansion of the isoprenoid chemical space to identify molecules with improved properties and the biotechnological production of valuable compounds.

Microbe Engineering to Provide Drimane-Type Building Blocks for Chiral Pool Synthesis of Meroterpenoids

Drimane-type merosesquiterpenoids (DMT) are a class of natural products with diverse structures and broad biological activity. Classical DMT synthesis relies on atom-inefficient plant-derived chiral pool building blocks, while alternative drimane-type building blocks such as drimenol and albicanol offer more direct routes but face production challenges. In this study, we engineered a microbial platform for efficient production of these building blocks. By optimizing the PhoN-IPK system through rational engineering and incorporating a Nudix hydrolase, we achieved a drimenol production of 398 mg/L and high albicanol titers of 1805 mg/L in shake flasks and 3.5 g/L in a bioreactor. Structural analysis and molecular dynamics simulations of the engineered PhoN provided insights into its improved catalytic efficiency. We demonstrated the utility of this platform by synthesizing several DMT using albicanol as the starting material, reducing the number of synthetic steps and improving overall efficiency as compared to classical approaches.

Engineering terpene synthases and their substrates for the biocatalytic production of terpene natural products and analogues

Terpene synthases produce a wide number of hydrocarbon skeletons by controlling intramolecular rearrangements of allylic pyrophosphate subtrates via reactive carbocation intermediates. Here we review recent research focused on engineering terpene synthases and modifying their substrates to rationally manipulate terpene catalyisis. Molecular dynamic simulations and solid state X-ray crystallography are powerful techniques to identify substrate binding modes, key active site residues for substrate folding, and the location of active site water. Variants in specific 'hotspots' of terpene synthases including the G1/2, K/H and Hα-1 helices have been targeted to modify active site water management and yield new products. We discuss the potential of exploiting substrate analogues to synthesise novel compounds and briefly outline biphasic flow systems for biocatalysis of terpenes. We forsee greater applications for terpenes as the field converges on effective methods for engineering of terpene synthases by new computational and high throughput experimental methods and for high-yield production. It is crucial when engineering terpene synthases that both product distribution and enzyme activity are simultaneously optimised.

The utility of Streptococcus mutans undecaprenol kinase for the chemoenzymatic synthesis of diverse non-natural isoprenoids

Isoprene chemoenzymatic cascades (ICCs) overcome the complexity of natural pathways by leveraging a streamlined two-enzyme cascade, facilitating efficient synthesis of C5-isoprene diphosphate precursors from readily available alcohol derivatives. Despite the documented promiscuity of enzymes in ICCs, exploration of their potential for accessing novel compounds remains limited, and existing methods require additional enzymes for generating longer-chain diphosphates. In this study, we present the utility of Streptococcus mutans undecaprenol kinase (SmUdpK) for the chemoenzymatic synthesis of diverse non-natural isoprenoids. Using a library of 50 synthetic alcohols, we demonstrate that SmUdpK's promiscuity extends to allylic chains as small as four carbons and benzylic alcohols with various substituents. Subsequently, SmUdpK is utilized in an ICC with isopentenyl phosphate kinase and aromatic prenyltransferase to generate multiple non-natural isoprenoids. This work provides evidence that, with proper optimization, SmUdpK can act as the first enzyme in these ICCs, enhancing access to both valuable and novel compounds.

Enhancing structural diversity of terpenoids by multisubstrate terpene synthases

Terpenoids are one of the largest class of natural products with diverse structures and activities. This enormous diversity is embedded in enzymes called terpene synthases (TSs), which generate diverse terpene skeletons via sophisticated cyclization cascades. In addition to the many highly selective TSs, there are many promiscuous TSs that accept multiple prenyl substrates, or even noncanonical ones, with 6, 7, 8, 11, and 16 carbon atoms, synthesized via chemical approaches, C-methyltransferases, or engineered lepidopteran mevalonate pathways. The substrate promiscuity of TSs not only expands the structural diversity of terpenes but also highlights their potential for the discovery of novel terpenoids via combinatorial biosynthesis. In this review, we focus on the current knowledge on multisubstrate terpene synthases (MSTSs) and highlight their potential applications.

Divergent Biosynthesis of Bridged Polycyclic Sesquiterpenoids by a Minimal Fungal Biosynthetic Gene Cluster

The bridged polycyclic sesquiterpenoids derived from sativene, isosativene, and longifolene have unique structures, and many chemical synthesis approaches with at least 10 steps have been reported. However, their biosynthetic pathway remains undescribed. A minimal biosynthetic gene cluster (BGC), named bip, encoding a sesquiterpene cyclase (BipA) and a cytochrome P450 (BipB) is characterized to produce such complex sesquiterpenoids with multiple carbon skeletons based on enzymatic assays, heterologous expression, and precursor experiments. BipA is demonstrated as a versatile cyclase with (-)-sativene as the dominant product and (-)-isosativene and (-)-longifolene as minor ones. BipB is capable of hydroxylating different enantiomeric sesquiterpenes, such as (-)-longifolene and (+)-longifolene, at C-15 and C-14 in turn. The C-15- or both C-15- and C-14-hydroxylated products are then further oxidized by unclustered oxidases, resulting in a structurally diverse array of sesquiterpenoids. Bioinformatic analysis reveals the BipB homologues as a discrete clade of fungal sesquiterpene P450s. These findings elucidate the concise and divergent biosynthesis of such intricate bridged polycyclic sesquiterpenoids, offer valuable biocatalysts for biotransformation, and highlight the distinct biosynthetic strategy employed by nature compared to chemical synthesis.

Confirmation of the stereochemistry of spiroviolene

We confirm the previously revised stereochemistry of spiroviolene by X-ray crystallographically characterizing a hydrazone derivative of 9-oxospiroviolane, which is synthesized by hydroboration/oxidation of spiroviolene followed by oxidation of the resultant hydroxy group. An unexpected thermal boron migration occurred during the hydroboration process of spiroviolene that resulted in the production of a mixture of 1α-hydroxyspiroviolane, 9α- and 9β-hydroxyspiroviolane after oxidation. The assertion of the cis-orientation of the 19- and 20-methyl groups provided further support for the revised cyclization mechanism of spiroviolene.

Production of non-natural terpenoids through chemoenzymatic synthesis using substrate analogs

Chemoenzymatic synthesis of non-natural terpenes using the promiscuous activity of terpene synthases allows for the expansion of the chemical space of terpenoids with potentially new bioactivities. In this report, we describe protocols for the preparation of a novel aphid attractant, (S)-14,15-dimethylgermacrene D, by exploiting the promiscuity of (S)-germacrene D synthase from Solidago canadensis and using an engineered biocatalytic route to convert prenols to terpenoids. The method uses a combination of five enzymes to carry out the preparation of terpenoid semiochemicals in two steps: (1) diphosphorylation of five or six carbon precursors (prenol, isoprenol and methyl-isoprenol) catalyzed by Plasmodium falciparum choline kinase and Methanocaldococcus jannaschii isopentenyl phosphate kinase to form DMADP, IDP and methyl-IDP, and (2) chain elongation and cyclization catalyzed by Geobacillus stearothermophilus (2E,6E)-farnesyl diphosphate synthase and S. canadensis (S)-germacrene D synthase to produce (S)-germacrene D and (S)-14,15-dimethylgermacrene D. Using this method, new non-natural terpenoids are readily accessible and the approach can be adopted to produce different terpene analogs and terpenoid derivatives with potential novel applications.

Supramolecular Capsule Catalysis Enables the Exploration of Terpenoid Chemical Space Untapped by Nature

Terpenes represent the largest and the most diverse class of natural compounds. This is remarkable as the whole variety is accessed from just a handful of highly conserved linear precursors. Modification of the cyclization precursors would enable a dramatic expansion of the accessible chemical space. However, natural enzymes do not enable us to tap into this potential, as they do not tolerate larger deviations from the prototypical substrate structure. Herein we report that supramolecular capsule catalysis enables facile access to diverse and novel terpenoid skeletons that formally can be traced back to C3-phenyl, benzyl, and homoprenyl derivatives of farnesol. Novel skeletons related to the presilphiperfolane core structure, as well as novel neoclovene derivatives were accessed efficiently in only four synthetic steps. Importantly, the products obtained carry functional groups that may be readily derivatized further.

Reactivity and mechanism in chemical and synthetic biology

Physical organic chemistry and mechanistic thinking provide a strong intellectual framework for understanding the chemical logic of evolvable informational macromolecules and metabolic transformations in living organisms. These concepts have also led to numerous successes in designing and applying tools to delineate biological function in health and disease, chemical ecology and possible alternative chemistries employed by extraterrestrial life. A symposium at the 2020 Pacifichem meeting was scheduled in December 2020 to discuss designing and exploiting expanded genetic alphabets, methods to understand the biosynthesis of natural products and re-engineering primary metabolism in bacteria. The COVID-19 pandemic led to postponement of in-person discussions, with the symposium eventually being held on 20-21 December 2021 as an online event. This issue is a written record of work presented on biosynthetic pathways and enzyme catalysis, engineering microorganisms with new metabolic capabilities, and the synthesis of non-canonical, nucleobases for medical applications and for studies of alternate chemistries for living organisms. The variety of opinion pieces, reviews and original research articles provide a starting point for innovations that clarify how complex biological systems emerge from the rules of chemical reactivity and mechanism. This article is part of the themed issue 'Reactivity and mechanism in chemical and synthetic biology'.

In vitro Applications of the Terpene Mini-Path 2.0

In 2019 four groups reported independently the development of a simplified enzymatic access to the diphosphates (IPP and DMAPP) of isopentenol and dimethylallyl alcohol (IOH and DMAOH). The former are the two universal precursors of all terpenes. We report here on an improved version of what we call the terpene mini-path as well as its use in enzymatic cascades in combination with various transferases. The goal of this study is to demonstrate the in vitro utility of the TMP in, i) synthesizing various natural terpenes, ii) revealing the product selectivity of an unknown terpene synthase, or iii) generating unnatural cyclobutylated terpenes.

In situ synthesis of artificial lipids

Lipids constitute one of the most enigmatic family of biological molecules. Although the importance of lipids as basic units of compartmental structure and energy storage is well-acknowledged, deciphering the biosynthesis and precise roles of specific lipid species has been challenging. To better understand the structure and function of these biomolecules, there is a burgeoning interest in developing strategies to produce noncanonical lipids in a controlled manner. This review covers recent advances in the area of in situ generation of synthetic lipids. Specifically, we report several approaches that constitute a powerful toolbox for achieving noncanonical lipid synthesis. We describe how these methodologies enable the direct construction of synthetic lipids, helping to address fundamental questions related to the cell biology of lipid biosynthesis, trafficking, and signaling. We envision that highlighting the current advances in artificial lipid synthesis will pave the way for broader interest into this emerging class of biomimetic molecules.

Heterocyclic and Alkyne Terpenoids by Terpene Synthase-Mediated Biotransformation of Non-Natural Prenyl Diphosphates: Access to New Fragrances and Probes

Two terpene cyclases were used as biocatalytic tool, namely, limonene synthase from Cannabis sativa (CLS) and 5-epi-aristolochene synthase (TEAS) from Nicotiana tabacum. They showed significant substrate flexibility towards non-natural prenyl diphosphates to form novel terpenoids, including core oxa- and thia-heterocycles and alkyne-modified terpenoids. We elucidated the structures of five novel monoterpene-analogues and a known sesquiterpene-analogue. These results reflected the terpene synthases' ability and promiscuity to broaden the pool of terpenoids with structurally complex analogues. Docking studies highlight an on-off conversion of the unnatural substrates.

Current Progress in the Chemoenzymatic Synthesis of Natural Products

Natural products, with their array of structural complexity, diversity, and biological activity, have inspired generations of chemists and driven the advancement of techniques in their total syntheses. The field of natural product synthesis continuously evolves through the development of methodologies to improve stereoselectivity, yield, scalability, substrate scope, late-stage functionalization, and/or enable novel reactions. One of the more interesting and unique techniques to emerge in the last thirty years is the use of chemoenzymatic reactions in the synthesis of natural products. This review highlights some of the recent examples and progress in the chemoenzymatic synthesis of natural products from 2019–2022.

Metabolic Engineering of the Isopentenol Utilization Pathway Enhanced the Production of Terpenoids in Chlamydomonas reinhardtii

Eukaryotic green microalgae show considerable promise for the sustainable light-driven biosynthesis of high-value fine chemicals, especially terpenoids because of their fast and inexpensive phototrophic growth. Here, the novel isopentenol utilization pathway (IUP) was introduced into Chlamydomonas reinhardtii to enhance the hemiterpene (isopentenyl pyrophosphate, IPP) titers. Then, diphosphate isomerase (IDI) and limonene synthase (MsLS) were further inserted for limonene production. Transgenic algae showed 8.6-fold increase in IPP compared with the wild type, and 23-fold increase in limonene production compared with a single MsLS expressing strain. Following the culture optimization, the highest limonene production reached 117 µg/L, when the strain was cultured in a opt2 medium supplemented with 10 mM isoprenol under a light: dark regimen. This demonstrates that transgenic algae expressing the IUP represent an ideal chassis for the high-value terpenoid production. The IUP will facilitate further the metabolic and enzyme engineering to enhance the terpenoid titers by significantly reducing the number of enzyme steps required for an optimal biosynthesis.

Chemo-enzymatic synthesis of natural products and their analogs

Enzymes continue to gain recognition as valuable tools in synthetic chemistry as they enable transformations, which elude conventional organochemical approaches. As such, the progressing expansion of the biocatalytic arsenal has introduced unprecedented opportunities for new synthetic strategies and retrosynthetic disconnections. As a result, enzymes have found a solid foothold in modern natural product synthesis for applications ranging from the generation of early chiral synthons to endgame transformations, convergent synthesis, and cascade reactions for the rapid construction of molecular complexity. As a primer to the state-of-the-art concerning strategic uses of enzymes in natural product synthesis and the underlying concepts, this review highlights selected recent literature examples, which make a strong case for the admission of enzymatic methodologies into the standard repertoire for complex small-molecule synthesis.

Thermoadaptation in an Ancestral Diterpene Cyclase by Altered Loop Stability

Thermostability is the key to maintain the structural integrity and catalytic activity of enzymes in industrial biotechnological processes, such as terpene cyclase-mediated generation of medicines, chiral synthons, and fine chemicals. However, affording a large increase in the thermostability of enzymes through site-directed protein engineering techniques can constitute a challenge. In this paper, we used ancestral sequence reconstruction to create a hyperstable variant of the ent-copalyl diphosphate synthase PtmT2, a terpene cyclase involved in the assembly of antibiotics. Molecular dynamics simulations on the μs timescale were performed to shed light on possible molecular mechanisms contributing to activity at an elevated temperature and the large 40 °C increase in melting temperature observed for an ancestral variant of PtmT2. In silico analysis revealed key differences in the flexibility of a loop capping the active site, between extant and ancestral proteins. For the modern enzyme, the loop collapses into the active site at elevated temperatures, thus preventing biocatalysis, whereas the loop remains in a productive conformation both at ambient and high temperatures in the ancestral variant. Restoring a Pro loop residue introduced in the ancestral variant to the corresponding Gly observed in the extant protein led to reduced catalytic activity at high temperatures, with only moderate effects on the melting temperature, supporting the importance of the flexibility of the capping loop in thermoadaptation. Conversely, the inverse Gly to Pro loop mutation in the modern enzyme resulted in a 3-fold increase in the catalytic rate. Despite an overall decrease in maximal activity of ancestor compared to wild type, its increased thermostability provides a robust backbone amenable for further enzyme engineering. Our work cements the importance of loops in enzyme catalysis and provides a molecular mechanism contributing to thermoadaptation in an ancestral enzyme.

Set of Cytochrome P450s Cooperatively Catalyzes the Synthesis of a Highly Oxidized and Rearranged Diterpene-Class Sordarinane Architecture

Cytochrome P450s are one of the most versatile oxidases that catalyze significant and unique chemical transformations for the construction of complex structural frameworks during natural product biosynthesis. Here, we discovered a set of P450s, including SdnB, SdnH, SdnF, and SdnE, that cooperatively catalyzes the reshaping of the inert cycloaraneosene framework to form a highly oxidized and rearranged sordarinane architecture. Among them, SdnB is confirmed to be the first P450 (or oxidase) that cleaves the C-C bond of the epoxy residue to yield formyl groups in pairs. SdnF selectively oxidizes one generated formyl group to a carboxyl group and accelerates the final Diels-Alder cyclization to furnish the sordarinane architecture. Our work greatly enriches the enzyme functions of the P450 superfamily, supplies the missing skills of the P450 synthetic toolbox, and supports them as biocatalysts in further applications toward the synthesis of new chemical entities.

Alternative metabolic pathways and strategies to high-titre terpenoid production in Escherichia coli

Covering: up to 2021Terpenoids are a diverse group of chemicals used in a wide range of industries. Microbial terpenoid production has the potential to displace traditional manufacturing of these compounds with renewable processes, but further titre improvements are needed to reach cost competitiveness. This review discusses strategies to increase terpenoid titres in Escherichia coli with a focus on alternative metabolic pathways. Alternative pathways can lead to improved titres by providing higher orthogonality to native metabolism that redirects carbon flux, by avoiding toxic intermediates, by bypassing highly-regulated or bottleneck steps, or by being shorter and thus more efficient and easier to manipulate. The canonical 2-C-methyl-D-erythritol 4-phosphate (MEP) and mevalonate (MVA) pathways are engineered to increase titres, sometimes using homologs from different species to address bottlenecks. Further, alternative terpenoid pathways, including additional entry points into the MEP and MVA pathways, archaeal MVA pathways, and new artificial pathways provide new tools to increase titres. Prenyl diphosphate synthases elongate terpenoid chains, and alternative homologs create orthogonal pathways and increase product diversity. Alternative sources of terpenoid synthases and modifying enzymes can also be better suited for E. coli expression. Mining the growing number of bacterial genomes for new bacterial terpenoid synthases and modifying enzymes identifies enzymes that outperform eukaryotic ones and expand microbial terpenoid production diversity. Terpenoid removal from cells is also crucial in production, and so terpenoid recovery and approaches to handle end-product toxicity increase titres. Combined, these strategies are contributing to current efforts to increase microbial terpenoid production towards commercial feasibility.

Extension of the Terpene Chemical Space: the Very First Biosynthetic Steps

The structural diversity of terpenes is particularly notable and many studies are carried out to increase it further. In the terpene biosynthetic pathway this diversity is accessible from only two common precursors, i. e. isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Methods recently developed (e. g. the Terpene Mini Path) have allowed DMAPP and IPP to be obtained from a two-step enzymatic conversion of industrially available isopentenol (IOH) and dimethylallyl alcohol (DMAOH) into their corresponding diphosphates. Easily available IOH and DMAOH analogues then offer quick access to modified terpenoids thus avoiding the tedious chemical synthesis of unnatural diphosphates. The aim of this minireview is to cover the literature devoted to the use of these analogues for widening the accessible terpene chemical space.

The Terpene Mini-Path, a New Promising Alternative for Terpenoids Bio-Production

Terpenoids constitute the largest class of natural compounds and are extremely valuable from an economic point of view due to their extended physicochemical properties and biological activities. Due to recent environmental concerns, terpene extraction from natural sources is no longer considered as a viable option, and neither is the chemical synthesis to access such chemicals due to their sophisticated structural characteristics. An alternative to produce terpenoids is the use of biotechnological tools involving, for example, the construction of enzymatic cascades (cell-free synthesis) or a microbial bio-production thanks to metabolic engineering techniques. Despite outstanding successes, these approaches have been hampered by the length of the two natural biosynthetic routes (the mevalonate and the methyl erythritol phosphate pathways), leading to dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP), the two common universal precursors of all terpenoids. Recently, we, and others, developed what we called the terpene mini-path, a robust two enzyme access to DMAPP and IPP starting from their corresponding two alcohols, dimethylallyl alcohol and isopentenol. The aim here is to present the potential of this artificial bio-access to terpenoids, either in vitro or in vivo, through a review of the publications appearing since 2016 on this very new and fascinating field of investigation.

Nature Chose Phosphates and Chemists Should Too: How Emerging P(V) Methods Can Augment Existing Strategies

Phosphate linkages govern life as we know it. Their unique properties provide the foundation for many natural systems from cell biology and biosynthesis to the backbone of nucleic acids. Phosphates are ideal natural moieties; existing as ionized species in a stable P(V)-oxidation state, they are endowed with high stability but exhibit enzymatically unlockable potential. Despite intense interest in phosphorus catalysis and condensation chemistry, organic chemistry has not fully embraced the potential of P(V) reagents. To be sure, within the world of chemical oligonucleotide synthesis, modern approaches utilize P(III) reagent systems to create phosphate linkages and their analogs. In this Outlook, we present recent studies from our laboratories suggesting that numerous exciting opportunities for P(V) chemistry exist at the nexus of organic synthesis and biochemistry. Applications to the synthesis of stereopure antisense oligonucleotides, cyclic dinucleotides, methylphosphonates, and phosphines are reviewed as well as chemoselective modification to peptides, proteins, and nucleic acids. Finally, an outlook into what may be possible in the future with P(V) chemistry is previewed, suggesting these examples represent just the tip of the iceberg.

Amorpha-4,11-diene synthase: a key enzyme in artemisinin biosynthesis and engineering

Amorpha-4,11-diene synthase (ADS) catalyzes the first committed step in the artemisinin biosynthetic pathway, which is the first catalytic reaction enzymatically and genetically characterized in artemisinin biosynthesis. The advent of ADS in Artemisia annua is considered crucial for the emergence of the specialized artemisinin biosynthetic pathway in the species. Microbial production of amorpha-4,11-diene is a breakthrough in metabolic engineering and synthetic biology. Recently, numerous new techniques have been used in ADS engineering; for example, assessing the substrate promiscuity of ADS to chemoenzymatically produce artemisinin. In this review, we discuss the discovery and catalytic mechanism of ADS, its application in metabolic engineering and synthetic biology, as well as the role of sesquiterpene synthases in the evolutionary origin of artemisinin.

Synthetic biology, combinatorial biosynthesis, and chemo‑enzymatic synthesis of isoprenoids

Isoprenoids are a large class of natural products with myriad applications as bioactive and commercial compounds. Their diverse structures are derived from the biosynthetic assembly and tailoring of their scaffolds, ultimately constructed from two C5 hemiterpene building blocks. The modular logic of these platforms can be harnessed to improve titers of valuable isoprenoids in diverse hosts and to produce new-to-nature compounds. Often, this process is facilitated by the substrate or product promiscuity of the component enzymes, which can be leveraged to produce novel isoprenoids. To complement rational enhancements and even re-programming of isoprenoid biosynthesis, high-throughput approaches that rely on searching through large enzymatic libraries are being developed. This review summarizes recent advances and strategies related to isoprenoid synthetic biology, combinatorial biosynthesis, and chemo-enzymatic synthesis, focusing on the past 5 years. Emerging applications of cell-free biosynthesis and high-throughput tools are included that culminate in a discussion of the future outlook and perspective of isoprenoid biosynthetic engineering.

Enzymatic control of product distribution in terpene synthases: insights from multiscale simulations

In this opinion, we review some recent work on terpene biosynthesis using multiscale simulation approaches, with special focus on contributions from our group. Terpene synthases generate terpenes employing rich carbocation chemistry, including highly specific ring formations, proton, hydride, methyl, and methylene migrations, followed by reaction quenching. In these enzymes, the main catalytic challenge is not rate enhancement, but rather control of intrinsically reactive carbocations and the resulting product distribution. Herein, we review multiscale simulations of selected mono-, sesqui-, and diterpene synthases. We point to the many tools adopted by terpene synthases to achieve correct substrate fold, carbocation formation, carbocation reaction environment, and reaction quenching. A better understanding of the toolbox employed by terpene synthases is expected to aid in the search for new enzymatic and biomimetic synthetic routes to natural and unnatural terpenes.