polymers Review Antioxidant Activity of Synthetic Polymers of Phenolic Compounds Subhalakshmi Nagarajan 1,*, Ramaswamy Nagarajan 2, Jayant Kumar 3, Adele Salemme 4, Anna Rita Togna 4, Luciano Saso 4 and Ferdinando Bruno 5,* 1 Department of Natural and Social Sciences, Bowling Green State University-Firelands, Huron, OH 44839, USA 2 Department of Plastics Engineering and Center for Advanced Materials, University of Massachusetts, Lowell, MA 01854, USA; [email protected] 3 Department of Physics and Center for Advanced Materials, University of Massachusetts, Lowell, MA 01854, USA; [email protected] 4 Department of Physiology and Pharmacology “Vittorio Erspamer”, Sapienza University of Rome, P.le Aldo Moro 5, 00185 Rome, Italy; [email protected] (A.S.); [email protected] (A.R.T.); [email protected] (L.S.) 5 Combat Capabilities Development Command Soldier Center, Natick, MA 01760, USA * Correspondence: [email protected] (S.N.); [email protected] (F.B.) Received: 1 January 2020; Accepted: 17 July 2020; Published: 24 July 2020 Abstract: In recent years, developing potent antioxidants has been a very active area of research. In this context, phenolic compounds have been evaluated for their antioxidant activity. However, the use of phenolic compounds has also been limited by poor antioxidant activity in several in vivo studies. Polymeric phenols have received much attention owing to their potent antioxidant properties and increased stability in aqueous systems. To be truly effective in biological applications, it is important that these polymers be synthesized using benign methods. In this context, enzyme catalyzed synthesis of polymeric phenols has been explored as an environmentally friendly and safer approach. This review summarizes work in enzymatic syntheses of polymers of phenols. Several assays have been developed to determine the antioxidant potency of these polymeric phenols. These assays are discussed in detail along with structure-property relationships. A deeper understanding of factors affecting antioxidant activity would provide an opportunity for the design of versatile, high performing polymers with enhanced antioxidant activity. Keywords: antioxidant activity of phenolic polymers; enzymatic polymerization; polymeric flavonoids; peroxidase catalyzed polymerization; lipase catalyzed polymerization 1. Introduction Reactive oxygen species (ROS) are known to play a key role in a variety of physiological processes. Production of ROS is continuously balanced by antioxidant (AO) defense mechanisms; however, this delicate balance can be disrupted. Oxidative stress is a physiological condition produced by an imbalance due to excess production of oxidants called ROS and reactive nitrogen species (RNS). When produced in excess, ROS or RNS can interact and damage several cellular membranes and impede normal functioning of the cells. ROS can include a series of oxidants including but not limited to hydrogen peroxide (H O ), peroxyl radical (ROO ), singlet oxygen (1O ) and hydroxyl radical 2 2 • 2 ( OH). The ability to alleviate oxidative stress makes antioxidants an important class of molecules. • Several approaches have been utilized to alleviate oxidative stress including delivery of antioxidants, either small molecules (e.g., vitamin C) or antioxidant enzymes (e.g., superoxide dismutase, catalase) and polymeric antioxidants. Small molecule antioxidants scavenge a variety of Polymers 2020, 12, 1646; doi:10.3390/polym12081646 www.mdpi.com/journal/polymers PolymersPolymers2020 2020, 12, ,12 1646, x FOR PEER REVIEW 2 of2 27 of 27 dismutase, catalase) and polymeric antioxidants. Small molecule antioxidants scavenge a variety of ROSROS and and RNS RNS in in a non-selectivea non-selective fashion fashion andand onlyonly inin stoichiometricstoichiometric ratios. ratios. Hence, Hence, large large sustained sustained dosesdoses are are required required to observe to observe a significant a significant therapeutic therapeutic effect. effect. Antioxidant Antioxidant enzymes enzymes have beenhave explored been asexplored alternatives, as alternatives, but with limitedbut with success, limited success, since they since are they prone are prone to pH to dependentpH dependent inactivation inactivation and proteolyticand proteolytic cleavage. cleavage. Enzyme Enzyme mimics mimics possessing possessing catalytic catalytic activities activities have have been been designed, designed, however however the synthesisthe synthesis protocol protocol often involvesoften involves multiple multiple steps steps [1]. While [1]. While natural natural antioxidants antioxidants are stillare still used used in many in foodmany applications, food applications, use of synthetic use of synthetic AO is controversialAO is controversial due to due toxicity to toxicity concerns. concerns. MixedMixed results results from from decades decades of anti-oxidantof anti-oxidant research research has indicatedhas indicated that forthat an for antioxidant an antioxidant therapy to betherapy efficient, to be three efficient, conditions three conditions need to be need met—(1) to beThe met—(1) antioxidant The antioxidant must be able must to be scavenge able to scavenge the correct radicalsthe correct (2) the radicals radical (2) scavenging the radical mustscavenging be targeted must tobe thetargeted cells duringto the cells the during appropriate the appropriate time (3) AO musttime remain (3) AO active must remain and not active degrade and fornota degrade significant for a amount significant of time. amount In thisof time. context In this the context exploration the of polymersexploration of of phenols polymers as potent of phenols and long-lasting as potent AOand islong-lasting relevant. This AO review is relevant. summarizes This review work in thesummarizes enzymatic syntheseswork in the of polymericenzymatic syntheses phenols for of antioxidantpolymeric phenols applications. for antioxidant Enzymatic applications. methods are discussedEnzymatic in detailmethods to enableare discussed design in of detail high-performance to enable design polymers. of high-performance Relevant assays polymers. for analyzing Relevant AO assays for analyzing AO activity are discussed along with potential pitfalls. AO activity of activity are discussed along with potential pitfalls. AO activity of enzymatically synthesized polymeric enzymatically synthesized polymeric phenols and their analogues are summarized and compared phenols and their analogues are summarized and compared with commercial antioxidants. with commercial antioxidants. Phenolic Antioxidants Phenolic Antioxidants Phenolic compounds are one group of widely studied compounds useful as antioxidants [2–4]. Phenolic compounds are one group of widely studied compounds useful as antioxidants [2–4]. Several hindered phenols used as commercial AO (Figure1) work by donating their phenolic hydrogen Several hindered phenols used as commercial AO (Figure 1) work by donating their phenolic to the generated free radical. Phenoxy radicals generated are resonance stabilized and react with other hydrogen to the generated free radical. Phenoxy radicals generated are resonance stabilized and react freewith radicals. other free Chemical radicals. structure Chemical and structure position and of posi –OHtion groups of –OH influence groups influence the AO activitythe AO activity of phenolic of compounds.phenolic compounds. The antioxidant The antioxidant activity ofactivity phenolic of ph compoundsenolic compounds has been has been studied studied in several in severalin vitroin studiesvitro [studies5,6]. [5,6]. FigureFigure 1. 1.Structures Structures ofof somesome commercially used used antioxidants. antioxidants. However,However, widespread widespread use use of phenol-basedof phenol-based compounds compounds has beenhas limitedbeen limited due to due their to low their solubility low in watersolubility and in poor waterin and vivo poorresults in vivo (low results bioavailability (low bioavailability and physiological and physiological stability, limitedstability, absorption limited andabsorption high doses and required high doses for antioxidant required for activity). antioxid Inant organic activity). chemistry, In organic the chemistry, word “polyphenol” the word is used“polyphenol” to describe is the used presence to describe of two the or morepresence hydroxyl of two groups or more on hydroxyl an aromatic groups compound. on an aromatic However, in publicationscompound. However, involving in synthesis publications of polymeric involving phenols synthesis from of polymeric monomers, phenols the term from “polyphenol” monomers, isthe also employed.term “polyphenol” While this isis notalso completely employed. incorrect, While this this is oftennot completely leads to confusion. incorrect, Throughout this often leads this paper, to polymericconfusion. phenols Throughout will be this used paper, to describe polymeric polymers phenols of will phenols be used synthesized to describe from polymers enzyme of phenols catalysis. synthesized from enzyme catalysis. Polymers 2020, 12, 1646 3 of 27 2. Polymer Antioxidants While small molecule antioxidants mount sufficient defense against many oxidative processes, they have some disadvantages such as leaching which lower the antioxidant performance. This has been compounded with disappointing results from in vivo studies. Interest in use of polymers in antioxidants was spurred by recognition that increasing antioxidant shelf-life in vivo was important to enable these