processes Article Adsorption/Desorption Characteristics and Simultaneous Enrichment of Orientin, Isoorientin, Vitexin and Isovitexin from Hydrolyzed Oil Palm Leaf Extract Using Macroporous Resins Mohamad Shazeli Che Zain 1 , Soo Yee Lee 1 , Chian Ying Teo 2,* and Khozirah Shaari 1,* 1 Natural Medicines and Products Research Laboratory (NaturMeds), Institute of Bioscience, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; [email protected] (M.S.C.Z.); [email protected] (S.Y.L.) 2 Department of Pharmaceutical Chemistry, School of Pharmacy, International Medical University, Bukit Jalil 57000, Kuala Lumpur, Malaysia * Correspondence: [email protected] (C.Y.T.); [email protected] (K.S.); Tel.: +60-3-27317742 (C.Y.T.); +60-3-8942148 (K.S.) Abstract: Oil palm leaves (OPL) containing flavonoid C-glycosides are abundantly generated as oil palm byproducts. The performances of three macroporous resins with different physical and chemical properties for the enrichment of isoorientin, orientin, vitexin, and isovitexin from acid- hydrolyzed OPL (OPLAH) extract were screened. The XAD7HP resin exhibited the best sorption capacities for the targeted flavonoid C-glycosides and was thus selected for further evaluation. Static adsorption using the XAD7HP resin under optimal conditions (extract adjusted to pH 5, Citation: Che Zain, M.S.; Lee, S.Y.; shaken at 298 K for 24 h) gave adsorption kinetics that fit well with a pseudo-second-order kinetic Teo, C.Y.; Shaari, K. Adsorption/ model. The adsorption of isoorientin and orientin was well described by Langmuir isotherms, while Desorption Characteristics and vitexin and isovitexin fit well with the Freundlich isotherms. Dynamic sorption trials using the Simultaneous Enrichment of Orientin, column-packed XAD7HP resin produced 55–60-fold enrichment of isovitexin and between 11 and Isoorientin, Vitexin and Isovitexin from Hydrolyzed Oil Palm Leaf 25-fold enrichment of isoorientin, vitexin, and orientin using aqueous ethanol. The total flavonoid Extract Using Macroporous Resins. C-glycoside-enriched fractions (enriched OPLAH) with isoorientin (247.28–284.18 µg/mg), orientin Processes 2021, 9, 659. https:// (104.88–136.19 µg/mg), vitexin (1197.61–1726.11 µg/mg), and isovitexin (13.03–14.61 µg/mg) showed doi.org/10.3390/pr9040659 excellent antioxidant free radical scavenging activities compared with their crude extracts, with IC50 values of 6.90–70.63 µg/mL and 44.58–200.00 µg/mL, respectively. Hence, this rapid and efficient Academic Editor: Zongbi Bao procedure for the preliminary enrichment of flavonoid C-glycosides by using macroporous resin may have practical value in OPL biomass waste utilization programs to produce high value-added Received: 12 March 2021 products, particularly in the nutraceuticals, cosmeceuticals, pharmaceuticals, and fine chemicals Accepted: 7 April 2021 industries. Published: 9 April 2021 Keywords: oil palm leaves; total flavonoid C-glycosides; acid hydrolysis; macroporous resin; enrichment; Publisher’s Note: MDPI stays neutral antioxidant free radical scavenging activity with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction The oil palm (Elaeis guineensis Jacq.) tree was introduced into Malaysia in 1875, with the first oil palm tree plantation established at Tennamaran Estate in Kuala Selangor [1]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Fueled by full support from Malaysian government agricultural diversification initiatives, This article is an open access article palm oil plantations expanded tremendously and now cover a large acreage of agricultural distributed under the terms and land areas [2]. Currently, Malaysia is the second-largest oil palm producer in the world after conditions of the Creative Commons Indonesia [3]. In fact, the latest statistics show that Malaysia was reaching 20 million tons of Attribution (CC BY) license (https:// crude palm oil production in 2020 [4]. However, in the wake of this massive cultivation, a creativecommons.org/licenses/by/ huge amount of oil palm biomass is generated as agricultural waste. Apart from mesocarp 4.0/). fibers (MF), empty fruit bunches (EFB), and palm kernel shells (PKS) from downstream Processes 2021, 9, 659. https://doi.org/10.3390/pr9040659 https://www.mdpi.com/journal/processes Processes 2021, 9, x FOR PEER REVIEW 2 of 18 Processes 2021, 9, 659 2 of 18 cultivation, a huge amount of oil palm biomass is generated as agricultural waste. Apart from mesocarp fibers (MF), empty fruit bunches (EFB), and palm kernel shells (PKS) from downstreamprocessing in processing oil palm mills, in oil in parallel,palm mills, oil palm in parallel, trunks (OPT),oil palm oil trunks palm fronds (OPT), (OPF), oil palm and frondsoil palm (OPF), leaves and (OPL) oil palm were leaves also generated, (OPL) were presenting also generated, a huge presenting environmental a huge problem environ- if mentalleft unutilized problem [5 if]. left unutilized [5]. LikeLike many other speciesspecies inin thethe plantplant kingdom, kingdom, OPL OPL byproduct byproduct is is an an excellent excellent source source of ofphytochemicals phytochemicals which which could could be usedbe used for somefor some applications. applications. OPL OPL have have in fact in been fact reportedbeen re- portedto contain to contain bioactive bioactive compounds compounds that are that responsible are responsible for various for various medicinal medicinal properties, proper- such ties,as treating such as kidney treating diseases, kidney cancer,diseases, cardiovascular cancer, cardiovascular diseases, anddiseases, wounds and[ 6wounds]. A previous [6]. A previousstudy on study OPL revealed on OPL therevealed presence the ofpresence both flavonoid of both Oflavonoid- and C-glycosides O- and C-glycosides [7]. In general, [7]. Inflavonoid general,C flavonoid-glycosides C are-glycosides not widely are present not widely in plants, present and in due plants, to this, and they due have to this, received they haveless attentionreceived less in comparison attention in with comparison their O -glycosylwith their counterparts. O-glycosyl counterparts. Nevertheless, Neverthe- several less,recent several biological recent and biological pharmacological and pharmacological studies have studies shown have that shown flavonoid that Cflavonoid-glycosides C- glycosidesalso possess also a wide possess spectrum a wide of spectrum biological of properties, biological which properties, include which anticancer, include hepato- anti- cancer,protective, hepatoprotective, antioxidant, and antioxidant, antidiabetic and properties antidiabetic [8]. FlavonoidpropertiesC [8].-glycosides Flavonoid differ C-glyco- from sidesflavonoid differO from-glycosides flavonoid in thatO-glycosides they are more in that resistant they are to more hydrolysis, resistant since to hydrolysis, the aglycone since is thelinked aglycone to the anomericis linked to carbon the anomeric of the sugar carbon moiety of the via sugar an acid-resistant moiety via an C-C acid-resistant bond. Figure C-1 Cshows bond. the Figure structures 1 shows of four the flavonoidstructuresC of-glycosides four flavonoid of OPL, C-glycosides which include of OPL, orientin, which isoori- in- cludeentin, orientin, vitexin, andisoorientin, isovitexin. vitexin, These and flavonoid isovitexin.C-glycosides These flavonoid were presentC-glycosides in considerable were pre- sentamounts in considerable in comparison amounts with in other comparison luteolin andwith apigenin other luteolin derivatives and apigenin in OPL derivatives [7,9–12]. It C inis worthOPL [7,9–12]. noting thatIt is worth the global noting demand that the for global flavonoids, demand including for flavonoids, flavonoid including-glycosides, flavo- is forecasted to reach USD 1.2 billion by 2024 [13]. Therefore, their presence in a widely noid C-glycosides, is forecasted to reach USD 1.2 billion by 2024 [13]. Therefore, their pres- available and abundant biomass material warrants further investigation into the develop- ence in a widely available and abundant biomass material warrants further investigation ment of efficient methods for the preparative purification for downstream purposes and into the development of efficient methods for the preparative purification for downstream applications. purposes and applications. Figure 1. (A) Structures of C-glycosyl flavonoids identified in oil palm leaves (OPL): orientin (1), vitexin (2), isoorientin Figure 1. (A) Structures of C-glycosyl flavonoids identified in oil palm leaves (OPL): orientin (1), vitexin (2), isoorientin (3), (3), isovitexin (4). (B) Structure of the XAD7HP resin. R = polyfunctional aliphatic residue. isovitexin (4). (B) Structure of the XAD7HP resin. R = polyfunctional aliphatic residue. UtilizationUtilization of of macroporous macroporous resins (MARs) in separating and purifying the flavonoid flavonoid C-glycosides-glycosides present present in in plant plant extracts extracts has has been been pr practicedacticed in in recent recent years. years. It It offers offers an an alter- alter- nativenative to conventional methods, which often start with solid-liquid extraction, followed byby liquid-liquid extraction and eventually column chromatography [14]. [14]. These These conven- conven- tionaltional approaches approaches are are not not only only time-consuming time-consuming and and inefficient, inefficient, but they also require high consumptionconsumption of of solvents and and energy [15,16]. [15,16]. The chemical nature of MARs