CAN GRAPHENE BILAYERS BE the MEMBRANE MIMETIC MATERIALS? ION CHANNELS in GRAPHENE-BASED NANOSTRUCTURES Oleg V
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154 NANOSYSTEMS CAN GRAPHENE BILAYERS BE THE MEMBRANE MIMETIC MATERIALS? ION CHANNELS IN GRAPHENE-BASED NANOSTRUCTURES Oleg V. Gradov Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, http://www.inepcp.ru 117829 Moscow, Russian Federation [email protected] Margaret A. Gradova Semenov Institute of Chemical Physics, Russian Academy of Sciences, http://www.chph.ras.ru 119991 Moscow, Russian Federation Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, http://www.inepcp.ru 117829 Moscow, Russian Federation [email protected] Abstract. The prospects of application of graphene and related structures as the membrane mimetic materials, capable of reproducing several biomembrane functions up to the certain limit, are analyzed in the series of our papers. This paper considers the possibility of the ion channel function modeling using graphene and its derivatives. The physical mechanisms providing selective permeability for different membrane mimetic materials, as well as the limits of the adequate simulation of the transport, catalytic, sensing and electrogenic properties of the cell membrane ion channels using bilayered graphene-based structures are discussed. Keywords: ion channels, graphene, channel mimetics, nanopores, kapillarchemie, size-dependent electrochemistry, electric double-layer capacitor, ISFET & ChemFET PACS: 81.05.ue Bibliography - 171 references Received 14.11.2016 RENSIT, 2016, 8(2):154-170 DOI: 10.17725/rensit.2016.08.154 CONTENTS analogical functional similarity criteria for the 1. INTRODUCTION (154) analysis of possibility of consideration of 2. SEMIPERMEABILITY AND THE ION CHANNELS (154) graphene, particularly the two-layer graphene as 3. PORES AND ION CHANNELS IN GRAPHENE-BASED the membrane mimetic material - the subject of NANOSTRUCTURES (158) this work, with a focus on the analysis of it key 4. CONCLUSION (161) property – permeability for ions, water and other REFERENCES (161) organic matter. 1. INTRODUCTION 2. SEMIPERMEABILITY AND THE ION In [1] provides an overview of membrane CHANNELS models – semi-synthetic, synthetic, biogenic, A well known flexibly adjustable semipermeability semiconducting, superconducting, ferroelectric towards various agents is characteristic for the - and the related membrane mimetic materials graphene-oxide membranes in organic solvents, from phospholipid bilayers and Langmuir films with the size of the nanochannels providing to multilayer nanofilms and ferromagnetic either transport or blocking of several agents, structures, along with the consideration of the which can be narrowed down by the thermal problems of ambiguity models, completeness annealing or extended by solvation, thereby the membrane mimetic materials modeling, changing the boundary of the transmitted functional and substrate equivalence of the agent’s size selectivity [2]. This corresponds to membrane mimetic materials. The use of No. 2 | Vol. 8 | 2016 | RENSIT CAN GRAPHENE BILAYERS BE THE MEMBRANE 155 NANOSYSTEMS MIMETIC MATERIALS? ION CHANNELS... the well known models of the “size / solubility” Thus, it is possible to work out the criteria – regulation of the sodium ion channels [3] and for verification of the biomimetic nature of good approximations of the kinetic regimes the membrane mimetic models, based on the depending on the ionic radius [4]. As a particular idea about the gradations of the ion channel case of the applicability of the quantitative efficiency according to the conductivity and relations between the structure and biological bioelectric response generated, rather than on activity (QSAR) [5], the analysis principles for the conventional statement about the discrete the functional adjustability of the ion channels transition between the excited states («all-or- by the sink parameters, with the membrane none law» [11, 12]), typical for the deterministic pores corresponding to the drains, and the size model of the electrophysiological potential of particles penetrating through those pores, in generation. the case of the graphene layers fail to describe It is reasonable, since it has been shown the membrane permeability, since the membrane earlier that the ion channel states (which are the considered performs a biological function despite prototypes of the membrane mimetic, or even the fact that its chemical composition is far from channel mimetic models [13-15]) with a low ion bioorganic one, and hence, is not included into conductivity in fact are not “closed”, but are the ion channel databases [6] which could be changing within several gradations providing used to obtain QSAR data. qualitatively (“ion-selectively”) and quantitatively It is noteworthy that similar problems with (according to the permeability) distinguishable the QSAR approaches earlier emerged in the types of the membrane-electrophysical or analysis of the lipid nanopores operating as the electropysiological response, which is the ion channels of the cell membranes without any main cause for the specific channelome noises conventional ion channel components [7] due at the patch-clamp registrograms [16]. Of to their size corresponding to the ion radius. course, this does not mean the close similarity (Typically even at the dependence of the several between the biological and synthetic membrane ion channel group input on the ionic radius structures or the possibility to reduce the complex biochemical and crystallographic channelome molecular machinery to the models are applied up to the homohexamer one simple porous structure, but it indicates the [8], i.e. compatible with the QSAR principles general operation principles of the non-specific based on the analysis of the conformation physical mechanisms at the nanoscale where and steric accessibility of the channelome the size effects are physically-determined and biomacromolecules). chemically-independent, i.e. are little if any In connection with the above contradiction influenced by the chemical composition of the it is also worth to mention the known data medium and macromolecular morphology of on the ionic permeability of the synthetic the biopolymers (in a conventional meaning filters which have never been analyzed using of this term introduced by P.J. Flory [17, 18], a QSAR methods for the similar reasons: it Nobel prize winner and the author of the Flory- was shown that the ions penetrating through Huggins equation which is formally similar to the synthetic PET filters are selected by the the Van der Waals equation, and hence, can be mechanisms providing the biomimetic and applied at the physical scale considered in this membrane mimetic kinetics with the discrete paper). fast transitions between the conductivity levels For the above reasons, biomimetic and parametric selectivity of the ion fluxes, as interpretation and assigning of the specific well as inhibition by divalent cations, similar to membrane properties to graphene and the cell ion channels [9, 10]. other layered membrane mimetic materials RENSIT | 2016| VOL. 8 | No. 2 156 OLEG V. GRADOV, MARGARET A. GRADOVA NANOSYSTEMS with the pore size corresponding to the ion different gases [25] and electrochemical radii, although does not contradict to the properties of the nanoelectrode graphene layers experimental data, in fact is a logical error of the [25] also depend on the pore diameter. The inversion type, arising from the mixing of the above phenomenon known as “size-dependent deductive and inductive predicates: graphene electrochemistry” is a conceptual continuation and the cell membrane with the similar pore of the Freundlich’s “Kapillarchemie”. This size obviously subject to the same physical and approach is widely applicable not only in the chemical principles (which is manifested in chemistry of the carbon structures, but also the similarity of the solvation regulation and in the nanostructural materials science as a the electrophysical response), and hence, in whole. this aspect “graphene can be considered as a Regarding the carbon structures related to membrane mimetic material to the same extent graphene, nanoelectrochemistry based on the as the biological membrane can be considered size effects was applied in the studies on the as a graphene-mimetic”. multilayered carbon nanotubes [27] and a fully This suggests that the similarity problem or partially reduced graphite oxide [28] which (including the above mentioned QSAR) for such is an electrode material for electrochemical structures, if considered from the standpoint double-layer capacitors [29] often considered of molecular biology rather than biophysical as the biological membrane bilayer model [30- and biocolloid chemistry, becomes incorrect. 34], as well as the layered supercapacitors of According to the classical colloid and capillary EDLC-type (“electric double-layer capacitor”) chemistry principles [19, 20] it is possible to with the pronounced relation between the ion determine the efficiency limits of the sorption and pore sizes [35] similar to the biological processes within the pores and channels of the membranes and membrane mimetic materials particular diameter, which is usually applied in considered above. By the way, this is a reduced cytophysiology [21] (the Freundlich adsorption graphite oxide which is used as an electrode isotherm [22] is named after Herbert Freundlich material in such supercapacitors [36], allowing – the founder of the capillary chemistry and a the EDLC-based membrane models to mimic pioneer of the capillary-chemical approaches not only the energy storage