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Emerging Applications of Porphyrins and Metalloporphyrins in Biomedicine and Diagnostic Magnetic Resonance Imaging

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Emerging Applications of Porphyrins and Metalloporphyrins in Biomedicine and Diagnostic Magnetic Resonance Imaging biosensors Review Emerging Applications of Porphyrins and Metalloporphyrins in Biomedicine and Diagnostic Magnetic Resonance Imaging Muhammad Imran 1,*, Muhammad Ramzan 2,*, Ahmad Kaleem Qureshi 1, Muhammad Azhar Khan 2 and Muhammad Tariq 3 1 Department of Chemistry, Baghdad-Ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan; [email protected] 2 Department of Physics, Baghdad-Ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan; [email protected] 3 Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan; [email protected] * Correspondence: [email protected] (M.I.); [email protected] (M.R.) Received: 26 September 2018; Accepted: 17 October 2018; Published: 19 October 2018 Abstract: In recent years, scientific advancements have constantly increased at a significant rate in the field of biomedical science. Keeping this in view, the application of porphyrins and metalloporphyrins in the field of biomedical science is gaining substantial importance. Porphyrins are the most widely studied tetrapyrrole-based compounds because of their important roles in vital biological processes. The cavity of porphyrins containing four pyrrolic nitrogens is well suited for the binding majority of metal ions to form metalloporphyrins. Porphyrins and metalloporphyrins possess peculiar photochemical, photophysical, and photoredox properties which are tunable through structural modifications. Their beneficial photophysical properties, such as the long wavelength of emission and absorption, high singlet oxygen quantum yield, and low in vivo toxicity, have drawn scientists’ interest to discover new dimensions in the biomedical field. Applications of porphyrins and metalloporphyrins have been pursued in the perspective of contrast agents for magnetic resonance imaging (MRI), photodynamic therapy (PDT) of cancer, bio-imaging, and other biomedical applications. This review discusses photophysics and the photochemistry of porphyrins and their metal complexes. Secondly, it explains the current developments and mode of action for contrast agents for MRI. Moreover, the application of porphyrin and metalloporphyrin-based molecules as a photosensitizer in PDT of cancer, the mechanism of the generation of reactive oxygen species (ROS), factors that determine the efficiency of PDT, and the developments to improve this technology are delineated. The last part explores the most recent research and developments on metalloporphyrin-based materials in bio-imaging, drug delivery, and the determination of ferrochelatase in bone marrow indicating their prospective clinical applications. Keywords: porphyrins; metalloporphyrins; photophysics; photochemistry; photodynamic therapy; photosensitizer; magnetic resonance imaging; contrast agents; bio-imaging; drug delivery 1. Introduction Porphyrins represent a unique class of heterocyclic tetrapyrrolic organic molecules which are the most ubiquitous compounds found in nature. These macrocycles derive their name from “porphura” which was used for the first time by ancient Greeks for the intense purple color [1]. From the structural viewpoint, porphyrin is composed of four pyrrolic units that are linked in a coplanar fashion by four methene bridges that give a planar macrocyclic structure to the porphyrin molecule (Figure1). It has an extended conjugated 18 π-electron system which is responsible for its aromatic behavior, Biosensors 2018, 8, 95; doi:10.3390/bios8040095 www.mdpi.com/journal/biosensors Biosensors 2018, 8, x FOR PEER REVIEW 2 of 17 Biosensors 2018, 8, 95 2 of 17 (Figure 1). It has an extended conjugated 18 π-electron system which is responsible for its aromatic behavior, and its limited size cavity enables the accommodation of large metal cations [2,3]. Porphyrinsand its limited are of size fundamental cavity enables significance the accommodation on this planet of to large sustain metal life cations in various [2,3 ].ways Porphyrins like storage are andof fundamental transport of oxygen, significance chlorophyll on this for planet photosynthes to sustainis, life different in various types ways of enzymes like storage and vitamins and transport [4,5]. Syntheticof oxygen, porphyrins chlorophyll forand photosynthesis, metalloporphyrins different have types inspiring of enzymes biological, and vitamins photophysical, [4,5]. Synthetic and photochemicalporphyrins and properties metalloporphyrins and are promising have inspiringcandidates biological, for diseases photophysical, treatment [6], biological and photochemical imaging [7], industrialproperties and[8], areanalytical promising [9], candidates photocatalytic for diseases [10], treatmentnonlinear [6 ],optics biological (NLO) imaging [11], [7and], industrial molecular [8], photovoltaicsanalytical [9], [12,13]. photocatalytic [10], nonlinear optics (NLO) [11], and molecular photovoltaics [12,13]. InIn the the last last few few decades, decades, researchers researchers have have expanded expanded the the use use of porphyrin-based of porphyrin-based compounds compounds for medical,for medical, drug drugdelivery, delivery, bio-sensing, bio-sensing, and bio-imaging and bio-imaging purposes purposes [14,15]. Porphyrin-based [14,15]. Porphyrin-based molecules havemolecules been pioneering have been theranostic pioneering agents theranostic not only agents for MRI, not and only photodynamic for MRI, and cancer photodynamic therapy but cancer also fortherapy drug butdelivery also for and drug single delivery cell imaging and single [16–19]. cell imaging Moreover, [16 –these19]. Moreover,compounds these have compounds potential applicationshave potential as applicationschemosensors as [20]. chemosensors Dong et al. [20 [18]]. Dongexplored et al. the [18 application] explored of the the application porphyrin- of incorporatedthe porphyrin-incorporated hydrogel with hydrogelfour arm-copolymer with four arm-copolymer as promising asdrug promising carrier system. drug carrier The clinical system. applicationsThe clinical applications based on photophysical based on photophysical properties are properties embedded are embeddedin the aromatic in the macrocyclic aromatic macrocyclic structure ofstructure porphyrins of porphyrins and metalloporphyrins and metalloporphyrins [1]. In the [1 free]. In base the free porphyrins base porphyrins out of 22 out π-electrons of 22 π-electrons 18 are supposed18 are supposed to be toconjugated, be conjugated, which which are areresponsibl responsiblee for forthe the characteristic characteristic redox redox and and electronic properties. SomeSome other other compounds compounds such such as chlorinas chlo andrin bacteriochlorinand bacteriochlorin showresemblances show resemblances in structure in structurewith porphyrins with porphyrins [21]. Due to [21]. the conjugation Due to the of conjugationπ-electrons onof the π-electrons frontier orbitals, on the porphyrins frontier orbitals, possess porphyrinsdistinctive UV–VISpossess distinctive spectra because UV–VIS metalloporphyrins spectra because metalloporphyrins have four-fold symmetry have four-fold and four symmetry nitrogen andatoms four directed nitrogen towards atoms thedirected center towards of the porphyrin the center coreof the [22 porphyrin]. Extensive core conjugation [22]. Extensive of 18 conjugation electrons in ofporphyrins 18 electrons gives in porphyrins rise to facile givesπ!π rise* transitions to facile π that→π give* transitions rise to two that distinct give rise bands to two within distinct the visiblebands withinregion ofthe electromagnetic visible region of radiations. electromagnetic An intense radiations. absorption An bandintense called absorption B band band or Soret called band B betweenband or Soret350 and band 500 between nm resulting 350 and from 500 a nm ground resulting state from to second a ground excited state singlet to second state (Sexcited0!S2 )singlet with molar state − − 5 −1 −1 (Sabsorption0→S2) with coefficient molar absorption 105 M 1 cm coefficient1 and a 10 less M intense cm band and knowna less intense as Q-band band observed known fromas Q-band 500 to observed750 nm resulting from 500 from to 750 ground nm resulting state to firstfrom excited ground state state (S 0to! firstS1) withexcited molar state absorption (S0→S1) with coefficient molar absorption104 M−1 cm coefficient−1 [23]. Insertion 104 M− of1 cm a metal−1 [23]. ion Insertion into the porphyrinof a metal cavity ion into or the the protonation porphyrin ofcavity the nitrogen or the protonationatoms or variation of the ofnitrogen the peripheral atoms or substituents variation of may the result peripheral in a change substituents in the wavelength may result and in a intensity change inof the the wavelength absorptionspectrum and intensity [24]. of the absorption spectrum [24]. FigureFigure 1. 1. TypicalTypical structures, structures, i.e., ( A)) pyrrole, pyrrole, and and ( (BB)) porphyrin porphyrin consist consist of of four four pyrrole pyrrole rings rings joined joined by by methenemethene bridges, and ( C)) metalloporphyrin metalloporphyrin (M = Fe, Fe, Mn, Mn, Cr). Cr). Freebase porphyrins can form a complex with various metal cations and can adopt a wide variety Freebase porphyrins can form a complex with various metal cations and can adopt a wide of conformation: planar, domed, saddled, ruffled, etc. Depending on the size of coordinating metal variety of conformation: planar, domed, saddled, ruffled, etc. Depending on the size of coordinating cations, metalloporphyrins are of two types.
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