Chiral Nanostructures from Helical Copolymer-Metal Complexes: Tunable Cation-π Interactions and Sergeants and Soldiers Effect

Significance Statement

Dynamic helical polymers such as poly(phenylacetylene)s (PPAs) are an important source of chiral nanomaterials because their helices can be tuned once they have been prepared, and therefore helical structures with either P or M helical sense or elongated/compressed helical scaffolds become available. In this way, novel materials with intrinsic properties (e.g., chiral sensors, asymmetric synthesis, chiral separations, etc.) connected to their helical structure can be generated.

Recently, we have developed a chiral helical poly(phenylacetylene) that although bears a chiral pendant —the anilide of (R)-a-methoxy-a-phenylacetic acid, (R)-MPA—, produces an equal population of P and M helices in equilibrium and therefore, it can be considered as axially racemic. Nevertheless, this racemic helix can be shifted to the left sense by addition of tiny amounts of monovalent metal ions such as Na+ or Ag+. However, if the metal ion is delivered in the presence of a donor cosolvent such as MeOH or MeCN, or these cosolvents are added once the helical polymer metal complex (HPMC) is formed, the polymer evolves to a right handed helical sense. Thus, using Ag+/MeCN or Na+/MeOH as external stimuli, the originally racemic helix is selectively converted into a P or an M helix. Moreover, when the polymer/metal ratio is appropriate, the metal ion acts also as a crosslinking agent of the polymer chains producing nano-aggregates with precise macroscopic P or M helicity.

In this paper we show that the combination of these concepts with the Sergeants and Soldiers effect allows to extend those results to different copolymers and therefore, to the preparation of a wide scope of macroscopically chiral nanostructures with different functionalities and P or M helical senses.

The mechanistic explanation is based on the addition of Ag+ or Na+ ions that are complexed by the scarce (R)-MPA units in the copolymer (minor component). The Sergeant character acquired by those units command the adoption of either the P or the M helix in the rest of the chain formed by the non-chiral monomers (i.e., the Soldiers), depending on the amount of MeOH or MeCN employed.

Chiral Nanostructures from Helical Copolymer-Metal Complexes. Advances in Engineering

 

About the author

Prof. Ricardo Riguera is full Professor in Organic Chemistry at the Universidad de Santiago de Compostela (Spain) and PI at the CIQUS Research Center of the USC. He has authored more than two hundred papers covering Bioactive Natural Products, Medicinal Chemistry and NMR Methods for determination of Absolute Configuration. In the last few years he has concentrated on Polymeric Nanostructures for Biomedical Applications and on Stimuli Responsive Dynamic Helical Polymers.

As academic, he has been Director of Department, Dean of the Faculty of Chemistry and Vice Chancellor of the University. In 2013 he was awarded the Félix Serratosa Medal by the Real Sociedad Española de Química.

 

About the author

Dr. Felix Freire obtained his B. Sc. (2000), M. Sc. (2002) and Ph. D. (2005) degrees in Chemistry from the University of Santiago de Compostela (USC) working on determining the absolute configuration of polyfunctional compounds by NMR. During 2005-2008 he performed two postdoctoral stays at Prof. Jesús Jiménez Barbero  (CSIC-Madrid) and Prof. Samuel H. Gellman (Univ. of Wisconsin-Madison) groups interested in the folding of biomolecules. Since 2009 he works at the University of Santiago as “Ramón y Cajal” researcher interested in stimuli-responsive polymers, polymer self-assembly and chiral polymer particles. 

About the author

Prof. Emilio Quiñoá obtained his Ph.D. degree in Chemistry at the University of Santiago de Compostela in the subject of Marine Natural Products. After a postdoctoral stay at the University of California (Santa Cruz) with Prof. Phillip Crews, he became Associate Professor (1989) and later Full Professor in Organic Chemistry (1998). His research addressed the development of Methods for the Assignment of Absolute Configuration by NMR. He has coauthored more than 100 scientific publications, a number of textbooks and patent applications, and has received several research awards. In 2010 he joined the Center for Research in Biological Chemistry and Molecular Materials (CIQUS) at USC. His current research interests are focused on the study of Dynamic Helical Polymers and their applications in Nanotechnology.  

About the author

Dr. Julian Bergueiro received his BS and MS in Chemistry from the University of Santiago de Compostela (Spain) in 2007 and 2008, respectively. In 2013 he received his PhD from USC under the supervision of Prof. S. Lopez. In 2012 he joined the group of Prof. R. Riguera to work on the synthesis and characterization of stimuli-response helical polymers and poly(phenylacetylene)[email protected] nanoparticle nanocomposites.

He joined Prof. M. Calderón group in 2013 to carry out his postdoctoral research at Freie Universität Berlin. Recently he was awarded with a Dahlem International Network Postdocs Fellowship to develop gold-based thermoresponsive nanogels as nanocarriers based on supramolecular interactions in collaboration with Prof. T. Aida group in Tokyo University. 

About the author

Sandra Arias obtained her B. Sc. (2012) and M. Sc. (2013) degrees in Chemistry from the University of Santiago de Compostela (USC). Currently she is developing her Ph.D in Nanobiomol group at the Center for Research in Biological Chemistry and Molecular Materials (CIQUS) at USC. In 2015 she did a predoctoral stay in the group of Prof. Marcelo Calderón at Freie University, where she worked in “Chiroplasmonic Thermoresponsive Nanogels Based on Elastine Based Helical Polymers and Non Spherical Gold Nanoparticles for Biomedical Application”. Since 2015 she is been awarded with a Gil Dávila fellowship to continue with her research in Dynamic Helical Polymers. 

Journal Reference

Small. 2016 Jan; 12(2):238-44. 

Arias S, Bergueiro J, Freire F, Quiñoá E, Riguera R.

Centre for Research in Biological Chemistry and Molecular Materials (CIQUS), University of Santiago de Compostela, Santiago de Compostela, Spain.

Abstract

Poly(phenylacetylene) (PPA) copolymers containing (R)- or (S)-MPA as minor chiral pendant can be forced to selectively adopt the right- o left-handed helix, in the presence of small amounts of Na(+) or Ag(+) (“Sergeants and Soldiers Effect”) by addition of a donor cosolvent. The helical sense depends exclusively on the chiral monomer/donor cosolvent ratio, and this allows a perfect on/off tuning of the helicity of the copolymer. When the amount of the donor cosolvent is low, the metal ion complex is stabilized by a cation-π interaction, which is selectively cleaved when the amount of cosolvent is higher. Macroscopically chiral nanospheres and nanotubes composed by helical copolymers with P or M helical sense are also described. Our results demonstrate that it is possible to obtain the two enantiomeric helical structures (P and M helicities) and the corresponding nanospheres and nanotubes from a single helical copolymer, by controlled activation/deactivation of the Sergeant and Soldiers Effect with a donor cosolvent.

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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