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Chiral Separation of Rac-Propylene Oxide on Penicillamine Coated Gold Nps

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Chiral Separation of Rac-Propylene Oxide on Penicillamine Coated Gold Nps nanomaterials Article Chiral Separation of rac-Propylene Oxide on Penicillamine Coated Gold NPs Nisha Shukla 1,2, Zachary Blonder 2 and Andrew J. Gellman 2,3,* 1 Institute of Complex Engineered Systems, Carnegie Mellon University, Pittsburgh, PA 15213, USA; [email protected] 2 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA; [email protected] 3 W.E. Scott Institute for Energy Innovation, Carnegie Mellon University, Pittsburgh, PA 15213, USA * Correspondence: [email protected] Received: 16 July 2020; Accepted: 20 August 2020; Published: 30 August 2020 Abstract: The surfaces of chemically synthesized spherical gold NPs (Au-NPs) have been modified using chiral L- or D-penicillamine (Pen) in order to impart enantioselective adsorption properties. These chiral Au-NPs have been used to demonstrate enantioselective adsorption of racemic propylene oxide (PO) from aqueous solution. In the past we have studied enantioselective adsorption of racemic PO on L- or D-cysteine (Cys)-coated Au-NPs. This prior work suggested that adsorption of PO on Cys-coated Au-NPs equilibrates within an hour. In this work, we have studied the effect of time on the enantioselective adsorption of racemic PO from solution onto chiral Pen/Au-NPs. Enantioselective adsorption of PO on chiral Pen/Au-NPs is time-dependent but reaches a steady state after ~18 h at room temperature. More importantly, L- or D-Pen/Au-NPs are shown to adsorb R- or S-PO enantiospecifically and to separate the two PO enantiomers from racemic mixtures of RS-PO. Keywords: chiral; nanoparticles; enantioselective adsorption; penicillamine; cysteine 1. Introduction Chirality has attracted enormous interest in the field of chemistry due to the enantiospecific responses of living organisms to the two enantiomers of chiral compounds that they have ingested. These differences in response are important in pharmaceutical industries because it is often the case that only one enantiomer of a chiral drug is therapeutically active. While one enantiomer of a drug can treat disease, the other enantiomer can have fatal or long-lasting side effects. Consequently, chiral pharmaceuticals that are produced synthetically must be prepared in enantiomerically pure form. The same is true of other chiral bio-active chemicals such as agrochemicals, pesticides and flavors. In recent years, researchers have also focused on the role of chirality in other areas such as catalysis [1], chiroptical applications [2,3], chiral magnetism [4], chiral semiconductors [5], cellular imaging [6], sensing [7], biomedical applications [8], chiral molecular assembly on surface [9], supramolecular chirality [10] and synthesis of chiral nanoparticles (NPs) [11]. One emerging field is chiral luminescent nanomaterials which have potential applications in chiral sensing [7]. Although a number of these studies have required synthesis of nanomaterials, it is still challenging to synthesize well-defined, monodispersed and inherently chiral NPs. In previous work, we have shown that L- and D-cysteine (Cys, HO2CCH(NH2)CH2SH) modified surfaces of Au-NPs are capable of enantiospecific adsorption of racemic propylene oxide (PO) [11,12] and the chiral pharmaceutical propranolol hydrochloride [13]. In these studies, Au-NPs were either spherical or tetrahexahedral (24-sided) in shape. A systematic study has also been performed to understand the effects of temperature and of Au-NP size on enantiospecific adsorption from racemic Nanomaterials 2020, 10, 1716; doi:10.3390/nano10091716 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1716 2 of 8 Nanomaterials 2020, 10, x 2 of 9 PO [[14].14]. These These studies studies made made use use of of optical polarimetry as a means of detectingdetecting enantioselectiveenantioselective adsorption and used a model developed by us to to analyze analyze optical optical rotatio rotationn measurements measurements and quantify enantiospecificenantiospecific adsorption equilibrium constants.constants. In this work, we havehave usedused sphericalspherical Au nanoparticles,nanoparticles, which were synthesized based on our previous published work [[11].11]. We We chose to use spherical Au nanoparticlesnanoparticles based on our previousprevious work [[14]14],, which showed that the size of Au-NPsAu-NPs has an influenceinfluence on their adsorption of chiral compounds andand subsequent subsequent optical optical rotation. rotation. Smaller Smaller sized sized chiral chiral Au-NPs Au yield-NPs higheryield opticalhigher rotationoptical duringrotation exposure during exposure to racemic to mixtures racemic ofmixtures chiral probe of chiral molecules. probe molecules. This indicates This that indicates smaller that chiral smaller NPs yieldchiral moreNPs yield effective more enantiomer effective enantiomer separations separations than large than NPs, large presumably NPs, presumabl as a resulty as a of result their of higher their surfacehigher surface area. In area. comparison In comparison to our to past our work, past work, we have we developed have developed this work this inwork two in directions. two directions. First, weFirst, have we changed have changed the chiral the surface chiral modifier surface used modifier to render used the to surfaces render of the Au-NPs surfaces enantioselective. of Au-NPs Dienantioselective.fferent chiral ligands Different have chiral diff ligandserent optical have rotationsdifferent optical and di ffrotationserent capacities and different for adsorption capacities and for interactionadsorption withand interaction chiral probe with molecules. chiral probe We have molecules. substituted We the have D- /substitutedL-Cys (Figure the1a) D used-/L-Cys as a(Figure chiral surface1a) used modifier as a chiral in our surface past work modifier [11–13 ]in with our D- past/L-penicillamine work [11–13] (Pen, with HO D2-CCH(NH/L-penicillamine2)C(CH3 )(Pen,2SH) (FigureHO2CCH(NH1b). In our2)C(CH notation,3)2SH) D-(Figure/L-denotes 1b). In either our notation, pure enantiomer, D-/L- denotes while either DL-denotes pure enantiomer use of the racemic, while mixture.DL- denotes Penicillamine use of the was racemic chosen mixture. as chiral Penici modifierllamine because was itschosen chemical as chiral functionality modifier and because structure its arechemical similar functionality to those of and Cys. structure Both are are amino similar acids to those with alkylof Cys. thiol Both side are groups. amino acids Cys iswith one alkyl of the thiol 20 naturallyside groups. occurring Cys is chiralone of amino the 20 acidsnaturally that isoccurring polar in naturechiral amino and is theacids only that naturally is polar occurringin nature aminoand is acidthe only with naturally a thiol group occurring (-SH) [amino15]. Adsorption acid with ofa thiol Cys on group Au surfaces (-SH) [15]. is dependent Adsorption on of the Cys pH on of theAu solutionsurfaces is [16 dependent]. At an acidic on the pH, pH Cys of the bonds solution to the [16]. Au At surface an acidic mainly pH, viaCys deprotonation bonds to the Au of thesurface -SH bond.mainly Bonding via deprotonation via the carboxylic of the -SH acid bond. group Bonding or amine via group the ca isrboxylic minimal, acid although group theor amine –NH2 group is + + convertedminimal, although to –NH3 the. On –NH the2 othergroup hand, is converted in alkaline to – pH,NH3 Cys. On still the bonds other with hand, the in Au alkaline surface pH, mainly Cys viastill -SH bonds bond with deprotonation the Au surface but alsomainly via via the – –NHSH 2bondgroup. deprotonation In alkaline pH, but the also carboxyl via the group–NH2 isgroup. ionized, In andalkaline some pH, studies the carboxy suggestl group that it is ionized aligned, paralleland some to thestudies Au surface.suggest Somethat it DFTis aligned studies parallel suggest to that the CysAu surface. can bond Some with DFT the Austudies surface suggest via both that thiolateCys can and bond amino-thiolate with the Au modes.surface Thevia both bond thiolate strength and of N–Auamino- andthiolate S–Au modes. were calculated The bond tostrength be 6 and of 47N– kcalAu/ andmol, S respectively–Au were calculated [17]. Another to be di6 ffanderence 47 kcal/mol, between Penrespectively and Cys is[17]. their Another specific difference optical rotation. between The Pen specific and optical Cys is rotation their specific of Pen inoptical water rotation. is ~9 higher The × thanspecific that optical of Cys. rotation Some studies of Pen in have water shown is ~9 that× higher Pen isthan a strong that of chelating Cys. Some agent studies and bonds have shown with many that metals,Pen is a particularlystrong chelating with agent Cu [18 and–20 ].bonds with many metals, particularly with Cu [18–20]. a) b) Figure 1. The molecular structures of (a) L-Cys and (b) L-Pen differ only in the addition of two methyl Figure 1. The molecular structures of (a) L-Cys and (b) L-Pen differ only in the addition of two methyl groups to the terminal carbon. groups to the terminal carbon. The second direction in which this work expands on our prior accomplishments is by documenting The second direction in which this work expands on our prior accomplishments is by the effect of adsorption time on enantioselective adsorption of racemic RS-PO on chirally modified documenting the effect of adsorption time on enantioselective adsorption of racemic RS-PO on Au-NPs. In the past, enantioselective adsorption was studied within one hour of exposure of the chiral chirally modified Au-NPs. In the past, enantioselective adsorption was studied within one hour of NPs to the chiral probe PO [11–14]. Here, we find that the equilibration of PO on Pen/Au-NP requires exposure of the chiral NPs to the chiral probe PO [11–14]. Here, we find that the equilibration of PO ~18 h of exposure. Our work shows that when using Pen as a chiral ligand, only weak enantiomeric on Pen/Au-NP requires ~18 h of exposure. Our work shows that when using Pen as a chiral ligand, separation of PO is observed within one hour of PO exposure; however, there is an almost ten-fold only weak enantiomeric separation of PO is observed within one hour of PO exposure; however, increase in enantiomeric separation after 18 h of PO exposure.
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