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Probing the Interactions of Porphyrins with Macromolecules Using NMR Spectroscopy Techniques

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Probing the Interactions of Porphyrins with Macromolecules Using NMR Spectroscopy Techniques molecules Review Probing the Interactions of Porphyrins with Macromolecules Using NMR Spectroscopy Techniques Ilche Gjuroski, Julien Furrer and Martina Vermathen * Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland; [email protected] (I.G.); [email protected] (J.F.) * Correspondence: [email protected]; Tel.: +41-(31)-631-3948 Abstract: Porphyrinic compounds are widespread in nature and play key roles in biological pro- cesses such as oxygen transport in blood, enzymatic redox reactions or photosynthesis. In addition, both naturally derived as well as synthetic porphyrinic compounds are extensively explored for biomedical and technical applications such as photodynamic therapy (PDT) or photovoltaic systems, respectively. Their unique electronic structures and photophysical properties make this class of compounds so interesting for the multiple functions encountered. It is therefore not surprising that optical methods are typically the prevalent analytical tool applied in characterization and pro- cesses involving porphyrinic compounds. However, a wealth of complementary information can be obtained from NMR spectroscopic techniques. Based on the advantage of providing structural and dynamic information with atomic resolution simultaneously, NMR spectroscopy is a powerful method for studying molecular interactions between porphyrinic compounds and macromolecules. Such interactions are of special interest in medical applications of porphyrinic photosensitizers Citation: Gjuroski, I.; Furrer, J.; that are mostly combined with macromolecular carrier systems. The macromolecular surrounding Vermathen, M. Probing the typically stabilizes the encapsulated drug and may also modify its physical properties. Moreover, the Interactions of Porphyrins with interaction with macromolecular physiological components needs to be explored to understand and Macromolecules Using NMR control mechanisms of action and therapeutic efficacy. This review focuses on such non-covalent Spectroscopy Techniques. Molecules interactions of porphyrinic drugs with synthetic polymers as well as with biomolecules such as 2021, 26, 1942. https://doi.org/ phospholipids or proteins. A brief introduction into various NMR spectroscopic techniques is given 10.3390/molecules26071942 including chemical shift perturbation methods, NOE enhancement spectroscopy, relaxation time Academic Editors: measurements and diffusion-ordered spectroscopy. How these NMR tools are used to address Constantinos K. Zacharis and porphyrin–macromolecule interactions with respect to their function in biomedical applications is Paraskevas D. Tzanavaras the central point of the current review. Received: 24 February 2021 Keywords: porphyrin; NMR spectroscopy; interaction; phospholipids; proteins; nucleic acids; drug Accepted: 24 March 2021 delivery; polymer; cyclodextrin; surfactant; micelles Published: 30 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- Porphyrinic compounds stand out among the organic molecules found in nature iations. due to their unique properties associated with their common scaffold, a planar macro- cycle consisting of four pyrrole rings linked by methine bridges [1]. The tetrapyrrole ring system forms a cavity that can accommodate many different metal ions, typically forming bidentate complexes that give rise to the widespread metalloporphyrins [2,3]. Copyright: © 2021 by the authors. It is remarkable that a wide range of living systems including animals, plants and mi- Licensee MDPI, Basel, Switzerland. croorganisms make use of this common concept for fulfilling key functions in biological This article is an open access article processes [4–6]. The major classes of naturally occurring porphyrinic compounds can be distributed under the terms and subdivided into porphyrins (I) in a narrower sense (often the whole group of porphyrinic conditions of the Creative Commons compounds is referred to as porphyrins), chlorins (II), bacteriochlorins (III), and corrins Attribution (CC BY) license (https:// (IV) (Figure1)[ 3,4]. Heme forms the iron complex of protoporphyrin IX (PPIX) and its creativecommons.org/licenses/by/ 4.0/). protein complex hemoglobin is the major constituent of red blood cells, imparting them Molecules 2021, 26, 1942. https://doi.org/10.3390/molecules26071942 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 45 Molecules 2021, 26, 1942 2 of 43 1) [3,4]. Heme forms the iron complex of protoporphyrin IX (PPIX) and its protein com‐ plex hemoglobin is the major constituent of red blood cells, imparting them their red color. Thetheir ability red color. of hemoglobin The ability ofand hemoglobin myoglobin and to bind myoglobin molecular to bind oxygen molecular plays an oxygen important plays rolean important in oxygen role transport in oxygen and transportstorage in and living storage systems. in living Heme systems. also functions Heme alsoas cofactor functions or prostheticas cofactor group or prosthetic in a vast group number in a vast of hemoprotein number of hemoprotein-based‐based enzymes such enzymes as the such cyto as‐ chromesthe cytochromes that are that involved are involved in electron in electron transfer transfer and catalytic and catalytic reactions reactions [7,8]. [ 7Chlorins,8]. Chlorins are partlyare partly reduced reduced dihydro dihydro-porphyrins‐porphyrins and their and theirmagnesium magnesium complexes complexes form the form core the struc core‐ turestructure of chlorophylls, of chlorophylls, rendering rendering the green the green color color to plants to plants [4]. [The4]. The chlorophylls chlorophylls are are part part of theof the light light-harvesting‐harvesting complexes complexes of all of photosynthetic all photosynthetic organisms organisms and and thus thus fulfill fulfill one oneof the of mostthe most important important functions functions in life in lifewith with their their ability ability to use to use solar solar energy energy and and transferring transferring it toit toreaction reaction centers centers so that so that molecular molecular oxygen oxygen can canbe formed be formed [9,10]. [9, 10In ].bacteriochlorins, In bacteriochlorins, two oppositetwo opposite pyrrole pyrrole rings rings are reduced, are reduced, leading leading to tetrahydro to tetrahydro-porphyrins‐porphyrins (two (two adjacently adjacently re‐ ducedreduced pyrrole pyrrole rings rings yield yield isobacteriochlorins). isobacteriochlorins). They They are are part part of of bacteriochlorophylls bacteriochlorophylls in anoxygenicanoxygenic phototropic bacteriabacteria in in the the form form of of their their Mg Mg or or Zn Zn complexes complexes [9, 11[9,11].]. Finally, Finally, the thecorrins corrins have have to be to pointed be pointed out as out modified as modified tetrapyrroles tetrapyrroles whose Co(III)whose complexesCo(III) complexes forming formingvitamin B12vitamin (cobalamin) B12 (cobalamin) are part ofare key part metabolic of key metabolic enzymes enzymes [12]. [12]. (I) (II) (III) (IV) (V) (VI) (VII) (VIII) Figure 1. Structures of porphyrins (I), chlorins ((IIII),), bacteriochlorins ((IIIIII),), corrinscorrins ((IVIV),), tetraphenylporphyrinstetraphenylporphyrins ( (VV),), phthalo- phthal‐ ocyaninescyanines ( VI(VI),), texaphyrins texaphyrins ( VII(VII),), and and corroles corroles (VIII (VIII).). TheThe many many different different aspects aspects of of porphyrins porphyrins such such as as synthesis, synthesis, coordination coordination chemistry, chemistry, biochemistry,biochemistry, andand applicationsapplications have have been been compiled compiled in in a detaileda detailed compendium compendium of of numerous numer‐ ousvolumes volumes devoted devoted to this to this class class of compounds, of compounds, “The “The Porphyrin Porphyrin Handbook” Handbook” [13 [13].]. Owing Owing to totheir their unique unique electronic electronic structures, structures, redox-, redox‐, photochemical photochemical andand photophysical properties, porphyrinic compounds compounds have have gained gained much much interest interest in in a broad range of applications in technologytechnology and and biomedicine. biomedicine. Technical Technical applications applications mainly mainly rely rely on the on capability the capability of por of‐ phyrinsporphyrins for energy for energy conversion, conversion, making making them attractive them attractive materials materials for photovoltaic for photovoltaic systems, photocatalystssystems, photocatalysts and energy and storage energy systems storage [14–16]. systems [14–16]. TheThe most most prevalent prevalent applications applications of of porphyrinic porphyrinic compounds compounds can can be found be found in the in bio the‐ medicalbiomedical field field [17–19]. [17–19 This]. This is mainly is mainly due due to the to fact the factthat thatporphyrins porphyrins combine combine many many advan ad-‐ tageousvantageous properties properties such such as aslight light absorption absorption in in the the visible visible and and near near-infrared‐infrared wavelength wavelength region,region, intense intense fluorescence, fluorescence, ability ability to to form toxic singlet oxygen from excited electronic triplettriplet states followingfollowing lightlight irradiation irradiation (phototoxicity), (phototoxicity), preferential preferential accumulation accumulation in tumorin tu‐ mortissue,
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