Tolyporphins-Exotic Tetrapyrrole Pigments in a Cyanobacterium-A Review

Tolyporphins were discovered some 30 years ago as part of a global search for antineoplastic compounds from cyanobacteria. To date, the culture HT-58-2, comprised of a cyanobacterium-microbial consortium, is the sole known producer of tolyporphins. Eighteen tolyporphins are now known-each is a free base tetrapyrrole macrocycle with a dioxobacteriochlorin (14), oxochlorin (3), or porphyrin (1) chromophore. Each compound displays two, three, or four open β-pyrrole positions and two, one, or zero appended C-glycoside (or -OH or -OAc) groups, respectively; the appended groups form part of a geminal disubstitution motif flanking the oxo moiety in the pyrroline ring. The distinct structures and repertoire of tolyporphins stand alone in the large pigments-of-life family. Efforts to understand the cyanobacterial origin, biosynthetic pathways, structural diversity, physiological roles, and potential pharmacological properties of tolyporphins have attracted a broad spectrum of researchers from diverse scientific areas. The identification of putative biosynthetic gene clusters in the HT-58-2 cyanobacterial genome and accompanying studies suggest a new biosynthetic paradigm in the tetrapyrrole arena. The present review provides a comprehensive treatment of the rich science concerning tolyporphins.

Development of Biotechnological Photosensitizers for Photodynamic Therapy: Cancer Research and Treatment-From Benchtop to Clinical Practice

Photodynamic therapy (PDT) is a noninvasive therapeutic approach that has been applied in studies for the treatment of various diseases. In this context, PDT has been suggested as a new therapy or adjuvant therapy to traditional cancer therapy. The mode of action of PDT consists of the generation of singlet oxygen (¹O₂) and reactive oxygen species (ROS) through the administration of a compound called photosensitizer (PS), a light source, and molecular oxygen (³O₂). This combination generates controlled photochemical reactions (photodynamic mechanisms) that produce ROS, such as singlet oxygen (¹O₂), which can induce apoptosis and/or cell death induced by necrosis, degeneration of the tumor vasculature, stimulation of the antitumor immune response, and induction of inflammatory reactions in the illuminated region. However, the traditional compounds used in PDT limit its application. In this context, compounds of biotechnological origin with photosensitizing activity in association with nanotechnology are being used in PDT, aiming at its application in several types of cancer but with less toxicity toward neighboring tissues and better absorption of light for more aggressive types of cancer. In this review, we present studies involving innovatively developed PS that aimed to improve the efficiency of PDT in cancer treatment. Specifically, we focused on the clinical translation and application of PS of natural origin on cancer.

Fluorescence Assay for Tolyporphins Amidst Abundant Chlorophyll in Crude Cyanobacterial Extracts

Tolyporphins are distinctive tetrapyrrole natural products found singularly in a filamentous cyanobacterial-microbial holobiont (termed HT-58-2) from Micronesia. The absorption and fluorescence features of tolyporphins resemble those of chlorophyll a, complicating direct analysis of culture samples. Treatment of the crude (unfractionated) organic extract (CH₂ Cl₂ /2-propanol, 1:1) of HT-58-2 cultures with NaBH₄ in methanol causes reduction of the peripheral ketone auxochromes, whereupon tolyporphins (predominantly 7,17-dioxobacteriochlorins) exhibit a bathochromic shift (λ_abs ~ 676 → ~ 700 nm) and chlorophyll a (a 13¹ -oxochlorin) exhibits a hypsochromic shift (λ_abs 665 → 634 nm). Fluorescence excitation spectroscopy (at 368 and 491 nm with λ_em 710 nm) enabled detection of reduced tolyporphins amidst abundant reduced chlorophyll a (1:19 ratio), a detection sensitivity >5 times that without reduction. The resulting assay combines simple sample preparation from non-axenic cultures at microscale quantities (2 mL, 2 μm), absence of any fractionation procedures, and fluorescence detection. Tolyporphins were readily detected in cultures of HT-58-2 at reasonable growth periods in the absence of environmental stressors, which was not possible previously.

Imaging mass spectrometry for natural products discovery: a review of ionization methods

Covering: 2009-2019 Over the last decade, methods in imaging mass spectrometry (IMS) have progressively improved and diversified toward a variety of applications in natural products research. Because IMS allows for the spatial mapping of the production and distribution of biologically active molecules in situ, it facilitates phenotype and organelle driven discovery efforts. As practitioners of IMS for natural products discovery, we find one of the most important aspects of these experiments is the sample preparation and compatibility with different ionization sources that are available to a given researcher. As such, we have focused this mini review to cover types of ionization sources that have been used in natural products discovery applications and provided concrete examples of use for natural products discovery while discussing the advantages and limitations of each method. We aim for this article to serve as a resource to guide the broader natural product community interested in IMS toward the application/method that would best serve their natural product discovery needs given the sample and analyte(s) of interest. This mini review has been limited to applications using natural products and thus is not exhaustive of all possible ionization methods which have only been applied to image other types of samples such as mammalian tissues. Additionally, we briefly review how IMS has been coupled with other imaging platforms, such as microscopy, to enhance information outputs as well as offer our future perspectives on the incorporation of IMS in natural products discovery.

Cellular localization of tolyporphins, unusual tetrapyrroles, in a microbial photosynthetic community determined using hyperspectral confocal fluorescence microscopy

The cyanobacterial culture HT-58-2, composed of a filamentous cyanobacterium and accompanying community bacteria, produces chlorophyll a as well as the tetrapyrrole macrocycles known as tolyporphins. Almost all known tolyporphins (A-M except K) contain a dioxobacteriochlorin chromophore and exhibit an absorption spectrum somewhat similar to that of chlorophyll a. Here, hyperspectral confocal fluorescence microscopy was employed to noninvasively probe the locale of tolyporphins within live cells under various growth conditions (media, illumination, culture age). Cultures grown in nitrate-depleted media (BG-11₀ vs. nitrate-rich, BG-11) are known to increase the production of tolyporphins by orders of magnitude (rivaling that of chlorophyll a) over a period of 30-45 days. Multivariate curve resolution (MCR) was applied to an image set containing images from each condition to obtain pure component spectra of the endogenous pigments. The relative abundances of these components were then calculated for individual pixels in each image in the entire set, and 3D-volume renderings were obtained. At 30 days in media with or without nitrate, the chlorophyll a and phycobilisomes (combined phycocyanin and phycobilin components) co-localize in the filament outer cytoplasmic region. Tolyporphins localize in a distinct peripheral pattern in cells grown in BG-11₀ versus a diffuse pattern (mimicking the chlorophyll a localization) upon growth in BG-11. In BG-11₀, distinct puncta of tolyporphins were commonly found at the septa between cells and at the end of filaments. This work quantifies the relative abundance and envelope localization of tolyporphins in single cells, and illustrates the ability to identify novel tetrapyrroles in the presence of chlorophyll a in a photosynthetic microorganism within a non-axenic culture.

Total synthesis campaigns toward chlorophylls and related natural hydroporphyrins - diverse macrocycles, unrealized opportunities

Covering: up to 2018 Chlorophylls, bacteriochlorophylls and related hydroporphyrins constitute invaluable natural products but have largely remained outside the scope of viable syntheses. The campaign toward chlorophyll a by Woodward and coworkers is a deservedly celebrated landmark in organic synthesis yet the route entailed 49 steps, relied on semisynthetic replenishment of advanced intermediates, and then pointed to (but did not implement) uncertain literature procedures for the final transformations. Indeed, the full synthesis at any scale of any (bacterio)chlorophylls - conversion of small-molecule starting materials to the product - has never been accomplished. Herein, the reported syntheses of (±)-bonellin dimethyl ester (0.93 mg) and tolyporphin A O,O-diacetate (0.38 mg), as well as the never-fully traversed route to chlorophyll a, have been evaluated in a quantitative manner. Bonellin and tolyporphin A are naturally occurring chlorin and bacteriochlorin macrocycles, respectively, that lack the characteristic fifth ring of (bacterio)chlorophylls. A practical assessment is provided by the cumulative reaction mass efficiency (cRME) of the entire synthetic process. The cRME for the route to chlorophyll a would be 4.3 × 10-9 (230 kg of all reactants and reagents in total would yield 1.0 mg of chlorophyll a), whereas that for (±)-bonellin dimethyl ester or tolyporphin A O,O-diacetate is approximately 6.4 × 10-4 or 3.6 × 10-5, respectively. Comparison of the three syntheses reveals insights for designing hydroporphyrin syntheses. Development of syntheses with cRME > 10-5 (if not 10-4), as required to obtain 10 mg quantities of hydroporphyrin for diverse physicochemical, biochemical and medicinal chemistry studies, necessitates significant further advances in tetrapyrrole chemistry.