Photosensitive Cationic Azobenzene Surfactants: Thermodynamics of

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Photosensitive Cationic Azobenzene Surfactants: Thermodynamics of Hydration and the Complex Formation with Poly(methacrylic acid) Maria Montagna, and Olga Guskova Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03638 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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Photosensitive Cationic Azobenzene Surfactants: Thermodynamics of Hydration and the Complex Formation with Poly(methacrylic acid) Maria Montagna∗,† and Olga Guskova∗,†,‡ Institute Theory of Polymers, Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden. E-mail: [email protected]; [email protected]

Abstract In this computational work we investigate the photosensitive cationic surfactants with the trimethylammonium or polyamine hydrophilic head and the azobenzenecontaining hydrophobic tail. The azobenzene-based molecules are known to undergo a reversible trans-cis-trans isomerization reaction when subjected to UV-Visible light irradiation. Combining the density functional theory and the all-atom molecular dynamics simulations, the structural and the hydration properties of the trans- and the cis-isomers and their interaction with the oppositely charged poly(methacrylic acid) in aqueous solution are investigated. We establish and quantify the correlations of the molecular structure and the isomerization state of the surfactants and their hydrophilicity/hydrophobicity and the self-assembling altered by light. For this reason, ∗

To whom correspondence should be addressed Institute Theory of Polymers, Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden. †

‡

Dresden Center for Computational Materials Science (DCMS), Technische Universit¨at Dresden, D-01062 Dresden.

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we compare the hydration free energies of the trans- and the cis-isomers. Moreover, the investigations of the interaction strength between the azobenzene molecules and the polyanion provide additional elucidations of the recent experimental and theoretical studies on the light triggered reversible deformation behaviour of the microgels and the polymer brushes loaded with azobenzene surfactants.

Introduction Functional polymers have received considerable attention in recent years as promising and versatile materials in a large variety of applications covering different fields of biomedicine and nanotechnology. In particular, the stimuli-responsive polymeric chains can switch reversibly between two conformations, globular or swollen one under the action of an external stimulus, e.g. temperature, pH, different fields, including light, which are often used to manipulate sensitive nanostructures. Among these responsive materials, the polymers loaded with the azobenzene-containing molecules are a representative class of the light-driven materials. 1–4 Azobenzene (azo) is an organic photoactive molecule known to undergo a reversible photoisomerization reaction between the trans- (the most stable state) and the cis- (the metastable state) isomers. The trans-cis isomerization occurs upon excitation with UV light while the cis-trans transition is triggered by the irradiation at longer wavelengths or is induced thermally. The photoisomers are associated with largely altered properties, such as geometry, electronic characteristics (frontier orbital levels, band gap, red-ox potentials) and the optical properties. In the light-responsive polymers, the azobenzene derivatives can be dispersed in a matrix 5 or can be chemically or non-covalently attached to the macromolecules as side chains, e.g. through a hydrogen or halogen bonding 6–8 or electrostatically. 9,10 The effect of strength and type of the non-covalent interaction between the azobenzene and the polymer backbone has been shown to play a crucial role in the light-induced manipulation of polymers. 8,9,11 The application of azobenzenes electrostatically attached to a polymer backbone forming 2


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a bottle-brush supramolecular complex, 12,13 is a very promising approach to the formulation of the light-responsive polymers. The most widely used charged azobenzene derivatives are the anionic and the cationic azo surfactants, i.e. amphiphilic molecules containing azobenzene fragment in their hydrophobic tail, which is linked by a short alkyl chain to the charged head group. 9,14–20 First of all, the approach is based on the self-assembly of two oppositely charged components - an azo molecule and a polymer - via multiple weak intermolecular interactions to form a stable yet stimulus-responsive material. Secondly, the charged polymers, i.e. polyelectrolytes, could endow the system with additional features, such as pH- or temperature-dependence, rendering the material excellent dual sensitivity. Finally, the electrostatically driven complex formation of the azo surfactants with different biomacromolecules can be used to actuate by light their secondary and tertiary structures and therefore the biological activity. 21–30 The latter aspect is related to the problem of the molecular design of the azo surfactants for a target application. For example, to improve the interactions with biomacromolecules, the multicharged synthetic polyamine cations are utilised instead of conventional trimethylammonium (TMA) as the polar heads of the surfactants because of their well-known role in regulating important aspects of the DNA physiology, such as conformation, protection and packaging. 31 The polyamine-based azo surfactants mimic the behaviour of natural polyamines (putrescine, spermidine, spermine) and have been used in DNA compaction for the transfection and the gene delivery applications. 32–34 Another object of the molecular tuning of azo amphiphiles is the length of alkyl spacers and the position of the azobenzene fragment in the hydrophobic tail. 35,36 The variations in the non-polar part of the molecule dramatically influence the thermal behaviour, the solubility, the hydrophilic/lipophilic balance and therefore, their self-organised structures and the light-response. Besides the photosensitivity, the azo surfactants possess all the common properties of the conventional surface-active compounds: They lower the surface tension at the interfaces and self-assemble into various micellar aggregates above the critical micelle concentration

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(CMC), and hence are used as foaming and wetting agents, detergents, and emulsifiers. However, through azobenzene fragment incorporated into the hydrophobic tail, the above mentioned processes can be remotely controlled by the light stimulus. For instance, the micelle formation, 37–39 the foam stability and the surface forces, 40–44 the manipulation of complex fluids, the motion of microparticles and the control of interfacial movements 45 are now becoming light-triggered in a modern nanotechnology. Despite the rapid experimental progress, the field of photosensitive azo-containing nanomaterials is still far from being fully unveiled. Further fundamental insights into the structural and energetic properties of the amphiphilic photoisomers, including their light-driven aggregation mechanisms can be accomplished by the methods of computational modelling. Several studies have been published recently in attempt to bridge theories/simulations of the cationic azo surfactants and experimental results. 20,30,38,46–52 The fully atomistic models, for instance, have been applied to understand the photostability of the isolated micelle, 38 to consider the light-controlled unfolding of some protein domains 47 or the DNA-fragment compaction 30,48 and to explain the photoresponsive assembly/disassembly of the host-guest complexes. 50 The coarse-grained simulation models 49,51,53 and the thermodynamic theories 20,37,52,54 have been focused on larger systems aiming at characterization of the trans- and cis-azo surfactant aggregation at various concentrations into spherical, disc-like or worm-like micelles in solutions, on their interfacial behaviour and on the interactions with the oppositely charged polymers and microgels. This work represents the first systematic computational study of two groups of the cationic azobenzene-containing surfactants either with conventional trimethylammonium or with multicharged polyamine polar head. Combining the density functional theory and the all-atom molecular dynamics simulations within the framework of a hierarchical multiscaling scheme, the structural and the energetic properties of the trans- and the cis-isomers and their interaction with the oppositely charged poly(methacrylic acid) (PMAA) in aqueous solution are investigated. We establish and quantify the correlations of the molecular struc-

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ture and the isomerization state of the surfactants and their self-assembling ability altered by light. Using the thermodynamic integration technique, we evaluate the hydrophilicity/hydrophobicity of the trans- and the cis-isomers of each azo surfactant. The comparison of the hydration free energies of the photoisomers provide additional elucidations of the recent experimental and theoretical studies on the light triggered reversible deformation behaviour of the microgels and the polymer brushes loaded with azobenzene surfactants. These data will describe quantitatively the light-controlled hydrophilicity/hydrophobicity changes of the azo amphiphiles. The remainder of this paper is divided into three sections: In section Models and Methods we describe the objects of the study in detail, introduce the simulation models and the methodologies. In section Results and Discussion we report on the structural and energetic properties of the photoisomers and their complexation with PMAA. Finally, in section Conclusions we summarize the main findings of the computational study.

Models and Methods In the theoretical papers mentioned above, 20,30,38,46–52 the light-induced isomerization reaction of the azobenzene fragment itself is not considered, otherwise it would require the application of the quantum-chemical methods, 55,56 but the primary focus lies on the transand the cis-photoisomers and how the differences in the structure affect their properties. Here, we focus on two isomers of each surfactant without modelling the isomerization reaction.

Objects of the Study The first group of compounds consists of six surfactants denoted as AzoC6TMA, AzoC8TMA, AzoC10TMA, AzoC12TMA, Azo-Lee-TMA, Azo-Nitro-TMA, having the same TMA polar head and differing in the length of the hydrophobic tail (see Figure 1). The symbols C6-

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C12 code the number of methylene groups in the alkyl linker between the TMA head and the azobenzene fragment. The abbreviation Azo-Lee-TMA refers to the azo surfactant synthesized by Lee and co-authors. 27 A short title Azo-Nitro-TMA indicates the presence of nitro group NO2 as a substituent of one of the azobenzene rings. For the second group of compounds, three azo amphiphiles with En, Deta and Tren polyamine-based polar heads are modelled, differing in the number of the amino groups and hence in the net charge of the surfactant head. The nomenclature for these molecules is adapted from the publications of Santer and co-authors. 9,17–20,45,52,54 The structural formulae of the cationic surfactants are reported in Figure 1. The panel A shows six TMA surfactants: AzoC6TMA, AzoC8TMA, AzoC10TMA, AzoC12TMA, Azo-Lee-TMA, Azo-Nitro-TMA with Cl− anions. Three polyamine-based pH-sensitive amphiphiles Azo-En, Azo-Deta and Azo-Tren in their non-protonated forms are listed in the panel B. The model for poly(methacrylic acid) represents a residue with one carboxylic group (CH3 )3 C(COO)− . This simplistic model, however, is sufficient to imitate the binding of the surfactant and the polymer fragment and to evaluate the association energies, as has been demonstrated in paper by Nagy and Erhardt. 57 It should also be noted that in fully-atomistic modelling of PMAA, the relatively short oligomers are simulated, and such a polymer representation is also adequate for studying, for example, the conformational transitions associated with the formation of complexes with surfactants. 58 For the visualization of the molecular structures and their analysis, GaussView 59 , Avogadro 60 , QuteMol 61 and VMD 62 software are used. For all the snapshots reported in this paper, we used the following colour scheme: Carbon (C), oxygen (O), nitrogen (N), hydrogen (H) and chlorine (Cl) atoms are shown in grey, red, blue, white and light-green, respectively.

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Figure 1: Objects of the study and their structural formulae. The polyamines are shown in a non-protonated form.

Details of the DFT calculations of the isolated surfactants in implicit water The geometries of all photoisomers depicted in Figure 1 have been optimized without any constraints applying B3LYP functional with 6-31g(d,p) basis set as employed in Gaussian 09 Revision A.01 suite of programs. 63 This combination of the functional and the basis set has been widely and successfully employed in studies of azobenzenes delivering accurate ground state geometries. 64–71 The optimization of the molecular geometries is performed in vacuum for the trans- and cis-states, in their neutral and cationic forms with a tight self-consistent field convergence threshold (10−8 -10−10 a.u.). Further, the solvent effects were taken into account implicitly applying two different polarizable continuum models: the integral equation

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formalism (IEF-PCM/UFF) and the SMD continuum solvation model. 72–74 The vibrational spectra have been analytically determined to confirm that the structure of each isomer in vacuum or in water is a true minimum. The electrostatic potential (ESP) has been calculated at DFT level using the Merz-SinghKollman procedure 75 and mapped onto a surface with an electron density isovalue of 0.02 a.u. The electrostatic potential is defined as the interaction energy between the electrical charge generated from the molecule electrons and a positive point probe located at the position r as reported in Equation 1: N X

zA − V (r) = |R A − r| A=1

Z

0

0

ρ(r )d3 r |r − r0 |

(1)

Here zA is the charge of the nucleus A, which is a point charge located at RA while the term ρ(r) is the electron density function. The sign of the electrostatic potential is correlated to the partial charges on the atoms/atomic groups, i.e. the value at the minimum of V (r) quantifies the electron-rich character of that region, and vice versa. The ESP-derived atomic charges were further used in the all-atom simulations of the surfactant/PMAA complexes and in thermodynamic integration calculations.

Molecular Dynamics Simulations Protocols Classical Molecular Dynamics (MD) simulations were performed using the Universal Force Field (UFF) 76 implemented in Materials Studio 9.0. 77 This force field has been successfully used to model various azobenzene molecules. 78–80 The water molecules were modelled using UFF with TIP3P 81 charges (oxygen -0.834e and hydrogen +0.417e). To account for the van der Waals interactions we used the atom-based summation method and the cubic spline truncation approach with a cut-off at 12.5 ˚ A. For the calculation of the electrostatic interactions, the Ewald summation method was applied. First, the MD simulations were conducted to investigate the complex formation in implicit

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water between the PMAA residue and the cationic surfactant. The procedure consisted of two stages. At the beginning, we considered only the polar heads of the surfactants trimethylammonium or charged polyamines - and the PMAA residue in the simulation box with the edge length of 50 ˚ A. In case of multicharged polyamines, one or two chloride ions were added to ensure the electroneutrality of the system. All systems were equilibrated for 1 ns with a subsequent production run of 10 ns in the NVT ensemble at T =298 K. From the MD trajectories, the complexes have been extracted every 2000 steps and further analysed. All complexes have demonstrated comparable radial distribution functions (RDFs) of the oppositely charged groups, therefore the selected complexes had similar structures and could be used for further DFT analysis. During the second stage of the implicit MD, the surfactant molecules in the trans- or the cis-state have been modelled with the PMAA fragment and Cl− counter-ions for polyamines. Here, the selection of the most probable mutual orientation of the PMAA residue and the surfactant’s polar head becomes a non-trivial task, since the alkyl segments of the non-polar tails are conformationally flexible, and many conformations of the molecules can be realized. Therefore, we have performed additional simulations to extract the most probable structures of the complexes. From the analysis of the RDFs of the negatively and positively charged groups, the most probable orientations have been defined. The representative snapshots were taken from the MD trajectories for subsequent DFT calculations of the association energies ∆E. The latter ones were estimated in a single point energy calculation of the complexes at B3LYP/6-31g(d,p) level using the equation:

∆E = Epair − (Ecat + Eani )

(2)

where E pair is a single point energy of the complex, E cat and E ani are, respectively, the single point energies of the cationic surfactant or its head and the negatively charged PMAA fragment calculated in implicit water.

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The MD simulations of the azo surfactants in explicit water were also performed with the aim to study the hydration properties of the molecules. In this case, the simulations were carried out in a cubic box containing one cationic surfactant and 2000 water molecules. The correct number of Cl− counter-ions was also included to keep the electroneutrality of the systems. Then, the systems were equilibrated during the NPT run at P =1 atm until the density value of 0.9 g/cm3 was achieved (the density of organic systems in water at ambient conditions). Starting from these configurations, the simulations in the NVT ensemble for 1 ns were conducted. Temperature was set at 298 K and controlled by a Nos´e thermostat. The integration step in MD was 1 fs, the coordinates of all atoms were saved every 0.1 ps. The energy and temperature were monitored to conclude about the equilibration of the systems. The RDFs g(r) showing the hydration shells around the solute molecules were analysed. The coordination numbers nc of water in the first solvation shell around the amino groups of polyamines, the TMA head, and the azo group were calculated by integration the g(r) functions from r =0 to the distance corresponding to the first local minimum, r min (see Equation 3): rmin

Z

r2 g(r)dr

nc = 4πρ

(3)

0

The changes in hydration free energies (∆HFE) of the cationic surfactants, i.e. the energy to transfer the amphiphile from its vapour phase (vacuum) to water were obtained by using a thermodynamic integration scheme implemented in Materials Studio 9.0. 77 The first term is the ideal contribution - starting from a surfactant molecule in vacuum, the charges are gradually reduced to zero, whilst keeping all other interactions the same. The second one, called van der Waals term, is calculated when the non-charged amphiphile is coupled to the solvent by switching on the van der Waals interactions. Finally, the electrostatic contribution i.e. the free energy of charging the cavity once it has been placed into solution charges, is calculated. The total ∆HFE is the sum of ideal, van der Waals, and electrostatic terms. The calculation of each contribution consists of a series of molecular dynamics calculations 10


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in which the interaction strength is effectively modified through the coupling parameter λ. 82 The λ value defines the strength of the interaction between the solute and the solvent being represented by a number between 0 and 1, increasing its value by 0.01 for each run (λ=0 means no coupling while λ=1 denotes a fully coupled solute and solvent). Once the systems consisting of water, surfactant in the cis- or trans-state and counter-ions were preequilibrated, the simulation was followed by a new equilibration for 50 ps and productive NVT-MD simulation for 100 ps at each λ value. The ∆HFE (here the Helmholtz free energy) is calculated in each molecular dynamics run. At least three independent calculations of the Helmholtz free energy of hydration were performed for each photoisomer of the surfactants listed in Figure 1. More details on the ∆HFE calculations can be found in paper by Varanasi et al. 83 The changes in the hydration free energies (the Gibbs free energy) were also estimated applying the DFT approach. In this case, the energies have been estimated for all isomers as the differences between the energies of the system in vacuum and in IEF-PCM/UFF or SMD water. Once the ∆HFE values have been collected for the photoisomers, the relative hydration free energies have been estimated to compare the hydrophobicity/hydrophilicity of the surfactant in the trans- and the cis- state:

∆HFErelative = ∆HFEcis − ∆HFEtrans

(4)

These data will allow us to conclude about the hydrophobicity/hydrophilicity of the azobenzene surfactants which can be adjusted by illumination with light of appropriate wavelength and to quantify this difference.

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Results and Discussion Structural Properties of the Azo Surfactants in Implicit and Explicit Water While trimethylammonium azo surfactants always have a positively charged quaternary ammonium head, the polyamines demonstrate the pH-dependent protonation. Here we refer to the experimental set-up and the data from the Ref. 20, where the measurements of photosensitive microgels were conducted at pH=7. Using the empirical Henderson-Hasselbalch equation and the pKa values of the primary, secondary and the tertiary amino groups, the following net charges of the Azo-En, Azo-Deta and Azo-Tren molecules at pH=7 were defined: +1.5 for Azo-En, +2 for Azo-Deta and +3 for Azo-Tren amphiphile. 20 In order to properly simulate the distribution of the point-like charges for the MD simulations, the identification of the protonated amino groups in multi-charged polyamines should be performed. For the Azo-En, having one primary and one secondary amino group, we assume the charge +2, and therefore both groups are protonated (Figure 2A). The Azo-Deta has one primary and two secondary amines and the net charge +2. The Azo-Tren has a net charge +3 and four possible protonation sites: two primary, one secondary and one tertiary group. Therefore, all possible protonated structures for Azo-Deta and Azo-Tren should be calculated, and the structure with the lowest energy will be adopted for the further classical simulations. The most stable protonated Azo-Deta and Azo-Tren heads are reported in Figure 2B,C where added protons are highlighted in orange. As follows from Figure 2, for Azo-Deta and Azo-Tren all primary amines are protonated. Then the protonation affects the secondary groups. The tertiary amine stays non-protonated, which can be related first of all to the steric hindrance of the neighbouring methylene groups, the hydrogen atom experiences being attached to the central nitrogen. Secondly, it can be also due to a strong electrostatic repulsion effect of the equally charged functional groups in a very close proximity to each other. The same behaviour is seen for Azo-Deta, where two protonated sites are distant 12


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Figure 2: The Azo-En (A), Azo-Deta (B) and Azo-Tren (C) heads in their protonated states. The sites of protonation are highlighted in orange. from each other. Finally, such a protonation pattern could also favour the proton-induced cyclization of the chain and the formation of the proton-bridged structures. 84 The maps of the ESP are presented in Supporting Information (Figure S1), showing the positively charged polyamine-based heads which bind to the polyanion. A special case here is AzoDeta, which has not only the largest concentration of the positive charges at the periphery of the head, similarly to Azo-En and Azo-Tren, but has clearly distinguished non-protonated amino group between two protonated ones. It is worth noting here, that non-protonated amino groups of Azo-Deta and Azo-Tren may still play a role as a donor of the electronic density forming classical hydrogen bonding (HB) with water molecules or the carboxylic groups of PMAA. The analysis of the azo -N=N- bond lengths and the dihedral angles -CH2 -N=N-CH2 showed that the lengths and the torsions are not sensitive to the variations in the molecular structure, being 1.265 ˚ A, 1.255 ˚ A and 179.8°, 10.7° for the trans- and the cis-isomers, respectively. The values of the bond lengths and the dihedral angles correspond to the simulation results of other azobenzene compounds in water. 71 The optical properties of the surfactants and the comparison of calculated and measured UV-Vis spectra are described in Supporting Information. In order to a priori determine the shapes of the micellar aggregates of the azo surfactants and their self-assembling ability in water, the critical packing parameter (CPP) is calculated

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according to Israelachvili’s packing rule: 85,86

CPP =

V A·l

(5)

where V is the volume of the hydrophobic tail, A is the area of the polar head, and l is the chain length of the tail (Figure 3). For a typical surfactant in aqueous solutions, the value CPP< 1/3 indicates the presence of spherical micelles. The range 1/3

Photosensitive Cationic Azobenzene Surfactants: Thermodynamics of

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