Simulation of DBS, DBS-COOH, and DBS-CONHNH2 as Hydrogelators

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Simulation of DBS, DBS-COOH and DBS-CONHNH as Hydrogelators Dafna Knani, and David Alperstein J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11130 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017

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The Journal of Physical Chemistry

Simulation of DBS, DBS-COOH and DBS-CONHNH2 as Hydrogelators Dafna Knania* , David Alpersteinb* a

*

Department of Biotechnology Engineering, ORT Braude College, P.O. Box 78, Karmiel 2161002, Israel. b Department of Mechanical Engineering, ORT Braude College, P.O. Box 78, Karmiel 2161002, Israel.

E-mail: [email protected]., *E-mail: [email protected] ABSTRACT

The organic gelator 1,3(R):2,4(S)-dibenzylidene-D-sorbitol (DBS) self-organizes to form a 3D network at relatively low concentrations in a variety of nonpolar organic solvents and polymer melt. DBS could be transformed into a hydrogelator by introduction of hydrophilic groups, which facilitate its self-assembly in aqueous medium. In this work, we have investigated the hydrogelators DBS-COOH and DBS-CONHNH2 and the organogelator DBS by molecular modeling. We have used quantum mechanics (QM) to elucidate the preferred geometry of one molecule and a dimer of each of the gelators, and molecular dynamics (MD) to simulate the pure gelators and their mixtures with water. The results of the simulation indicate that the interaction between DBS-COOH molecules is the strongest of the three and its water compatibility is the highest. Therefore, DBS-COOH seems to be a better hydrogelator than DBS-CONHNH2 and DBS. Intermolecular H-bonding interactions are formed between DBS, DBS-COOH and DBS-CONHNH2 molecules as pure substances, and they dramatically decrease in the presence of water. In contrast, the intramolecular interactions increase in water. This result indicate that in aqueous environment the molecular structure tend to be more rigid and fixed in the preferred conformation. The most significant intramolecular interaction is formed between O3 Acetal and H-O6 groups. Due to the H-bonds, DBS, DBS-COOH and DBS-CONHNH2 molecules form a rigid structure similar to liquid crystal forming molecules, which might explain their tendency to create nanofibrils. It was found that the aromatic rings do not contribute significantly to the inter-and intra-molecular interactions. Their main role is probably to stiffen the molecular structure.

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1. Introduction Low molecular weight gelators (LMWGs) are molecules capable of forming gels in which they are self-assembled into a physical three-dimensional network of fibers, held together by noncovalent interactions like hydrogen bonds, van der Waals forces and π-π-interactions1. LMWGs have potential applications involving nanomaterials (such as sensors, molecular electronics, and catalysts) and delivery or modification agents for paints, inks, cleaning agents, cosmetics, polymers, drugs, etc.2,3 LMWGs can be divided into two types according to the medium they operate in, organogelators and hydrogelators. An organogelator is a molecule capable of self-organizing into molecular networks (organogels) within organic media such as organic solvents or polymeric melt4. Upon cooling, the organic gelator spontaneously forms a 3D thermoreversible network of crystalline

nanofibrils5.

Hydrogelators form gels in aqueous solutions. Whereas hydrogen bonds are a common driving force in aggregation of organogelators, hydrophobic forces play a major role in aggregation of hydrogelators in aqueous environments6. To achieve gelation, there must be a balance between the tendency of the molecules to dissolve or to aggregate. De Loos, Feringa and van Esch suggested guiding lines to design hydrogelators1. They claim that a key to the design of organic hydrogelators is the control of its hydrophobic interactions. In order to obtain effective LMW hydrogelators, fine-tuning of the balance between the hydrophilic (soluble) and hydrophobic (insoluble) parts is essential. LMW hydrogelators are usually composed of a hydrophilic moiety and a hydrophobic aromatic group or long hydrocarbon chain. The hydrophilic moieties provide the water compatibility of the molecules, whereas the hydrophobic part is generally providing the main driving force for the self-assembly of the molecules by hydrophobic interactions1. One class of organogelators of current interest is sugar-based gelators, especially those based on 1,3(R):2,4(S)-dibenzylidene-D-sorbitol (DBS).

DBS is an important low molecular

weight organic gelator since it is capable of (i) dissolving in a wide variety of organic solvents and polymers at elevated temperatures and (ii) inducing physical gelation at very low concentrations upon cooling7. DBS readily gels various organic solvents and polymer melts like polypropylene and polyethylene glycol8,9,10, PPG-PEG copolymers11, poly(L-lactic acid)12, polystyrene13, polyethylene14 and polypropylene15. The DBS molecule (Figure 1) is chiral and amphiphilic, and is often described as “butterfly-like", with a sorbitol body and two benzylidene “wings"16.


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DBS could be transformed into a hydrogelator by the introduction of hydrophilic groups. For example, Okesola and Smith17 developed a novel DBS gelator functionalized with hydrazide (DBS-CONHNH2), which is able to assemble into hydrogel across a wide pH range. This gelator exhibits pH-switchable dye adsorption–desorption dependent on protonation of the target dyes and their resulting interactions with the self-assembled gel nanofibres17. They also showed that DBS functionalized with carboxylic acid (DBS–COOH) retains its ability to assemble in the presence of an agarose gel network and can yield a hybrid hydrogel18. These materials can be used to remediate unwanted pollutants from the environment19. The aim of the present work is to study computationally the DBS derivatives DBS-COOH and DBS-CONHNH2, having the structures shown in figure 1, and compare them to DBS. This study may shade light on the difference between organo- and hydrogelator in aqueous medium. Only few works were published concerning investigation of DBS and its derivatives using computational tools. Wilder et al. elucidated equilibrium structure of DBS molecule and the molecular interactions that govern DBS self-assembly by using molecular mechanics (MM) and molecular dynamics (MD) simulations20. Nagarajan and Myerson used computer simulation to study nucleation and crystallization of isotactic polypropylene (iPP) in the presence of a number of derivatives of dibenzylidene sorbitol (DBS) as nucleators21. In recent work, we have used computer simulation to study the gelation of isotactic polypropylene (iPP) in the presence of dibenzylidene sorbitol (DBS) as a gelator22. We have used quantum mechanics (QM) to elucidate the preferred geometry of one molecule and a dimer of DBS, and molecular mechanics and molecular dynamics to simulate pure DBS, pure PP and mixture of DBS and PP as condensed phases, at various temperatures22. In the present work, we have applied similar computational tools to explore the hydrogelators DBS-COOH and DBS-CONHNH2. Okesola and Smith found experimentally that unlike DBS, DBS-COOH and DBS-CONHNH2 form stable hydrogel in water without adding organic solvents3,17. However, while DBS-CONHNH2 forms stable hydrogel by heating to dissolve and cooling the hydrosol at room temperature, DBS-COOH forms stable hydrogel by deprotonationprotonation cycle. DBS-COOH undergoes deprotonation at pH (8-12) to form a solution and the solution transforms into hydrogel when acidified. The computational study might account for that behavior.



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O

H2N HN

1 O

2 O

O 3

O 4

O O O O

O

HN

OH 5

HO 6

HO OH

DBS

DBS-COOH

DBS-CONHNH2

Figure 1: DBS, DBS-COOH and DBS-CONHNH2 molecular structure. The oxygen atoms in DBS are labeled for further discussion.

2. Experimental 2.1 Computational tools The simulation tools used were as follows: Material Studio 7.0 (by Biovia)23 molecular modeling package. Three simulation modules were used: • Forcite - A forcefield simulation tool performing molecular mechanics and molecular dynamics tasks. The forcefield used was COMPASS II (condensed-phase optimized molecular potentials for atomistic simulation studies). • Amorphous Cell - A simulation tool capable of building 3D periodic boundary cells. • DMOL3 - A quantum mechanics module, modeling the electronic structure and energetics of molecules using density functional theory (DFT). 2.2 Computation details 2.2.1 Quantum simulation The geometry of the gelator molecules was optimized using DMOL3. To ensure that the minimum energy conformation is obtained, prior to optimization the molecule was subjected to 100000 dynamics steps at 10000K and was minimized. This procedure was repeated for ten different conformations. The lowest energy conformation was then optimized using DMOL3. Angles and distances in the optimized structure were measured. The same procedure was repeated for a pair of gelator molecules and their inter- and intramolecular distances were measured. 2.2.2 Dynamic simulation Step 1: Building cubic cells Five simulation cubic boxes (about 20Å edge) were constructed using Amorphous cell module at a temperature of 3000K for each of the following: Pristine DBS, DBS-COOH and DBS-CONHNH2 and their mixture with water. 4 ACS Paragon Plus Environment

NH2

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Step 2: Molecular Dynamics simulation Dynamic simulation was performed at 3000K. The cells were subjected to 100,000 dynamic steps of 1fs each at NPT ensemble to determine their density. This stage was followed by a NVT ensemble refinement stage of 100,000 dynamic steps and a data collection stage of additional 400,000 NVT steps. All MD simulations were conducted using Forcite module with COMPASSII force field. Electrostatic term was considered using Ewald and van der Waals term using Atom based summation methods with an accuracy of -103 kcal/mol. The repulsive cut-off for Van der Waals term was chosen as 12.5 Å. For NPT molecular dynamic simulations, Nose thermostat and Berendsen barostat were chosen. Step 3: Analysis The resulted dynamic trajectories were analysed using Forcite module analysis tools.23 The following properties were calculated: Cohesive Energy Density (CED) and Solubility parameter δ Cohesive energy is the energy required to break the interactions between molecules. Generally, it is measured as the heat of vaporization of a liquid. The cohesive energy density (CED) corresponds to the cohesive energy per unit volume. Solubility parameter δ is the square root of the CED. The solubility parameter is a measure of the ability of materials to dissolve each other. Radial distribution function (RDF) Radial distribution function (also referred to as Pair correlation function) gives a measure of the probability that, given the presence of an atom at the origin of an arbitrary reference frame, there will be an atom with its center located in a spherical shell of infinitesimal thickness at a distance r from the reference atom. RDF may serve as a tool to estimate intermolecular interactions like hydrogen bonding.

3. Results and discussion Structural characterization and the interaction between two molecules of DBS, DBSCOOH and DBS-CONHNH2 DBS, DBS-COOH and DBS-CONHNH2 molecules were constructed and optimized using DMOL3 (DFT based) quantum mechanical software and conformer search. The lowest energy conformations (Figures 2, 3, 4) were found to be those in which the two aromatic rings are almost perpendicular in DBS and parallel in DBS-COOH and DBS-CONHNH2. The 5 ACS Paragon Plus Environment


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distances between various groups in the molecules were measured and are summarized in tables 1, 2 and 3. The hydrogen of the hydroxyl group O6 in DBS and DBS-COOH points towards the oxygen of the hydroxyl O5 group (distances of 2.038 and 2.074Å, respectively) but not in DBS-CONHNH2 (3.965Å). Whereas in DBS and DBS-CONHNH2 the hydroxyl group O5 is oriented towards O4, in DBS-COOH it points towards O3. Following the intramolecular analysis of one molecule, two DBS molecules were constructed, and their distances and conformation were optimized. As shown in figures 2, 3 and 4 and in tables 1, 2 and 3, the two molecules of DBS, DBS-CONHNH2 and DBS-COOH are attracted to each other by intermolecular hydrogen bonding formed between several groups. Whereas in DBS and DBS-CONHNH2 the most significant intermolecular interaction (the shortest distance) is formed between H-O6 / O5 (distances of 1.722 and 1.733Å, respectively), in DBS-COOH it is formed between H-O5 / O5 (2.403Å). Additional interactions are formed between the substituents, especially in DBS-CONHNH2 (distance of 1.866Å between H-NHNH / O_ Carbonyl). The distance between H-O Carboxylate / O_ Carbonyl in DBS-COOH is 3.532Å. The most significant intramolecular H-bonds are formed between either H-O6 and the acetal O3 or H-O5 and the acetal O4 for all three molecules. The calculated results obtained for DBS molecules are in good correlation with an experimental data presented by Yamasaki and co-workers24. They used IR, UV and CD spectroscopies to establish that the 6-OH group forms intermolecular H-bonds and the 5-OH group forms intramolecular H-bonds with an acetal oxygen. The phenyl groups are ordered side by side around the aggregate axis to form helical fibers. They showed that 6-OH is critical as a hydrogen bond donor for DBS self-assemble by selective conversion each of the 5-OH group and the 6-OH group into methoxy groups. While no gelation occurred when the 6-OH group was protected, gelation still occurred with the 5-OH group protected.24 These results support our observations that 6-OH participates in intermolecular hydrogen bonding and 5-OH in intramolecular hydrogen bonding. In contrast to Yamasaki and co-workers24 findings, our calculations indicate that H-O6 preferably forms intermolecular interaction with O5 and less with the acetal oxygen. That difference may be explained by the work of Wilder et al.20 that conducted a conformational search to explore structural variation in DBS. They found that the molecular configuration of DBS could potentially switch between several low energy structures, influenced by changes in solvent or derivatization. Watase et al.,7 investigated the dimerization of DBS molecules in alcoholic solvents using fluorescence spectroscopy. They concluded that DBS fibers are formed by the overlapping of each benzylidene group with a benzylidene group of another DBS molecule within a distance 6 ACS Paragon Plus Environment

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of 0.35 nm. Our calculations indicated a distance of 0.405, 0.353 and 0.473 nm between the aromatic rings for DBS, DBS-COOH and DBS-CONHNH2, respectively (tables 1, 2 and 3), which is in a good agreement with the experimental results.

Figure 2: DBS molecule (left) and two DBS molecules (right) after optimization

Figure 3: DBS-COOH molecule (left) and two DBS-COOH molecules (right) after optimization

Figure 4: DBS-CONHNH2 molecule (left) and two DBS-CONHNH2 molecules (right) after optimization



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Table 1: Interactions distances calculated for optimized one and two molecules of DBS Interaction between atoms

*

O6 / H-O6

One molecule Intramolecular interaction distance (Å) -

Two molecules Intermolecular Intramolecular interaction interaction distance (Å)* distance (Å) ** 4.575 -

O6 / H-O5

3.145

3.168

4.237

3.857

O5 / H-O6

2.038

1.722

3.894

4.095

O5 / H-O5

-

4.723

-

-

O3 Acetal / H-O5 O3 Acetal / H-O6 O4 Acetal / H-O5 O4 Acetal / H-O6 Aromatic rings

4.623

3.767

4.428

2.162

5.049

5.453

1.790

3.812

2.024

3.836

1.955

3.677

3.972

4.151

4.129

3.878

-

4.045

-

-

Shortest distances option A column for each of the two molecules

**

Table 2: Interactions distances calculated for optimized one and two molecules of DBS-COOH Interaction between atoms

O6 / H-O6

One molecule Intramolecular interaction distance (Å) -

O6 / H-O5

3.611

5.344

4.213

4.320

O5 / H-O6

2.074

3.616

5.616

3.889

O5 / H-O5

-

2.403

-

-

O3 Acetal / H-O5 O3 Acetal / H-O6 O4 Acetal / H-O5 O4 Acetal / H-O6 Aromatic rings

1.861

4.340

6.659

4.340

4.759

3.206

3.286

1.650

3.792

2.922

4.749

1.833

4.535

5.079

4.094

4.337

-

3.581

-

-

-

3.532

-

-

H-O Carboxylate / O_ Carbonyl *

Two molecules Intermolecular Intramolecular interaction interaction distance (Å)* distance (Å) ** 4.139 -


**


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Table 3:

Interactions distances calculated for optimized one and two molecules of DBS-

CONHNH2 Interaction between atoms

O6 / H-O6 O6 / H-O5 O5 / H-O6 O5 / H-O5 O3 Acetal / H-O5 O3 Acetal / H-O6 O4 Acetal / H-O5 O4 Acetal / H-O6 Aromatic rings

H-NHNH / O_ Carbonyl *

One molecule Intramolecular interaction distance (Å) 4.357 3.965 4.044

Two molecules Intermolecular Intramolecular interaction interaction distance (Å)* distance (Å) ** 4.015 3.254 4.289 4.257 1.733 4.113 3.903 5.477 4.011 3.806 4.584

4.492

4.509

3.737

1.734

2.426

4.161

2.717

1.760

4.574

3.793

4.044

4.074

-

4.729 1.866

-

-


**

Dynamic simulation Molecular dynamics simulations were performed at 3000K on pure DBS, DBS-COOH and DBS-CONHNH2, pure water and on their mixtures. Simulation boxes of pure DBS-COOH and DBS-CONHNH2 and their mixtures with water are shown in Figure 5 and Figure 6.

Density and Solubility parameter calculation Density, cohesive energy density (CED) and solubility parameter δ were calculated and the results are presented in table 4. Solubility parameter values of the pure gelators are much lower than that of pure water (47.39(J/cm3)0.5, because they are less polar than water. Therefore, it can be expected that the gelators will undergo phase separation when mixed with water, and this might explain the experimental observation that the gelator molecules self-organize into a nanofibrilar structure. The difference between the simulated and calculated values of the solubility parameters is an indication of the energetic benefit from mixing them together. The differences were quite small, indicating that the gelators do not mix well with water. The larger difference (1.37) was observed for DBS-COOH, then DBS-CONHNH2 (0.76) and the smallest for DBS (0.52). These results suggest that the solubility of DBS-COOH in water is the highest because the



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energetic benefit is the most pronounced. DBS-CONHNH2 is less soluble and DBS is even less soluble.

(a)

(b)

Figure 5: Cubic cell containing (a) 15 molecules of DBS-COOH ; (b) 2 molecules of DBSCOOH and 250 water molecules at 3000K after 500 ps dynamic steps

(a)

(b)

Figure 6: Cubic cell containing (a) 15 molecules of DBS-CONHNH2 ; (b) 2 molecules of DBS-CONHNH2 and 250 water molecules at 3000K after 500 ps dynamic steps Table 4: Calculated densities and solubility parameters of pure DBS, DBS-COOH and DBSCONHNH2 and their mixture with water at 3000K Densities (g/cm3)*

Solubility parameters δ [J/cm3]0.5

DBSCONHNH2 1.306

22.04±0.01

DBSCOOH 24.73±0.01

DBSCONHNH2 25.80±0.01

Pure gelator

1.228

DBSCOOH 1.310

Gelator with water (simulated) Gelator with water (expected)**

0.992

1.034

1.012

44.43±0.02

45.01±0.03

44.39±0.03

-

-

-

43.91

43.64

43.63

DBS

DBS


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Simulation results for water: Density: 0.983 g/cm3; Solubility parameters : 47.39 [J/cm3]0.5 ** Expected solubility parameters was calculated according to δ values and weight ratios of the constituents.

Radial distribution calculation (RDF) Inter- and intramolecular interaction, particularly hydrogen bonding, was calculated by RDF. The distance between the groups is measured, and the peaks height is proportional to the probability that the interacting groups will be at a certain distance from each other. Interaction intensities were calculated for the following pairs: 1. Between hydroxyl groups designated as: •

O6 / H-O6 (O6 and the hydrogen atom attached to O6)

•


•


•


2. Between hydroxyl groups and acetal group designated as: •

O3 Acetal / H-O5 (O3 acetal group and the hydrogen atom attached to O5)

•


•


•


3. Between aromatic rings 4. Between

H-O Carboxylate

/ O Carbonyl in DBS-COOH and H-NHNH

/ O

Carbonyl in DBS-CONHNH2 The results of the calculated inter- and intramolecular interaction distances and peak heights (correspond to the significance of the interaction) for DBS, DBS-COOH and DBSCONHNH2 and their mixture with water at 3000K appear in tables 5 and 6 and figures 7 and 8.

Intermolecular interactions between DBS, DBS-COOH and DBS-CONHNH2 molecules As can be seen in table 5, intermolecular H-bonding interactions are formed between O6 / HO6 (figure 7a), O5 / H-O6 (figure 7b), O5 / H-O5 and O6 / H-O5 of DBS, DBS-COOH and DBS-CONHNH2 molecules as pure substances. The interaction distance is between 1.651.85Å. On the other hand, Acetal / H-O5 and Acetal / H-O6 do not participate in intermolecular interactions. The interactions between aromatic rings were also elucidated, and no significant intermolecular π-π interaction was found. The interaction intensities of DBS-CONHNH2 are the lowest of the three, but an additional interaction is formed between



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the substituent H-NHNH / O Carbonyl groups. In DBS-COOH an interaction between H-O Carboxylate / O Carbonyl groups is also formed, in shorter distance and higher intensity than the interaction between H-NHNH / O Carbonyl groups of DBS-CONHNH2. Overall, the interactions between DBS-COOH molecules are stronger than the interactions between DBS-CONHNH2 molecules. The intermolecular interactions dramatically decrease in the presence of water. No significant interactions were detected for intermolecular H-bonding interactions between O6 / H-O6, O5 / H-O6, O5 / H-O5 and O6 / H-O5 of DBS and also between H-NHNH / O Carbonyl groups of DBS-CONHNH2 molecules. However, DBS-COOH molecules seem to interact in aqueous environment. H-bonds are formed especially between O6 / H-O5, and to a much lesser extent between O5 / H-O6 and H-O Carboxylate / O Carbonyl group, which also forms interactions with the water molecules.

Table 5: Intermolecular interaction distances (Å) and relative frequencies in brackets calculated for pure DBS, DBS-COOH, DBS-CONHNH2 and their mixtures with water in a condensed phase at 3000K

1.65 (3.30) 1.85 (3.66)

DBSCONHNH2 1.65 (1.62) 1.65 (2.21)

DBS with water No No

DBS-COOH with water No

DBS-CONHNH2 with water No

1.65 (3.97)

No

1.75 (3.30) 1.75 (2.23)

1.65 (4.49) 1.75 (2.50)

1.75 (2.42) 1.85 (1.05)

No No

3.15 (1.68) No

No No

No

No

No

No

3.65 (4.80)

No

No

No

No

No

4.70 (2.45)

No

No

No

No

No

No

No

No

No

4.20 (1.69)

No

No

2.10 (2.71)

No

No

No

4.05 (1.99)

4.55 (2.11)

No

-

1.75 (3.65)

-

-

3.00 (2.13)

-

-

-

2.00 (1.97)

-

-

No

-

-

-

No

1.65 (3.03)

No

-

-

-

No

No

No

Group

DBS

DBS-COOH

O6 / H-O6

1.65 (3.74) 1.75 (3.97)

O6 / H-O5 O5 / H-O6 O5 / H-O5 O3 Acetal / H-O5 O3 Acetal /H-O6 O4 Acetal / H-O5 O4 Acetal /H-O6 Aromatic rings H-O Carboxylate / O Carbonyl H-NHNH / O Carbonyl H-O Carboxylate / O water H-NHNH / O water

Intramolecular interactions between DBS, DBS-COOH and DBS-CONHNH2 molecules


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The intramolecular interactions are presented in table 6. Intramolecular H-bonding interactions are formed between O6 / H-O5, O5 / H-O6, O3 Acetal / H-O5, O3 Acetal / HO6 (figure 8a) and also O4 Acetal / H-O5 and O4 Acetal / H-O6 of DBS, DBS-COOH and DBS-CONHNH2 molecules as pure substances. The most significant intramolecular interaction appears to be formed between O3 Acetal / H-O6 groups (distance of 1.85Å and intensity of around 4). In contrast to the intermolecular interactions, the intramolecular interaction peak heights in water are much higher than for the pure gelators, though interaction distances do not significantly change. In the presence of water the most significant intramolecular interaction is also formed between O3 Acetal / H-O6 groups (distance of 1.65Å) (figure 8b). It is interesting to note that intensity of the intramolecular interactions of DBS-COOH molecules is lower than of DBS and DBS-CONHNH2.

Table 6: Intramolecular interaction distances (Å) and relative frequencies in brackets calculated for pure DBS, DBS-COOH, DBS-CONHNH2 and their mixture with water in a condensed phase at 3000K

O6 / H-O6

-

DBSCOOH -

2.65 (3.30)

2.55 (18.16)

1.75 (2.48) 4.15 (2.15)

2.45 (2.22) 3.65 (3.68) 1.85 (2.24) 3.95 (1.81)

1.85 (1.57) 4.25 (2.26)

1.75 (7.12) 4.15 (17.14)

DBS-COOH with water 2.45 (20.00) 2.75 (41.84) 2.45 (9.40) 4.25 (33.25) 2.75 (2.20) 4.15 (23.00)

O6 / H-O5

2.25 (2.64)

2.15 (4.30)

O5 / H-O6

2.35 (2.29)

O5 / H-O5 O3 Acetal / H-O5 O3 Acetal /H-O6 O4 Acetal / H-O5 O4 Acetal /H-O6

1.85 (4.43)

1.85 (4.00)

1.85 (4.00)

1.65 (34.50)

1.65 (10.03)

1.65 (30.14)

2.35 (3.93)

2.35 (1.94)

2.35 (2.33)

2.45 (33.43)

2.45 (33.43)

2.55 (35.81)

4.15 (1.30)

1.75 (1.34) 4.55 (1.31)

1.75 (1.42) 4.15 (1.55)

4.25 (12.77)

1.65 (1.68) 5.05 (15.28)

4.55 (17.87)

DBS

DBSCONHNH2 2.55 (2.90)

DBS with water 2.55 (21.26)


DBS-CONHNH2 with water 2.45 (19.00) 2.65 (21.14) 1.95 (8.02) 4.15 (19.53)


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(a)

Page 14 of 18

(b)

Figure 7: Intermolecular interaction intensities between (a) O6 and H-O6 and (b) O5 and HO6 hydroxyl groups of pure DBS, DBS-COOH and DBS-CONHNH2 and their mixture with water in a condensed phase at 3000K

(a)

(b)

Figure 8: Intramolecular interaction intensities between O3 acetal and H-O6 hydroxyl groups of (a) pure DBS, DBS-COOH and DBS-CONHNH2 and (b) their mixture with water in a condensed phase at 3000K 4. Conclusions In the present work, the interactions between DBS, DBS-COOH and DBS-CONHNH2 molecules were elucidated using computational tools, as pure substances and in a mixture with water. The intermolecular interactions are formed between the molecules of the three gelators in a similar manner. Intermolecular H-bonding interactions are formed between O6 / H-O6, O5 / H-O6, O5 / H-O5 and O6 / H-O5 of DBS, DBS-COOH and DBS-CONHNH2 molecules as pure substances, and they dramatically decrease in the presence of water. The


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interaction intensities between DBS-CONHNH2 molecules are lower than those formed between DBS-COOH molecules, which seem to interact in aqueous environment as well and form interactions with the water molecules. These results along with the solubility parameter calculations indicate that the interaction between DBS-COOH molecules is the strongest of the three and its water compatibility is the highest. Therefore, DBS-COOH seems to be a better hydrogelator than DBS-CONHNH2 and DBS. The intermolecular interactions between O6 / H-O6 groups of DBS were compared to those previously obtained for DBS in polypropylene (PP)22. Although somewhat different interaction distances were obtained in the present simulation, a higher peak value was obtained in PP whereas lower peak was observed in water compared to pure DBS. This may indicate that the H-bond interactions between the hydrophilic groups of DBS are more pronounced in the hydrophobic environment of PP and much less in water. The most significant intramolecular interaction appears to be formed between O3 Acetal / H-O6 groups. This result is in a good agreement with the quantum optimization conducted for one molecule of DBS-COOH (the same distance of 1.85 Å was measured), but for DBS and DBS-CONHNH2 the quantum-optimal geometry was found to be when the distance between O4 Acetal / H-O5 groups is the shortest. In both cases, those intramolecular interactions point to the formation of an additional ring held by H-bond. In contrast to the intermolecular interactions, the intramolecular interaction peaks obtained in water are much higher than for the pure gelators, though interaction distances do not significantly change. This result indicate that in the aqueous environment the molecular structure tend to be more rigid and fixed in the preferred conformation. The intramolecular interactions between DBSCOOH molecules are somewhat weaker than between DBS and DBS-CONHNH2. Due to the H-bonds, DBS, DBS-COOH and DBS-CONHNH2 molecules form a rigid structure similar to liquid crystal forming molecules, which might explain their tendency to create nanofibrils. It was found that the aromatic rings do not contribute significantly to the inter-and intra-molecular interactions. Their main role is probably to stiffen the molecular structure.



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TOC Graphic


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Simulation of DBS, DBS-COOH, and DBS-CONHNH2 as Hydrogelators

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