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Directional Supramolecular Assembly of #-Amphiphiles with Tunable Surface Functionality and Impact on the Antimicrobial Activity Amrita Sikder, Jayita Sarkar, Ranajit Barman, and Suhrit Ghosh J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b05193 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on August 3, 2019
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Directional Supramolecular Assembly of π-Amphiphiles with Tunable Surface Functionality and Impact on the Antimicrobial Activity Amrita Sikder,† Jayita Sarkar,† Ranajit Barman and Suhrit Ghosh* School of Applied and Interdisciplinary Sciences, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata, India-700032
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ABSTRACT: This article elucidates H-bonding regulated directional supramolecular assembly of naphthalene-diimide (NDI)-derived unsymmetric cationic bola-shape π-amphiphiles and systematic investigations on the thermodynamics of their interaction with bacteria mimic lipid vesicles and antimicrobial activity with mechanistic insights. Four NDI-amphiphiles (NDI-1, NDI-2, NDI-3, NDI-2a) have been studied, all of which contain a central NDI-chromophore, a non-ionic wedge, an amine containing head group and a hydrazide group. In NDI-2 and NDI-2a, the hydrophilic wedge and the head group (pyridine) are same but the location of the hydrazide group is different. Based on this difference, the pyridyl groups are displayed at the outer and inner wall of the vesicle, respectively. Isothermal calorimetry (ITC) studies revealed spontaneous interaction of NDI-2 assembly with bacteria membrane mimic DPPE liposome (G = -6. 35 Kcal/ mole) while NDI-2a assembly did not interact at all, confirming a strong influence of the H-bonding regulated functional group display. On the other hand, the location of the hydrazide group remains same in NDI-1, NDI-2 and NDI-3, but they differ in the head group structure. ITC binding studies confirmed spontaneous interaction of all of three assemblies with DPPE liposome with negative G values following the order NDI-1> NDI-2 > NDI-3, indicating significant influence of the structure of the head group on the interaction with model membrane. In fact, in all cases, the interaction was favourable both by enthalpy and entropy contribution, indicating dual involvement of the electrostatic interaction and hydrophobic effect. Notably S value for NDI-1 containing tertiary amine head group was found to be significantly higher than that for NDI-3 containing primary amine, which is attributed to the enhanced hydrophobic effect in the former case. Furthermore, ITC experiments revealed no interaction by any of these assemblies with the mammalian cell membrane mimic liposome, indicating their high selectivity towards bacterial membrane. Antimicrobial activity studies showed NDI-2 to be lethal
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selectively against gram positive bacteria while NDI-2a did not show any activity. NDI-3 with primary amine showed moderate activity, but no selectivity over the erythrocytes. NDI-1 with the tertiary amine group was found to be the most outstanding candidate exhibiting broad spectrum antimicrobial activity with very low MIC values of 15.8 and 62 g/ mL for S. aureus and E. coli, respectively, and high selectivity over erythrocytes. These results fully corroborate with the physical insights obtained from the ITC studies on their interaction with model liposome. Control molecules, lacking either the NDI chromophore or the hydrazide non-ionic containing wedge, did not exhibit any notable antibacterial activity. Live-dead assay with fluorescence microscopy studies indicated that the antimicrobial activity of NDI-1 operates through the membrane disruption pathway similar to that of the Host Defense Peptides (HDPs).
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INTRODUCTION Amphiphiles consisting of π-conjugated chromophores
1-16
produce diverse nanostructures by
directional molecular interaction. They exhibit low critical aggregation concentration (CAC), high thermal stability and rich photophysical properties and therefore appear to be better candidates than classical surfactants as supramolecular biomaterial.17-20 In this context, surface functional group display of the self-assembled structures is an important issue that strongly impacts the multivalent interaction with a desired biological target and therefore influences the biological functions.21-22 We have recently reported
23-26
a new class of unsymmetric π-
amphiphiles consisting of a hydrophobic naphthalene-diimide (NDI) central chromophore, appended with different hydrophilic wedge/ functional group in its two opposite arms. They contain a single hydrazide group in one of the two NDI arms, which by virtue of H-bonding enables control over the lateral orientation and display of a desired functional group in the surface. Inspired by these recent results, we sought to examine the potency of structurally similar π-amphiphiles (Scheme 1) containing different amine head groups for antimicrobial activity. Combating bacterial infection continues to remain a significant challenge in biomedicine because of the ever-evolving drug resistance of different pathogens against available antibiotics.27 Classically, the action of an antibiotic depends on targeting a specific intracellular organelle without disrupting the cell membrane. In contrast, Antimicrobial peptides (AMPs), also known as Host Defense Peptides (HDPs), kill bacteria primarily by less specific membrane disruption pathway enabling broad spectrum activity
28-30
and thus are considered to be important
components of the natural immunity response. They are short peptides consisting of cationic amino acids (arginine, lysine or histidine) and large hydrophobic residues, which segregate in the folded state. Inspired by the structure and function of HDPs, amphiphilic polymers
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31-45
and
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small molecules
46
have been investigated with great interest in the recent past for antimicrobial
activity. Nevertheless, examples on antimicrobial π-amphiphiles remain scarce.47
Scheme 1. a) H-bonding regulated unilateral orientation and functional group display in NDI amphiphiles; b) Structure of hydrazide containing cationic NDI amphiphiles and control molecules studied in this article. We envisaged that hydrazide group containing unsymmetric NDI-derived amphiphiles (Scheme 1) might be particularly promising in this context because H-bonding regulated supramolecular assembly with unilateral orientation and segregation of the amine groups in one side (similar to HDPs) might enable efficient multi-valent interaction with the negatively charged
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bacterial membrane. Furthermore, the hydrophobic NDI-stack and the presence of the hydrazide groups may also influence membrane disruption as reported in recent examples on antimicrobial polymers.48 To test these possibilities, we have studied supramolecular assembly of a few cationic NDI-amphiphiles and structural effect on the thermodynamics of their interaction with model liposomes, mimicking either bacterial or mammalian cell. We further correlate the physical insights obtained from the thermodynamic studies with the actual antimicrobial activity of these NDI assemblies.
EXPERIMENTAL Synthesis. Synthesis of the NDI-2 and NDI-2a and some intermediates has been reported by us earlier.25 NDI-1, NDI-3 and control molecules (C1 and C2) were prepared in multiple steps using the synthetic protocol as outlined in Scheme S1, S2 and S3, respectively, in the supporting information. Solution Preparation. In a typical method, a 5.0 mM stock solution of a given sample (NDI-1 /NDI-2 /NDI-2a /NDI-3) was prepared in THF. 300 µL of the stock solution was transferred to a glass vial and the THF was removed by heating with a hot air gun which produced a thin film. To this vial, 900 µL distilled water was added and the solid was dissolved by sonication for about 5-10 min. Then to this solution 100 µL 5.0 mM HCl solution was added to make the final solution (concentration 1.5 mM) and pH of this solution was measured to be ~5.5. The solutions were allowed to equilibrate for 1h at rt (25-30 °C) before doing any physical experiments. Calcein Encapsulation. A stock solution of Calcein in MeOH (20 μL, 1.0 mM) and NDI-1/ NDI-2/ NDI-2a/ NDI-3 in THF (100 μL, 10 mM) were mixed and the solvent was
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evaporated. To this 500 μL aqueous HCl solution (pH-5.5) was added and sonicated for a few minutes. Subsequently the solution was subjected to dialysis against water (pH-5.5) by using 35000 Da MWCO membrane for 48 h to remove un-encapsulated Calcein. The fluorescence spectrum of the dialyzed solution was recorded and compared with free Calcein solution of same concentration in aqueous solution (pH-5.5). Lipid Vesicles Preparation. Lipid [1.0 mM of DPPC (1000µL) or 1.0 mM DPPG: DPPE (880µL: 120µL)] solutions in chloroform were taken in glass vials. Thin films were made under dry argon gas, and dried under vacuum. Lipid films were hydrated with 10 mM HEPES buffer (pH =7.4) (1000µL) for overnight. Hydrated films were then heated to 70 °C and then cooled to 4 °C with intermittent vortexing. This freeze-thaw cycle was repeated for 10 times to get multilamellar vesicles. Then the solutions were sonicated at 70 °C for 15 minutes to get unilamellar vesicles.58 In a similar way, 2mM lipid solutions were prepared using a 2 mM stock solution in CHCl3. ITC Experiments. ITC dilution experiments for all the samples (NDI-1/ NDI-2/NDI-2a/ NDI-3) were performed at 25 °C using isothermal titration calorimeter (Micro Cal Inc.). The reference cell was filled with double distilled water. Concentration of the aqueous aggregated solution of a particular solution was adjusted in such a way so that when it was injected into pure water, the concentration reached lower than the critical aggregation concentration and thus the heat change would indicate heat of dissociation and when it reaches its CAC there would be a saturation of heat change as there would be no more disassembly but only dilution of the particular aggregates. The inflection point of the heat change vs concentration plot indicates the CAC of the corresponding system. Enthalpy of aggregation (∆Hagg) was determined from the inflection point in the heat change versus concentration.
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Interaction of NDI-1 with lipids were performed at 37 °C in 10 mM HEPES (pH = 7.4) buffer. Reference cell was filled with 10 mM HEPES. The liposomal suspensions (DPPC or DPPG: DPPE (88: 12)) were made in the above buffer at 1.0 mM and NDI-1 was also dissolved in the same buffer and used at a concentration of 0.1 mM which is above CAC. The lipid suspensions were taken in the syringe and the polymer solution was filled in the calorimetric cell. The experiment consisted of 40 injections of 1.0 µL each with 3 minutes interval with a stirring speed of 500 rpm to ensure that the titration peak returned to the baseline before the next injection was done. A background titration was performed under same conditions with the buffer placed in the calorimetric cell instead of the NDI-1 solution. This data was subtracted from each sample titration to account for the heat of dilution. The titration curves were analyzed using the “one-binding-site” model provided by ORIGIN® software in order to determine the apparent binding constant (Kapp), the binding enthalpy (ΔH) and the lipid/NDI binding stoichiometry (N). In this model, the NDI molecules are considered as ligands and it is considered that a given assembly has n number of independent and equivalent binding sites.53 The standard free energy (ΔG) and entropy (ΔS) of the binding were calculated using the equations: ΔG = - RTlnKapp ΔS = (ΔH-ΔG)/T
(1) (2)
In this model, the NDI molecules are considered as ligands and it is considered that a given assembly has n number of independent and equivalent binding sites.59 Similar experiments were done with NDI-2, NDI-2a and NDI-3, with injection volume of 2µL. For each experiment concentration was adjusted in such a way that it never reached below CAC value.
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Bacterial Preparation. S. aureus (ATCC- 25923) and E. coli (ATCC- 25922) were grown aerobically in Luria-Bertani (LB) Agar/Broth at 37 °C. Bacteria were harvested at the logarithmic growth phase with optical densities in between 0.6-1.0 at 600 nm. Minimum
Inhibitory
Concentration
(MIC)
Determination.
The
minimum
concentration of a solution required to completely inhibit the growth of bacteria (defined as the MIC) was determined by a broth microdilution assay according to the procedures outlined by the National Committee for Clinical Laboratory Standards (Wayne, PA) with the modifications proposed by Weigand et al. From the stock solutions of a given NDI molecule/ control molecule (Scheme 1), a serial dilution was done for getting different concentration of the sample in LB Broth and finally 50 μL sample solutions were added to each well of a non-coated polystyrene 96-well plates containing 50 μL of bacteria with the final inoculum of 5 X 105 cfu mL-1. The plates were incubated at 37 °C for 18 h with gentle shaking. MIC was taken as the lowest concentration of solution at which there was 100% reduction of growth. Only broth alone and the broth containing only cells were used as sterility control and growth control, respectively. All experiments were performed in triplicate. Hemolytic Activity Assay. HC50 value is a metric for measuring hemolytic activity of a compound which is defined as the concentration of antimicrobial compound that kills 50% red blood cells (RBCs). Human RBCs from healthy donor (1mL) were suspended in 9 mL of PBS buffer (pH 7.4) and centrifuged at 2000 rpm for 5 min. The supernatant was removed by pipetting and RBCs were re-suspended in PBS. This procedure was repeated for two additional times. Finally, RBC pellet was dissolved in 10 ml of PBS buffer and again diluted four times to get stock solution where RBC is 2.5% v/v. Afterwards serial dilutions of NDI-1/NDI-2/NDI2a/NDI-3 in PBS (50 μL) were prepared on a 96-well sterile round bottomed polypropylene
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plate, the RBC suspension (150 μL) was added and incubated at 37 °C with shaking at 180 rpm. Triton X-100 (0.1% v/v in water) was used as the positive lysis control and PBS was used as negative controls. Supernatant (100 μL) from each well was diluted with PBS buffer (100 μL) in a 96-well sterile flat-bottomed polystylene plate. The absorbance of the released haemoglobin at 414 nm was measured using Varioskan microplate reader (Thermo Fisher). The percentage of hemolysis was calculated relative to the positive control and negative control solvents. Zone Inhibition Test. A 100 μL volume of the S. aureus /E. coli suspension at logarithmic phase was spread on LB agar plate to prepare lawns of bacteria. Paper discs of 6 mm diameter soaked with 20 μL of compound dissolved in water were added to the agar plates and incubated at 37 °C for 24 h. Kill Kinetic Assay. Kill curves were generated for S. aureus to determine bacterial killing kinetics with the supramolecular assemblies of NDI-1, NDI-2, NDI-2a and NDI-3. Bacterial suspension was incubated at 37 °C with NDI-bolaamphiphiles at 2.5×MIC. At different time intervals 100 μL of the solution were taken and diluted 102 times and then spread on LA plates following overnight incubation at 37 °C. Next day, the colony forming units (CFU) were counted. The assay was repeated at three times to get the final curve. Fluorescence Microscopy. LIVE/DEAD viability testing was carried out using a standard protocol.40 Briefly, S. aureus and E. coli were grown to late-log phase (OD6002.5 >40 (E. coli) >157 (S. aureus) NDI-2 >2000 29.4 >2.5 >85 (S. aureus) NDI- 2a >2000 >2000 >2.5 N/A NDI-3 >2000 240 2.37 9.9 (S. aureus) C1 1700 1700 >1.7 >1.0 C2 >2100 >2100 >1.0 N/A
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It is well known
31-46
that gram negative bacteria are more difficult to kill than gram
positive bacteria due to the presence of an outer lipopolysaccharide membrane surrounding the cell wall. Furthermore, the gram negative cell membranes are primarily composed of zwitterionic phospholipids whereas gram positive cell membranes are mainly composed of anionic phospholipids. Therefore, the broad spectrum activity exhibited by NDI-1 is truly inspiring and may be attributed to the enhanced hydrophobicity of the head groups due to the presence of the two methyl groups in addition to the highly effective functional group display.
Figure 3. Zone inhibition assay against a) S. aureus and b) E. coli; NDI-amphiphiles are tested at 2.5 times of MIC value; c) Killing kinetics of the NDI-amphiphiles against S. aureus. NDIamphiphiles are tested at 2.5 times of MIC value. It is also noteworthy that the MIC values of the supramolecules listed in Table 3 were found to be greater than their corresponding CAC value which is indicative of the fact that the killing ability and the observed differences are characteristics of the aggregates and not of the
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discrete monomers. Now the antimicrobial activity of NDI-1, NDI-2 and NDI-2a was further confirmed by the zone inhibition assay. A clear zone formation was observed (Figure 3a-b) in presence of NDI-1 or NDI-2 (C = 5 x MIC) on LB agar plates with a lawn of S. aureus. For NDI1, zone formation was also prominent in the lawn of E. coli while NDI-2a did not form any zone in any of the two different bacterial lawns which fully corroborates with the data presented in Table 3. Further, to test the bactericidal activity, time-kill assay was performed with NDI amphiphiles on S. aureus (starting with 106 cells/ mL) which revealed (Figure 3c) a 5-log reduction in ~ 250 min by NDI-1, NDI-2 and NDI-3 (C = 2.5 × the MIC) suggesting all of them are lethal towards gram positive bacteria. However interestingly, for the NDI-2a, in which the pyridine group is directed at the inner wall of the vesicle, no reduction in cell count was noticed, instead it increased by 3-log which unambiguously confirm the decisive role of the functional group display on the antimicrobial activity.
Figure 4. Graph showing percentage of hemolysis with different concentrations (0.252.5mg/mL) of various NDI-amphiphiles.
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To examine the selectivity, we evaluated hemolytic toxicity of these compounds using human red blood cells (RBCs). NDI-1, NDI-2 or NDI-2a showed negligible hemolysis in contrast to NDI-3 which exhibited high toxicity towards RBCs with > 50% hemolysis at 2.5 mg/ mL (Figure 4). Comparing the HC50 and MIC values of each compound, the selectivity towards bacterial cell was found to be merely ~ 10 for NDI-3 containing the primary amine and therefore it was excluded from further investigation related to antimicrobial activity. However, for NDI-1 and NDI-2, high selectivity could be observed (Table 3). Amongst all, NDI-1 was found to be the best candidate with selectivity of > 40 and >157 fold was noted for S. aureus and E. coli, respectively (Table 3).
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Figure 5. Fluorescence microscopy of (a, b) S. aureus and (c, d) E. coli upon exposure NDI-1 at a concentration of 5 × MIC at 0h (a, c) and 2h (b, d). Green channel is SYTO-9 nucleic acid stain, red channel is propidium iodide (PI) which only enters membrane-compromised bacteria and their merged image. Scale bar is 2 μm. To elucidate the nature of NDI-1 assembly-bacterial membrane interaction, LIVE/DEAD bacterial viability test was employed by fluorescence microscopy studies. In this assay two dyes are used; a green emitting SYTO 9 which is able to internalize both live and dead cell and a red emitting propidium iodide dye which only can cross damaged membrane indicating dead cell. 40, 55
Fluorescence microscopy images of S. aureus and E. coli cells treated with these two dyes
showed intense green channel emission while the heat-treated cells showed prominent red
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channel emission which was considered as a positive control (Figure S8). Samples prepared soon after addition of NDI-1 also showed (Figure 5) intense green channel emission for both types of cells, but no notable red channel emission was noticed indicating presence of live cells. After 2 h, prominent red channel emission appeared and the merged red and green channel emission suggested that almost all the cells (both S. aureus and E. coli) were destructed by NDI-1. These results clearly support that for both types of cells, the antimicrobial activity of NDI-1 follows the membrane disruption pathway.
CONCLUSION Overall we have demonstrated programmable supramolecular assembly of NDI-derived unsymmetric bola-shape cationic π-amphiphile(s) by suitable molecular engineering and impact on the thermodynamic parameters for their interaction with bacteria and mammalian membrane mimicking liposome’s and finally with the actual antimicrobial activity. A single hydrazide group located in one side of the NDI chromophore ensures unilateral orientation in the π-stacking which creates vesicle like structures with excellent control over the display of the amine groups either at the inner or outer wall of the membrane. For two systems containing the same cationic head group and otherwise similar structure, showed fully contrasting ability to kill bacteria depending solely on the location of the hydrazide group. On the other hand, series of amphiphiles having the same location of the hydrazide group but differing on the nature of the cationic head group exhibit vastly different antimicrobial activity. Amongst all the tested compound one systems was identified by adjusting the functional group display and hydrophobicity which showed broad spectrum activity by membrane destruction pathway against gram positive as well as gram negative bacteria with remarkably high selectivity over
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mammalian cells. Antimicrobial activity results fully corroborate with the relative propensity of these systems for interaction with the model liposome’s, mimicking either bacteria or mammalian cell membrane. While enormous effort has been made to study cationic polymers, peptide amphiphiles and others with wide ranging structures, π-amphiphiles are yet to be explored for antimicrobial activity. The present result in this context is highly promising and the values obtained can be ranked among the top candidates reported in the literature. Unlike macromolecules, molecular assembly based systems like the present examples are advantageous in terms of precise control over the shape, size, rigidity and other parameters by well-defined molecular engineering and therefore they may provide new opportunities to tackle the global threat of ever emerging infectious disease
ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: Compound synthesis and characterization, additional information and physical data (PDF). AUTHOR INFORMATION Corresponding Author *Email:
[email protected] Funding Sources DST, Government of India, SwarnaJayanti Fellowship (DST/SJF/CSA-01/2-14-15). ACKNOWLEDGMENT AS thanks IACS Kolkata for a research fellowship. SG thanks DST, India, for funding.
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