Estudi de les propietats supramoleculars de porfirines sulfonades autoensamblants: Quiralitat i proves de desracemització

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2014, Tutor: Dr. Joaquim Crusats AliguerAlong this final grade project acid-base processes of porphyrin 1, supramolecular aggregation on acidic media and spontaneous symmetry breaking processes have been studied by UV-Vis and CD. An acid-base titration of porphyrin 1 has been done and the spectral shapes of the three monomeric species of the studied porphyrin (1-a, 1-b and 1-c) have been defined. Usually, the monoprotonated form of the porphyrin is undistinguishable from the diprotonated species due to the similar pKa values; Both in water and in DMSO it has been detected that the pKa3 and pKa4 values are much more differentiated than usual in structurally similar porphyrins (2 and 3). Even though, only the acidification process on DMSO has shown three different tendencies on the spectral changes. The pKa values for the protonation equilibria of 1 have been determined..

Caged-carvedilol as a new tool for visible-light photopharmacology of β-adrenoceptors in native tissues

Biochemistry; Cell biology; PharmacologyBioquímica; Biologia cel·lular; FarmacologiaBioquímica; Biología celular; FarmacologíaAdrenoceptors are G protein-coupled receptors involved in a large variety of physiological processes, also under pathological conditions. This is due in large part to their ubiquitous expression in the body exerting numerous essential functions. Therefore, the possibility to control their activity with high spatial and temporal precision would constitute a valuable research tool. In this study, we present a caged version of the approved non-selective β-adrenoceptor antagonist carvedilol, synthesized by alkylation of its secondary amine with a coumarin derivative. Introducing this photo-removable group abolished carvedilol physiological effects in cell cultures, mouse isolated perfused hearts and living zebrafish larvae. Only after visible light application, carvedilol was released and the different physiological systems were pharmacologically modulated in a similar manner as the control drug. This research provides a new photopharmacological tool for a wide range of research applications that may help in the development of future precise therapies.This work was supported by ERDF-FEDER European Fund (projects CTQ2017-89222-R) and by the Catalan government (2017SGR 1604) to AL. Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación (PID2020-120499RB-I00) supported XR and AL. XR research was financed by the Spanish Ministry of Economy, Industry and Competitiveness (SAF2015-74132-JIN). MF was supported by the “Agencia Estatal deInvestigación” from the Spanish Ministry of Science and Innovation and the IDAEA-CSIC, a Centre of Excellence Severo Ochoa (CEX2018-000794-S). ARS has a consolidated Miguel Servet contract and was financed by by the Catalan government (2017-SGR-1807). ADC received the support of a fellowship from “la Caixa” Foundation (ID 100010434) under the fellowship codeLCF/BQ/DE18/11670012

REGULATION OF BETA-ADRENOCEPTORS ACTIVITYUSING SYNTHETIC LIGHT-REGULATED MOLECULES

Beta-adrenoceptors (ß-AR) are prototypical G proteincoupled receptors and important pharmacological targets for many diseases. Indeed, a number of approved drugs target these receptors due to their key role on many physiological functions. Among other examples, we encounter ß1-AR antagonists (ß- Blockers), which constitute the first-line therapy for the treatment of heart diseases, and ß2-AR agonists, which act as bronchodilators for the treatment of breathing pathologies. Considering the relevance of these receptors, achieving a reversible and localised control of their activity would provide a powerful tool, both for its research applications and its clinical potential. In this context, photopharmacology arises as a potent approach. Photopharmacology is an emerging field based on the use of synthetic light-regulated molecules to allow reversible spatiotemporal control of target receptors in native tissues. These ligands have the potential to provide a precise and controllable therapeutic action with increased efficacy and reduced side effects. Moreover, the fine regulation on demand of the receptor activation state is of great interest for their study in non-modified cells, tissues and organisms. The present project provides the first proof of concept for beta-adrenoceptor photopharmacology. We first designed and synthesised libraries of lightregulated compounds in order to regulate ß-AR activity with spatiotemporal precision. Subsequent testing highlighted the successful development of compounds with promising pharmacological properties which can be reversibly and irreversibly controlled by light. The discovered molecules enable a fine control of ß-AR in their native environment that will certainly open the door to innovative research procedures and may inspire future personalized therapies targeting these receptors

In vitro and in vivo regulation of ß-Adrenoceptors signaling using synthetic light-regulated molecules

Beta-adrenoceptors (ß-AR) are prototypical G protein-coupled receptors (GPCR) and important pharmacological targets for numerous diseases. Indeed, a number of approved drugs target ß-AR, which are key regulators of many physiological functions. Among other examples, ß1-AR antagonists (known as ß-Blockers) are first-line therapies for the treatment of heart failure, and ß2-AR agonists, which act as bronchodilators, are widely used for the treatment of breathing pathologies. Considering the medical relevance of these receptors, achieving a reversible and localized control of their activity would provide a powerful research and clinical tool. GPCR signaling is currently recognized as a multidimensional process governed by molecular, spatial and temporal components. Uncovering the role of each of these dimensions is crucial to improve our knowledge on cell communication, to understand how different pathways give rise to cellular and physiological effects, and to know how can we interact with biological systems with precision using drugs. Photopharmacology is an emerging field in which light-sensitive molecules are used to control the function of a given target protein in native tissues. The modulation of the target activity is achieved by small, drug-like, photoregulated ligands. By the use of light, both spatial and temporal control of the compound activity can be achieved in unprecedented manners compared to conventional pharmacology. These ligands have the potential to provide highly precise and controllable therapeutic actions that may result in increased efficacies and reduced side effects. Importantly, photopharmacology may allow to gain mechanistic insight on the interplay between the activation time and the receptor location during signaling processes in non-modified cells, tissues and whole organisms. Our research focused on the generation of new molecular tools for beta-adrenoceptors photopharmacology will be presented in this communication. First, several libraries of light-sensitive compounds with the aim to regulate ß-AR activity with spatiotemporal precision were designed and synthesized. Subsequent testing in cell preparations demonstrated the successful development of compounds with promising pharmacological properties, which can be reversibly and irreversibly controlled by light. Among those, several hit compounds were identified as ligands for beta-1 and beta-2 adrenoceptors with low nanomolar activities. These libraries compounds were found to be active enough to become useful photopharmacological tools, so we also performed in vivo experiments to determine their research potential in physiological environments. Indeed, the discovered molecules enabled a fine control of ß-AR in their native environment. We believe that the results of these studies will certainly open the door to innovative research procedures and may inspire future therapies targeting ß-AR

Caged-carvedilol as a new tool for visible-light photopharmacology of β-adrenoceptors in native tissues

Adrenoceptors are G protein-coupled receptors involved in a large variety of physiological processes, also under pathological conditions. This is due in large part to their ubiquitous expression in the body exerting numerous essential functions. Therefore, the possibility to control their activity with high spatial and temporal precision would constitute a valuable research tool. In this study, we present a caged version of the approved non-selective β-adrenoceptor antagonist carvedilol, synthesized by alkylation of its secondary amine with a coumarin derivative. Introducing this photo-removable group abolished carvedilol physiological effects in cell cultures, mouse isolated perfused hearts and living zebrafish larvae. Only after visible light application, carvedilol was released and the different physiological systems were pharmacologically modulated in a similar manner as the control drug. This research provides a new photopharmacological tool for a wide range of research applications that may help in the development of future precise therapies.We thank Maria José Bleda (IQAC-CSIC, Barcelona), Ignacio Pérez (IQAC-CSIC, Barcelona), Yolanda Pérez (IQAC-CSIC, Barcelona) and Carme Serra (SimChemSiMChem, IQAC-CSIC, Barcelona) for technical support. We thank Dr. Kees Jalink (The Netherlands Cancer Institute, Amsterdam, the Netherlands) for providing the plasmids encoding for the Epac-SH188 biosensor. We thank the University of Vic-Central University of Catalonia (UVic-UCC), Dr. Malu Calle and Dr. Marta Otero for the material assignment which helped in some biological assays. This work was supported by ERDF-FEDER European Fund (projects CTQ2017-89222-R) and by the Catalan government (2017SGR 1604) to AL. Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación (PID2020-120499RB-I00) supported XR and AL. XR research was financed by the Spanish Ministry of Economy, Industry and Competitiveness (SAF2015-74132-JIN). MF was supported by the “Agencia Estatal deInvestigación” from the Spanish Ministry of Science and Innovation and the IDAEA-CSIC, a Centre of Excellence Severo Ochoa (CEX2018-000794-S). ARS has a consolidated Miguel Servet contract and was financed by by the Catalan government (2017-SGR-1807). ADC received the support of a fellowship from “la Caixa” Foundation (ID 100010434) under the fellowship codeLCF/BQ/DE18/11670012.Peer reviewe

Characterization of p38α autophosphorylation inhibitors that target the non-canonical activation pathway

16 pages, 10 figures, supplementary information https://doi.org/10.1038/s41467-023-39051-x.-- Data availability: The diffraction data and coordinates of the p38α complexes bound to NC-p38i compounds have been deposited in the Protein Data Bank under accession codes 7PVU, 7Z6I and 7Z9T. We have also used the following PDB structures: 4LOO, 1A9U, 3COI, 7N8T, 2ZOQ, 1PME, 3GC9, 1CM8, 4UX9. Source data are provided with this paperp38α is a versatile protein kinase that can control numerous processes and plays important roles in the cellular responses to stress. Dysregulation of p38α signaling has been linked to several diseases including inflammation, immune disorders and cancer, suggesting that targeting p38α could be therapeutically beneficial. Over the last two decades, numerous p38α inhibitors have been developed, which showed promising effects in pre-clinical studies but results from clinical trials have been disappointing, fueling the interest in the generation of alternative mechanisms of p38α modulation. Here, we report the in silico identification of compounds that we refer to as non-canonical p38α inhibitors (NC-p38i). By combining biochemical and structural analyses, we show that NC-p38i efficiently inhibit p38α autophosphorylation but weakly affect the activity of the canonical pathway. Our results demonstrate how the structural plasticity of p38α can be leveraged to develop therapeutic opportunities targeting a subset of the functions regulated by this pathwayThis work was supported by grants from the Spanish Ministerio de Ciencia e Innovación (MICINN, PID2019-109521RB-I00 and PID2021-122478NB-I00), the BioMedTec program of IRB-Fundació La Caixa, the European Research Council (Proof of Concept p38_InTh-825763), AGAUR (2016 LLAV 00043 and 2019 PROD 00138 supported by FEDER, and 2017 SGR-557, 2017 SGR-50, 2021 SGR-909, and 2021 SGR-866), BBVA Foundation, and the European Union’s Horizon 2020 research and innovation program (euCanSHare 825903 and BioExcel-3 101093290). L.G. and B.B. were funded by predoctoral contracts from MICINN (BES-2016-077122) and the Marie Skłodowska-Curie COFUND action of IRB Barcelona and the PREBIST Predoc Programme (PREBIST_754558), respectively. F.C. is a Ramon y Cajal Fellow (RYC2019-026768-I). Access to ALBA was granted through the BAG proposals 2018092972 and 2020094472. We gratefully acknowledge institutional funding from IRB Barcelona, the CERCA Programme of the Catalan Government, and the MICINN through the Centres of Excellence Severo Ochoa award. M.J.M. and A.R.N. are supported by the Institució Catalana de Recerca i Estudis Avancats (ICREA)With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

Photoswitchable ligands targeting beta-adrenoceptors for in vitro, in vivo and structural studies

[eng] G protein-coupled receptors (GPCRs) are a large family of membrane proteins responsible for signaling transduction processes. Due to their roles in modulating essential physiological functions, GPCRs are classical pharmacological targets and the focus of numerous research lines. Within this superfamily of receptors, beta-adrenoceptors (β-AR) have been widely studied. Their relevant contribution to the modulation of the cardiac output, among other physiological functions, has historically signaled these receptors as therapeutic targets. Many approved drugs modulate the activation of β-AR, which evidences the high potential of these GPCRs in clinical and research applications. On the other hand, photopharmacology has arisen as an innovative approach with therapeutic potential and many research applications. This technique uses synthetic light-regulated molecules to render light-controllable proteins without genetic manipulation. In this context, the main objective of the present thesis is the development of light-sensitive molecules that allow the modulation of beta-adrenoceptors through light application. In the first chapter, we report the development of a caged analogue of carvedilol, an approved inhibitor of β-AR. Upon illumination at 405 nm, Caged-Carvedilol photolitically releases the beta-blocker carvedilol. The caged ligand was used to explore light-dependent modulation of beta-adrenoceptors in several physiological systems, including native cardiac tissues and living zebrafish larvae. Overall, this novel caged compound provides an innovative molecular tool to precisely control the activation state of β-AR in space and time. Therefore, C-C could be used in future studies to better understand the complex role of beta-adrenoceptors in physiology. Caged ligands are valuable molecular tools but present one main limitation, their irreversible nature. Once the bioactive molecule is photolitically released, no compound deactivation is possible. To produce ligands that allow a reversible control of the target receptors with light, a series of azobenzene-based compounds were designed, synthesized, and photochemically characterized in Chapter 2. Additionally, we explored the light-dependent pharmacological properties of Photoazolols 1-3 (PZLs 1-3) in vitro against two different receptor subtypes, β1- AR and β2-AR. The pharmacological results obtained for the first series of photochromic ligands guided the design of two additional azobenzenes to modulate β1-AR selectively. Both ligands, designed with a para-substituted azobenzene (p-AB) scaffold, displayed good light-dependent properties and an excellent β1-AR selectivity profile. Additionally, in vivo assays were performed using zebrafish larvae. These experiments highlighted that light-dependent cardiac modulation was achieved when larvae were treated with p-AB 84 under different illumination conditions. Importantly, all photochromic ligands described in the second chapter of this thesis display trans→cis isomerization wavelengths within the UV range. This photochemical feature can limit the research applications for the developed photoswitches, considering that highly energetic ultraviolet light can introduce safety issues due to its phototoxicity. It could be especially problematic if the developed compounds were intended for therapeutic applications. For this reason, we developed red-shifted azobenzenes that enable modulation of β-AR with the application of visible light, reported in Chapter 3. Two series of aminoazobenzenes (aABs) were synthesized, which could be photoisomerized efficiently through the application of visible light. Photopharmacological properties of the red-shifted ligands were evaluated in vitro for both β1-AR and β2-AR subtypes. Finally, we intended to use the developed azobenzenes to perform structural studies on β-AR. The last chapter of this thesis describes the research conducted during a 6-months stay performed at the Paul Scherrer Institut (Switzerland), where a joint subproject was established with the Standfuss group. This subproject aimed to crystallize both β1-AR and β2-AR, bound to one of the trans-on compounds from our library of photochromic ligands. After crystallization, we intended to perform conventional X-Ray Crystallography experiments with the obtained crystals in the dark and after illumination with the appropriate wavelength to evaluate the light-induced structural changes in our target receptors. Firstly, receptor expression and purification protocols were optimized and established for different constructs of β1-AR and β2- AR. Finally, crystallization trials were set up using purified protein bound to PZL-1, and preliminary crystals were obtained for the ultra-stable (US) construct of β1-AR. However, we did not obtain the diffraction-quality crystals required to perform X-Ray Crystallography experiments. To sum up, the present thesis reports a variety of novel photopharmacological tools to study beta-adrenoceptors with spatiotemporal precision. We performed in vitro, in vivo, and structural studies using different molecules from the developed library to demonstrate the possibilities offered by the described compounds in therapeutic and research applications.[cat] Els receptors acoblats a proteïnes G (GPCRs) són una gran família de proteïnes de membrana responsables de modular la transducció de senyals a conseqüència de diversos estímuls. A causa del seu paper en la regulació d’importants funcions fisiològiques, els GPCR han estat establerts com a dianes farmacològiques clàssiques i constitueixen la base de nombroses línies de recerca. Dins d'aquesta superfamília de receptors, els receptors beta-adrenèrgics (β-AR) han estat àmpliament estudiats. La seva contribució a la modulació del ritme cardíac o del sistema respiratori, entre d’altres, ha assenyalat aquests receptors com a importants dianes terapèutiques. De fet, en l’actualitat existeixen molts fàrmacs aprovats que modulen l'estat d’activació dels β-AR, cosa que evidencia l'alt potencial d'aquests GPCRs en aplicacions clíniques i de recerca. D'altra banda, la fotofarmacologia constitueix un camp de recerca innovador, amb potencial terapèutic i una enorme varietat de possibles aplicacions en investigació bàsica. Aquesta tècnica, utilitza molècules sintètiques regulades per la llum que donen lloc a proteïnes controlables mitjançant la llum sense necessitat de manipular-les genèticament. En aquest context, l'objectiu principal de la present tesi doctoral és el desenvolupament de molècules fotocontrolables que permetin la modulació dels receptors beta-adrenèrgics mitjançant l'aplicació de la llum. En el primer capítol, es reporta el desenvolupament d'un anàleg “caged” del carvedilol, un inhibidor dels β-AR aprovat per a ús clínic. Es va observar que després d’il·luminar-lo a 405 nm, Caged-Carvedilol (C-C) alliberava fotolíticament la molècula bioactiva carvedilol. Així doncs, el lligand desenvolupat es va utilitzar per explorar la modulació, mitjançant l’aplicació de llum, dels receptors beta-adrenèrgics en una varietat de sistemes fisiològics, inclosos els teixits cardíacs natius i les larves de peix zebra. En vista dels resultats, el nou compost “caged” proporciona una eina molecular innovadora que permet controlar amb precisió l'estat d'activació dels β-AR en l'espai i el temps. Per tant, C-C es podria utilitzar en estudis futurs per a entendre millor el paper complex que desenvolupen els receptors beta-adrenèrgics en fisiologia. Els lligands “caged” són certament eines moleculars valuoses, però presenten una limitació clara, i és la seva naturalesa irreversible; un cop la molècula bioactiva s'allibera fotolíticament, no és possible desactivar el compost. Per tal de produir lligands que permetin un control reversible dels receptors diana amb la llum, es van dissenyar, sintetitzar i caracteritzar fotoquímicament una sèrie de compostos basats en l’azobenzè (capítol 2). A més, es van explorar les propietats farmacològiques depenents de la llum dels Photoazolols 1-3 ( PZLs 1-3) in vitro contra dos subtipus de receptors beta-adrenèrgics diferents, β1-AR i β2-AR. Els resultats farmacològics obtinguts per a la primera sèrie de lligands fotocròmics van ser imprescindibles per al disseny de dos azobenzens addicionals que permetessin modular β1-AR de forma selectiva. Tots dos lligands, basats en l’estructura d'un azobenzè substituït en la posició para- (p-AB), mostraven bones propietats dependents de la llum i un excel·lent perfil de selectivitat sobre els receptors β1-AR. A més, es van realitzar assaigs in vivo amb larves de peix zebra. Aquests experiments van destacar que la modulació cardíaca depenent de la llum es podia aconseguir quan les larves eren tractades amb p-AB 84 en diferents condicions d'il·luminació. És important destacar que tots els lligands fotocròmics descrits al segon capítol d'aquesta tesi mostren longituds d'ona d'isomerització trans→cis dins del rang UV. Aquesta característica fotoquímica pot limitar les aplicacions de recerca de les molècules fotoactivables desenvolupades, tenint en compte que la llum ultraviolada és altament energètica i pot introduir problemes de seguretat a causa de la seva fototoxicitat. Això podria ser especialment problemàtic si els compostos desenvolupats es volguessin utilitzar en aplicacions terapèutiques. Per aquest motiu, vam desenvolupar azobenzens desplaçats al vermell que permetessin la modulació dels receptors β-AR amb l'aplicació de llum visible (capítol 3). Es van sintetitzar dues sèries d'aminoazobenzens (aABs) que es podien fotoisomeritzar de manera eficient mitjançant l’aplicació de llum visible. Les propietats fotofarmacològiques dels lligands desplaçats al vermell es van avaluar in vitro per als dos subtipus de receptor d’interès, β1-AR i β2-AR. Finalment, es van voler utilitzar els azobenzens desenvolupats per realitzar estudis estructurals sobre els β-AR. L'últim capítol d'aquesta tesi descriu la recerca realitzada durant una estada de 6 mesos al Paul Scherrer Institut (Suïssa), on es va establir un subprojecte conjunt amb el grup del Dr. Standfuss. Aquest subprojecte tenia com a objectiu cristal·litzar tant β1-AR com β2-AR, units a un dels compostos trans-on de la nostra biblioteca de lligands fotocròmics. Després de la cristal·lització, es pretenia realitzar experiments cristal·lografia de raigs X convencional utilitzant cristalls mantinguts en condicions de foscor i cristalls il·luminats amb una longitud d'ona adequada. Aquests estudis ens haguessin permès avaluar els canvis estructurals induïts per la llum en els nostres receptors diana. En primer lloc, es van optimitzar i establir protocols d'expressió i purificació del receptor per a diferents plàsmids de β1-AR i β2- AR. Finalment, es van establir probes de cristal·lització utilitzant proteïna purificada unida al compost PZL-1. Es van obtenir cristalls preliminars per la proteïna β1-AR ultraestable (US) unida al lligand fotocròmic. Tanmateix, no vam aconseguir obtenir cristalls amb la qualitat necessària per a realitzar experiments de cristal·lografia de raigs X. En resum, la present tesi informa d'una sèrie de noves eines fotofarmacològiques que permeten estudiar els receptors beta-adrenèrgics amb precisió espacio-temporal. S'han realitzat estudis in vitro, in vivo i estructurals utilitzant diferents molècules de la biblioteca de compostos desenvolupada per tal de demostrar les possibilitats que ofereixen els lligands descrits tant en aplicacions terapèutiques com de recerca

Photoswitchable Antagonists for a Precise Spatiotemporal Control of β2-Adrenoceptors

β2-Adrenoceptors (β2-AR) are prototypical G-protein-coupled receptors and important pharmacological targets with relevant roles in physiological processes and diseases. Herein, we introduce Photoazolol-1–3, a series of photoswitchable azobenzene β2-AR antagonists that can be reversibly controlled with light. These new photochromic ligands are designed following the azologization strategy, with a p-acetamido azobenzene substituting the hydrophobic moiety present in many β2-AR antagonists. Using a fluorescence resonance energy transfer (FRET) biosensor-based assay, a variety of photopharmacological properties are identified. Two of the light-regulated molecules show potent β2-AR antagonism and enable a reversible and dynamic control of cellular receptor activity with light. Their photopharmacological properties are opposite, with Photoazolol-1 being more active in the dark and Photoazolol-2 demonstrating higher antagonism upon illumination. In addition, we provide a molecular rationale for the interaction of the different photoisomers with the receptor. Overall, we present innovative tools and a proof of concept for the precise control of β2-AR by means of light.We thank Montserrat Masoliver (UVic-UCC, Vic, Spain), Joan Bertrán (UVic-UCC, Vic, Spain), Jordi Serra (UVic-UCC, Vic, Spain), Lourdes Muñoz (SiMChem, IQAC-CSIC, Barcelona), Maria José Bleda (IQAC-CSIC, Barcelona), Ignacio Pérez (IQAC-CSIC, Barcelona), Yolanda Pérez (IQAC-CSIC, Barcelona) and Carme Serra (SiMChem, IQAC-CSIC, Barcelona) for technical support. We thank Dr. Kees Jalink (The Netherlands Cancer Institute, Amsterdam, the Netherlands) for providing the plasmids encoding for the Epac-SH188 biosensor. We thank the Cisbio Bioassays for their support and technical discussion, as well as for providing plasmids in the preliminary optimization of the biological assays. The project that gave rise to these results received the support of a fellowship from “la Caixa” Foundation (ID 100010434). The fellowship code is (LCF/BQ/DE18/11670012). This work was supported by FEDER/Ministerio de Ciencia, Innovación y Universidades–Agencia Estatal de Investigación (CTQ2017-89222-R), the Catalan government (2017SGR1604), PO FEDER of Catalonia 2014-2020 (project PECT Osona Transformació Social, Ref. 001-P-000382) and the Spanish Ministry of Economy, Industry and Competitiveness (SAF2015-74132-JIN).Peer reviewe

Hyperporphyrin effects extended into a Jaggregate supramolecular structure in water

The relationship between the acid-base chemistry and the supramolecular behavior of 5-(4-aminophenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin in acidic water is reported. A new species exhibiting a prominently red-shifted Q-absorption band at 742 nm is described which is in accordance with a hyperporphyrin-type spectrum of a J-aggregate in water. UV-vis spectroscopy and peak force microscopy reveal that depending on the pH value of the medium the porphyrin self-assembles into two structurally different mesophases which can be reversibly interconverted at will