Tandem Interplay of the HostâGuest Interaction and Photoresponsive

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Functional Nanostructured Materials (including low-D carbon)

Tandem Interplay of Host-guest Interaction and Photo-responsive Supramolecular Polymerization to 1D and 2D Functional Peptide Materials Jojo P Joseph, Ashmeet Singh, Deepika Gupta, Chirag Miglani, and Asish Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09690 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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ACS Applied Materials & Interfaces

Tandem Interplay of Host-guest Interaction and Photo-responsive Supramolecular Polymerization to 1D and 2D Functional Peptide Materials Jojo P. Joseph,‡ Ashmeet Singh,‡ Deepika Gupta, Chirag Miglani and Asish Pal* Institute of Nano Science and Technology, Phase 10, Sector 64, Mohali, Punjab-160062, India E-mail: [email protected] ‡ Jojo

P. Joseph and Ashmeet Singh contributed equally

KEYWORDS (Word Style “BG_Keywords”). Peptide nanosheet, 2-Dimensional materials, Seeded supramolecular polymerization, Pathway complexity, Amyloid-inspired peptide

ABSTRACT

Peptide 1 with an Aβ42 amyloid nucleating core and photodimerizable 4-methylcoumarin moiety at its N terminus demonstrates step-wise self-assembly in water to form nanoparticles, with eventual transformation into 1D nanofibers. Addition of -cyclodextrin (-CD) to 1 with subsequent irradiation with UV light at 320 nm resulted in morphological conversion to free standing 2D nanosheets mediated by the host-guest interaction. Mechanical agitation of the 1D

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and 2D nanostructures led to seeds with narrow polydispersity indices, which by mediation of seeded supramolecular polymerization found seamless control over the dimensions of the nanostructures. Such structural and temporal control to differentiate the pathway was exploited to tune the mechanical strength of hierarchical hydrogel materials. Finally, the dimensional characteristics of the positively charged peptide fibers and sheets were envisaged as excellent exfoliating agents for inorganic hybrid materials e.g. MoS2.

INTRODUCTION Over the recent decade, there has been a remarkable upsurge in designing complex functional supramolecular system in a bid to mimic natural biomolecular system.1-3 However, predominantly thermodynamically controlled artificial supramolecular systems have always lagged behind the natural systems with temporal and adaptive structural control owing to the pathway dictated outof-equilibrium self-assembly. Bottom-up self-assembly to one dimensional (1D) anisotropic materials e.g. nanofibers, nanorods and nanotubes have been of particular interest for their immense applications in nanomedicine, catalysis etc.4 Of late, two dimensional (2D) nanomaterials engineered in well-defined thickness and dimension has emerged with paramount importance, due to high order, flexibility and the augmented anisotropy of the materials owing to their molecular thickness and infinite planar dimensions.5 However, a precise control over the shape and size of the nanomaterials derived from the complex supramolecular systems remained elusive to chemists until a recent times when the concept of pathway complexity, with the coexistence of multiple competitive pathways in a self-assembly process, provided a new paradigm and tool to control the fate of supramolecular systems under a variety of conditions.6-10 Arresting the metastable state


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allows access to kinetically trapped species,11-14 which can be employed in seeded supramolecular polymerization mediated by nucleation-elongation process, similar to the living polymerization in covalent polymers.15 The model of pathway complexity in functionalized porphyrin, perylene bisimide,

naphthalene

diimide

and

corannulene

molecules

to

form

kinetically

or

thermodynamically controlled aggregates by tweaking the conditions, e.g. temperature, solvents and light, was successfully demonstrated by the groups of Sugiyasu, Takeuchi, Wurthner, Ghosh and Aida.16-21 In another interesting strategy, interplay of dynamic covalent chemistry and selfreplication was exploited by Otto et al. to grow kinetically controlled peptide fibers from metastable macrocycles.22 However, such control over dimension was limited only in 1D materials

Scheme 1. Molecular structure of peptide 1 and its cartoon representation. 1monomer in HFIP to form metastable nanoparticle, 1NP at low temperature with eventual formation of nanofiber, 1NF on increasing temperature (Pathway A). Addition of γ-CD to the metastable nanoparticles followed by UV irradiation ( = 320 nm) leads to formation of 2D nanosheet (di-1⊂γ-CD)NS (Pathway B).


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until recently, when Sugiyasu et al. reported J- and H-type aggregation in porphyrin derivatives to promote supramolecular 1D nanofibers and 2D nanosheets.23 Recently, our group also demonstrated pathway driven self-assembly of peptide amphiphile based on Aβ42 amyloid nucleating core i.e. 18VFFA21 to control the length dimension of the 1D nanofibers that dictated the mechanical stiffness of the resulting fibrous hydrogel.24 In quest for controlled 2D peptide materials through pathway driven self-assembly, we anchored a functional moiety, 4methylcoumarin, to the peptide sequence, NVFFAC, that is capable of photodimerization and hostguest interaction with -CD. Therefore, we intended to explore the tandem interplay of host-guest and hydrogen bonding interaction in the proposed molecule to dictate the self-assembly pathway leading to different nanostructures. Herein, we postulate stepwise self-assembly of short peptide amphiphile, 1 mediated by -sheet interaction to grow 1D nanofiber (1NF) in water through formation of metastable nanoparticles, 1NP (Scheme 1). Also, the addition of equimolar -CD and subsequent UV irradiation (= 320 nm) converts the 1NP to 2D nanosheet (di-1⊂γ-CD)NS via host-guest interaction and photodimerization of pendant 4-methylcoumarin moiety. For the first time, we report design and precise control of the template-less free standing peptide nanosheet mediated by seeded supramolecular polymerization. The dimensional variations of the supramolecular polymer are successfully exploited in generating hydrogels with tunable mechanical properties. Towards the end, we successfully demonstrate very efficient, environmentally benign method of MoS2 exfoliation using the advantages of surface functionality and dimensions of the peptide materials.


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RESULT & DISCUSSION The peptide, 1 was designed by tethering a hydrophobic 4-methylcoumarin group at the Nterminal and two hydrophilic lysine units at the C-terminal of a amyloid Aβ42 inspired peptide sequence, NVFFAC to maintain the hydrophobic–hydrophilic balance to make 1 self-assemble in water (pH = 6.5). The attractive hydrogen bonding interaction among the amide functionalities and

Figure 1. (A) Comparative 1H NMR spectra of the free peptide 1 (6 mM) and host-guest complex of peptide 1 with equimolar γ-CD in D2O showing upfield shift of a, b, c protons of the 4-methylcoumarin moiety, (B) UV spectra depicting photodimerization of 4-methylcoumarin moeity for equimolar mixture of 1 and γ-CD (0.125 mM) in water, (C) corresponding change in absorbance320nm and photodimerization degree (PD) with time of UV irradiation.


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π-π stacking interactions among the aromatic moieties promote parallel stacking amphiphiles to form secondary structures. The 4-methylcoumarin group is also capable of forming host-guest complex with -CD.11 Complexation behavior of 1 with -CD was investigated by 1H NMR spectroscopy. The protons marked as a, b, c of the 4-methylcoumarin moiety exhibited remarkable upfield shifts upon addition of equimolar -CD (Figure 1A, S2-3), that could be attributed to the shielding effects of -CD cavity. Job’s plot analysis by plotting the change of chemical shift of the coumarin protons vs. the mole fraction of 1, indicated 1:1 complexation stoichiometry (Figure S4). Upon UV irradiation, the NMR signals corresponding to a’ protons of 1-⊂-CD complex gradually diminished with appearance of new peak at  value of 3.6-3.7, indicating photodimerization of 4methylcoumarin moieties to form cyclobutane adduct. Prolong irradiation (12 h) decreased the ratio of integral area of Ha’ and Hc’+d’+e’ protons to estimate the photodimerization degree (PD) to be 44% (Figure S6), quite similar to 1 encapsulated in -CD (48%, Figure S7). UV spectra showed gradual disappearance of 4-methylcoumarin absorption at 320 nm with irradiation of 1-⊂γ-CD complex and free 1 to render PD values of 66% and 74% respectively (Figure 1B-C, S8), corroborating with the NMR results. Such a correlation of PD values in presence or absence of CD suggests a photodimerization process occurring exterior to the -CD cavity with subsequent penetration of the resulting dimers in the cavity to result pseudo-[2]-rotaxane like di-1⊂-CD.25-27 Peptide 1 was molecularly dissolved in hexafluoroisopropanol (HFIP) to erase any pre-assembly history. An aliquot from the HFIP stock solution of 1 was then injected in water at 10 oC (final HFIP content ~ 5 v/v%). Tapping mode AFM shows the presence of uniform sized nanoparticle (Figure 2A, S9), (diameter 45 nm, height ~4-6 nm) at 10 oC, which persisted in that shape for 20 min at 10 oC before rapidly converting into nanofibers. Solvent mixtures with higher v/v% of HFIP


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in water seemed to stabilize the nanoparticles for longer time (~ 5 h) even at 25 oC (Figure S10). The metastable 1NP eventually transformed into nanofibers (diameter ~7-8 nm, height ~ 6-7 nm) with the kinetics depending on temperature and solvent composition (Figure 2B). Addition of equimolar γ-CD solution to the 1NP followed by incubation for 3 days resulted in a mixed population of 1D nanofibers with a predominant 2D nanosheets morphology, (1-⊂-CD)NS-NF

Figure 2. Microscopic images showing nanostructures of 1NP, 1NF, (di-1⊂γ-CD)NS. AFM height images for (A) 1NP after incubating aqueous solution of 1 (0.25 mM) at 10 oC, (B) 1NF after incubating 1NP at 25 oC for 1 days, (C) (di1⊂γ-CD)NS after irradiating equimolar mixture of γ-CD with 1NP at 25 oC for 4 h. TEM images for (di-1⊂γ-CD)NS after mixing equimolar ratio of γ-CD with 1NP followed by UV irradiation ( = 320 nm) for (D) 2 h & (E) for 8 h. (F) Confocal fluorescence images for (di-1⊂γ-CD)NS upon binding with thioflavin-T (ex = 488 nm).

(Figure S9). Interestingly, upon UV irradiation ( = 320 nm) for 2 h, (1-⊂-CD)NS-NF resulted immediate formation of 2D nanosheet, (di-1⊂γ-CD)NS exclusively (height ~ 4-6 nm) with the area


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of the nanosheet increasing with the duration of irradiation (Figure 2C-E, S11). Confocal fluorescence images shows green fluorescence of thioflavin-T bound to (di-1⊂γ-CD)NS (Figure 2F, S12). Remarkably, the nanosheet could reversibly be transformed to nanoparticle upon UV irradiation ( = 254 nm) for 8 h (Figure S9F). The self-assembly was monitored with UV and CD spectra. In comparison to the nanofiber 1NF, (1⊂γ-CD)NS-NF showed a clearly distinct absorption spectrum with red-shifted band, that eventually diminished in (di-1⊂γ-CD)NS as a consequence of UV irradiation (Figure 3A). CD spectra of 1NF exhibited a CD band at 220 nm characteristic of -sheet structure, along with an induced CD band at 320 nm due to the π-π* transition of 4-methylcoumarin moiety (Figure 3B). However, host-guest complexation and irradiation as in (1⊂γ-CD)NS-NF and (di-1⊂γ-CD)NS diminished the ICD band of the 4-methylcoumarin, albeit retaining the -sheet structure. FT-IR peaks at 1626 cm-1 (amide I) and 1548 cm-1 (amide II) suggest parallel -sheet structures for 1D nanofibers and 2D nanosheets (Figure S13).28 X-ray diffraction peak of 1NF at d = 0.43 nm and

Figure 3. A) absorption and (B) circular dichroism spectra (CD) of 1monomer, 1NP, 1NF, (1⊂-CD)NS-NF and (di-1⊂γCD)NS. (Concentration = 0.1 mM).


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0.32 nm suggest -sheet distances between the amides and π-π distances between the aromatic amino acids, that shows minor increase in the -sheet distances (d = 0.46 nm) after incorporation of the γ-CD moieties.29 The fibers and sheets as observed from AFM are quite polydisperse in terms of length (PDI = 1.76) and area (PDI= 3.09) (Figure S15). The kinetic evolution of the nanostructures encouraged us to perform seeded supramolecular polymerization to circumvent the barrier/lag time for the nucleation-propagation to respective nanostructures. Shear-mediated breakage of long 1NF or (di1⊂γ-CD)NS produced short seeds with the reactive ends on either sides or peripheries, that promotes the growth of the nanostructures upon adding a solution of metastable species e.g. 1NP.

Figure 4. Seeded supramolecular polymerization to control the dimension of the fibers and sheets. (A) Cartoon representation showing nucleation mediated growth with 1NF seeds to 1NP of 1 : 2.34 ratio (top) and (di-1⊂γ-CD)NS to 1NP + γ-CD (bottom) in a 1 : 9 ratio and (B) corresponding AFM height images. (C) histogram analyses of the seeded nanofibers and seeded sheets after incubating the solution un-agitated for 8 h and 1 day respectively.


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1NF and (di-1⊂γ-CD)NS were either probe-sonicated for 10 min or stirred at ~600 rpm for 6 h respectively to obtain short seeds with narrow PDI (1.05-1.16) keeping the original shapes retained (Figure S15). Different ratios of 1NF seeds and metastable nanoparticles, 1NP were incubated to grow un-agitated at 15 oC. The fiber seeds directed the metastable nanoparticles towards fiber growth resulting in a linear relation between seed-nanoparticle ratio and average length of the nanofibers with narrow PDI (Figure 4A, S16). Incubation of different ratios of (di-1⊂γ-CD)NS seeds and a mixture of 1NP and γ-CD for 1 day showed increase in area for the seeded nanosheet, even without UV irradiation (Figure S17). However, unlike the linear growth of the length of 1NF with the amount of 1NP, the increase in area rather showed a non-linear relation to the amount of 1NP and γ-CD added to (di-1⊂γ-CD)NS seeds. Interestingly, when we performed the cross-seeding experiments, incubating 1NF seeds with a mixture of 1NP and γ-CD in dark, the seeds

Figure 5. (A) Cross-seeding experiment with incubating 1NF seed and monomer 1NP + γ-CD in 1:1 ratio at 15 oC for 1 day with (lower panel) or without UV irradiation (upper panel). (B) The AFM images and (C) histogram analysis in the upper panel suggest 1NF seed mediated growth of the fibers. However, upon irradiation 1NP + γ-CD forms nanosheet, while the 1NF seed does not show any significant fiber growth (lower panel).


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promoted fiber growth and resulted moderate length control (Figure 5). However, UV irradiation during the course of seeded supramolecular polymerization, altered the pathway towards formation of (di-1⊂γ-CD)NS in an uncontrolled manner while the dimension of the 1NF seeds remained unchanged. This suggests competition of the hydrogen bonding interaction among the peptide units and host-guest interaction between 1NP and CD, albeit hydrogen bonding interaction dominating over host-guest interaction. However, UV irradiation and subsequent formation of strong pseudorotaxane complex e.g. (di-1⊂γ-CD)NS enhanced the stability of host-guest interaction and resulted in mixed population of 1NF fiber seed and polydisperse nanosheets (di-1⊂γ-CD)NS. Similarly, incubation of (di-1⊂γ-CD)NS seeds with 1NP in absence of γ-CD led to mixed population of sheet seeds and nanofibers arising out of uncontrolled growth (Figure S18). This clearly indicates the choice of suitable seeds and monomer is crucial to reign control over the size and shape of the resulting nanostructures. Next, the functional aspects of such dimensionally different nanostructures were investigated. The surface of the peptide nanofibers and 2D nanosheets are positively charge at physiological pH owing to the protonation of lysine side chains. Addition of negatively charged counter anions as in sodium phosphate buffer (10 mM, pH = 7) promotes entanglement through physical crosslinking.24,30 While interfiber ion-bridging furnished hydrogel from 1NF, (di-1⊂γ-CD)NS rather resulted in a viscous solution. It may be presumably due to effective entanglement of the 1NF nanofibers as compared to minimal physical crosslinking of the nanosheets to form 3D hierarchical structures (Figure S19). The hydrogel from 1NF depicted frequency independent behaviour in oscillatory rheological study with G’ value 4 order of magnitude higher than that of the viscous solution of (di-1⊂γ-CD)NS and showed efficient thixotropic shear recovery character for both moduli G’,G’’ (Figure 6).


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Figure 6. (A) Representative image of an inverted vial containing hydrogel of 1NF in sodium phosphate buffer. (B) Frequency sweep oscillatory rheology for the hydrogels obtained by crosslinking of the 1NF and (di-1⊂γ-CD)NS in sodium phosphate buffer (10 mM, pH 7.0). (C) Thixotropic studies for the hydrogel of 1NF (8.46 mM) in SPB buffer (pH 7.0). Black and red color indicate storage modulus (G’, Pa) and is loss modulus (G”, Pa).

Further, we envisaged the cationic surfaces of the peptide materials for efficient exfoliation of inorganic chalcogenides e.g. MoS2. The layer-dependence of the MoS2 sheet renders profound alteration of its optoelectronic, magnetic and semiconducting properties with interesting application in energy storage, photovoltaics and thermoelectric materials31-32 and justifies its exfoliation in single or few layers. Eventhough mechanical exfoliation in presence of suitable ligands renders pristine, high quality structures of two-dimensional MoS2 sheet, it is limited by colloidal stability, use of toxic reagents, harsh exfoliation conditions etc.33-34 The environmentally benign supramolecular peptide fibers, 1NF and nanosheets, (di-1⊂γ-CD)NS were mechanically stirred with 0.16 mg of MoS2 for 12 h to result greenish black dispersions with characteristic absortion peaks of exfoliated MoS2 at 615 nm and 679 nm (Figure 7A). Raman active vibrational


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Figure 7. (A) UV-vis spectra of the exfoliated MoS2 in presence of 1NF and (di-1⊂γ-CD)NS (0.1 mM). Inset depicts the stable greenish black dispersion of MoS2 in 1NF (left) and (di-1⊂γ-CD)NS (right) after 7 days. (B) Raman spectra showing Raman active vibrational modes of the exfoliated MoS2. (C) AFM height image shows the exfoliated MoS2 sheet with inset depicting the height of 1-2 nm. (D) Phase contrast HRTEM images of exfoliated MoS2 in 1NF.

modes E12g and A1g at 386 cm-1 and 411 cm-1 respectively confirms the presence of exfoliated MoS2 for both 1NF and (di-1⊂γ-CD)NS solution (Figure 7B). AFM height image and HRTEM confirms the presence of fibers with layers of MoS2 in an undistorted lattice with hexagonal symmetry (height 1-2 nm), corresponding to mono- and bilayer of the chalcogenides and an avarage exfoliated chalcogenides area of 13 µm2 (Figure 7C-D, S20). Notably, the dispersions of MoS2 in 1NF is quite stable for ca. 1 month, while the composites from (di-1⊂γ-CD)NS precipitate after a week. This corroborates with increased exposure of cationic lysine units on the 1NF relative


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to (di-1⊂γ-CD)NS resisting the aggregation of dispersions of MoS2 and results a higher degree of colloidal stability. CONCLUSION In summary, we demonstrated an elegant pathway dependent self-assembly of peptide amphiphile to differentiate in 1D and 2D supramolecular polymers mediated by hydrogen bonding, host-guest interaction and photodimerization. Selection of the right kind of seeds promotes the unprecedented growth of nanofibers or nanosheets via seeded supramolecular polymerization with excellent control over the shape and dimension. Such structural history and adaptive control over the peptide materials could further be utilized for modulating the mechanical stiffness of the crosslinked hierarchical three-dimensional hydrogel network. The dimensional flexibility and surface charges were judiciously exploited for efficient exfoliation of MoS2 in single or bilayer of thickness and increased surface area. This work establishes a new strategy interplayed by multiple orthogonal interactions to dictate the fate of assembly in multiple pathways with unprecedented topological control over dimensions. It opens a new paradigm to relate the design and functional properties of nanocomposites for application in adaptive materials and devices. EXPERIMENTAL SECTION Multi-step self-assembly of peptides: Peptide, 1 was dissolved in HFIP to form stock solution of 6.25 mM at 10 °C. 10 L of the stock solution was added to 0.5 ml of water and mixture of HFIPwater in the ratio 1:3, 1:1 at 10 °C to a final concentration of 0.125 mM. At this temperature in all the cases, formation of the metastable nanoparticles were observed. The temperature was raised in a controlled manner using stuart waterbath (SWB15D) to 25 oC to observe transformation into nanofibers.


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16 L of the stock solution of 1 (6.25 mM) in HFIP was added to equimolar -CD (0.5 ml) at 10 °C to a final concentration of 0.2 mM. The temperature was raised in a controlled manner using stuart waterbath to 25 oC to observe transformation in to other nanostructures. The corresponding solution was kept for UV irradiation ( = 320 nm, 8 watts x 2 lamp) for 8 hours. Further microscopic analysis performed to observe formation of 2D nanosheets. Short seed formation using mechanical stirrer and probe sonicator: An aliquot of 0.5 mL of pre-assembled 1NF peptide solution (0.2 mM) was taken in 2 ml eppendorf which was fixed with a stand in a glass beaker having ice cold water. Then probe microtip (4417 number) was dipped into the sample and was sonicated using QSonica (model number Q700, power 700 watts and frequency 20 kHz) at an amplitude of 20% for ~10 minutes (5 sec on and 5 sec off to avoid heating due to the probe). An aliquot of 0.5 mL of peptide sheet solution (di-1γ-CD)NS (0.2 mM) was taken in 2 ml eppendorf and was kept on stirring at 600 rpm for 6 h using IKA magnetic stirrer. The formation of the short seeds were monitored by recording the AFM of the samples. Seeded supramolecular polymerization (SSP): Seeded supramolecular polymerization (SSP) was performed taking different ratio of the nanofiber (1NF) seeds and metastable nanoparticles (1NP) (Table S1) at 15 oC to furnish different length distribution of the fibers. In a separate experiment, seeds of 2D nanosheet, (di-1⊂γ-CD)NS were added in different ratio to equimolar mixture of monomers (1NP) with γ-CD at 15 oC in order to furnish sheets with growing area. Atomic Force Microscopy (AFM): Sample preparation at low temperature was performed by placing silicon wafer on a cold water maintained at 10 oC. 15 µL of 0.2 mM of the sample solution was drop-casted on Silicon wafer, placed on an ice-water bath maintained at 10 oC. After 5 minutes, the silicon wafer was washed with 500 µL water to remove excess peptide and it was then


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left to be air-dried in a desiccator. AFM height images were recorded by tapping mode on a Bruker Multimode 8 scanning probe microscope with silicon cantilever (Bruker) and analysed using the software NanoScope Analysis 1.5. Analysis of average length/area from AFM images: AFM images were recorded for the short seeds and growing fibers, sheets as obtained from different ratio of seeds and monomers. The length distributions of the fibers were analyzed using image-J software, from the U.S. National Institutes of Health. 100 random fibers or 50 random sheets were selected from different areas of the images and a histogram was generated by choosing bin and frequency in Microsoft Excel. The

average lengths, areas and PDI were estimated by calculating number average length, Area (Ln, An) and weight average fiber length, area (Lw, Aw) and the ratio Lw/Ln, Aw/An using eqn. 1-3, where Ni is the number of fibers and sheets of length Li and area Ai, and n is the number of fibers and sheets examined in each sample. MoS2 Exfoliation study: Stock dispersions were prepared by adding bulk MoS2 (0.16 mg) in powder form to 0.25 ml solutions of 1NF as well as (di-1-CD)NS (2 mM each). Afterwards the dispersions were stirred overnight at ~250 rpm. The dispersions were characterized by UV-vis


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spectroscopy at 0.1 mM. AFM samples were prepared by dropcasting 10 µL (0.1 mM) of 1NF + MoS2, (di-1⊂γ-CD)NS) + MoS2 on silicon wafer and dried overnight in desiccator. TEM samples were prepared by dropcasting 6 µL (0.1 mM) of 1NF + MoS2, (di-1⊂γ-CD)NS + MoS2 on 300 mesh carbon coated copper grid. Samples for Raman spectra were prepared by dropcasting 10 µL of stock solution on silicon wafer.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis and Characterization of peptide, 1; Monitoring host-guest complexation and photodimerization by 1H NMR and UV spectroscopy; Multi-step self-assembly of peptide, 1; Thioflavin-T binding and confocal studies; FTIR and X-ray diffraction studies; Generation of the Seeds and Seeded Supramolecular Polymerization, Formation of Hydrogel; MoS2 Exfoliation study (PDF) AUTHOR INFORMATION Corresponding Author Asish Pal *Email: [email protected] ORCID Asish Pal: 0000-0002-6158-1158 Author Contributions ‡ Jojo P. Joseph and Ashmeet Singh contributed equally.


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Funding Sources Department of Science & Technology (DST, SERB project ECR/2016/000401) Department of Biotechnology (BT/PR22067/NNT/28/1163/2016) Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors are thankful to INST, Mohali for providing infrastructure. A. Pal thanks Department of Science & Technology (DST, SERB project ECR/2016/000401) and Department of Biotechnology (BT/PR22067/NNT/28/1163/2016) for financial support. A. Singh thanks University Grants Commission (UGC), New Delhi for SRF fellowship (Sr. No 2121310082). J. P. Joseph, D. Gupta and C. Miglani gratefully acknowledge the INST fellowship.

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