Bapan Pramanik

Solvent Assisted Tuning of Morphology of a Peptide-Perylenediimide Conjugate: Helical Fibers to Nano-Rings and their Differential Semiconductivity

Understanding the regulatory factors of self-assembly processes is a necessity in order to modulate the nano-structures and their properties. Here, the self-assembly mechanism of a peptide-perylenediimide (P-1) conjugate in mixed solvent systems of THF/water is studied and the semiconducting properties are correlated with the morphology. In THF, right handed helical fibers are formed while in 10% THF-water, the morphology changes to nano-rings along with a switch in the helicity to left-handed orientation. Experimental results combined with DFT calculations reveal the critical role of thermodynamic and kinetic factors to control these differential self-assembly processes. In THF, P-1 forms right handed helical fibers in a kinetically controlled fashion. In case of 10% THF-water, the initial nucleation of the aggregate is controlled kinetically. Due to differential solubility of the molecule in these two solvents, elongation of the nuclei into fibers is restricted after a critical length leading to the formation of nano-rings which is governed by the thermodynamics. The helical fibers show superior semi-conducting property to the nano-rings as confirmed by conducting-AFM and conventional I-V characteristics.

Redox controlled reversible transformation of a supramolecular alternating copolymer to a radical cation containing homo-polymer

Research in the area of supramolecular polymeric materials has grown substantially in recent years owing to their potential applications in drug delivery, sensing, and catalysis. In this work, a supramolecular polymer has been prepared using the hetero-ternary complexation of cucurbit[8]uril (CB[8]). The CB[8] mediated ternary complexation of a viologen dimer and a tryptophan dimer led to the formation of a supramolecular alternating copolymer of (AB)n type. The polymer was characterized using UV-visible, fluorescence, 1H and DOSY NMR, DLS, and microscopy techniques. The copolymer led to the formation of well dispersed micro-globules of average diameter between 150 and 250 nm. Upon reduction of the viologen units of the copolymer, viologen radical cations were formed in the solution. These viologen radical cations displaced the second guest (tryptophan units) and formed dimers inside the CB[8] cavity. This complexation by viologen radical cations eventually led to the formation of a homo-polymer of (A)n type. The degree of polymerization was calculated to be approximately three times that of the copolymer, and larger micro-globules (∼950 nm) were obtained. Upon oxidation of the homo-polymer, the system reverted to the original co-polymer system. The reversible transformation can be achieved several times by controlling the redox reaction. A similar homo-polymer was also obtained by reducing the viologen dimer in the presence of CB[8].

A Viologen-Perylenediimide Conjugate as an Efficient Base Sensor with Solvatochromic Property.

A viologen-perylenediimide conjugate, denoted PDEV, is prepared for efficient base sensing. The conjugate shows solvatochromic behavior as well. The base sensitivity of viologen is purposefully coupled with the emission property of perylenediimide (PDI) to lower the detection limit. PDEV shows base-sensing ability at the ppb level, which is at least three orders of magnitude lower than those of previously reported sensors. The probe is sensitive toward solvent polarity and generates different shades of colors according to the polarity of the medium (solvent). The photophysical properties show a linear correlation with the solvent polarity, and this makes it an efficient solvatochromic agent. On the other hand, the generation of viologen radical cations by bases affects the aggregation and consequently the absorption and emission behavior of the PDI core. The effect of bases can also be visualized, because the probe generates different colors in the presence of bases, both under normal and under UV light. Organic amines can be detected even in the crystalline state, since the dark red color of the PDEV crystals changes to purple in a reversible fashion on exposure to amine vapors. An easy and practical paper-based tool created by using the probe can efficiently be used to detect solvent polarity and presence of bases optically.

Solvent Directed Morphogenesis and Electrical Properties of a Peptide–Perylenediimide Conjugate

Molecular organization of electron-deficient aromatic systems like perylenediimides (PDI) is extremely appealing, as they are potential candidates for organic electronics. The performance of these molecules in such applications primarily depends on the self-organization of the molecules. However, any correlation between the morphology of these self-assembled semiconducting molecules and their electrical performances has not yet been formulated. Herein, for the first time, we have made an effort to find such a correlation by studying the self-assembly, morphology, and their conducting properties for a peptide-PDI conjugate. The PDI conjugate formed fiber-like morphology in relatively nonpolar solvents (THF and CHCl3) while in more polar solvents (HFIP, MeOH, ACN, and acetone), spherical morphology could be found. Interestingly, the self-assembly and the morphologies showed a clear dependence on the solvent polarity. In polar solvents, the conjugate aggregates more efficiently than in the nonpolar solvents, and with decrease in solvent polarity, the dimension of the nanostructures increased. However, in all the tested solvents, irrespective of their polarity, the PDI-peptide conjugate adopts a right-handed helicity. To find a correlation between the morphologies with the conducting property, detailed electrical characterization of these nanostructures was carried out. While no significant change could be observed for the dc conductivities of these nanostructures, the ac conductivities show prominent difference at the low-frequency region. A dispersion of conductivity was observed for the nanospheres due to the polarization effect. A critical correlation between the nanostructures and the activation energy was observed as with decrease in radii of curvature of the aggregates the activation energy increases with an exception in the case of MeOH. The observed results suggest that the long-range transport of charge carriers is less favorable when the aggregates are small and closely packed.

Multiple Cross-Linking of a Small Peptide to Form a Size Tunable Biopolymer with Efficient Cell Adhesion and Proliferation Property

Development of biocompatible polymeric systems capable of cell adhesion and proliferation is a challenging task. Proper cross-linking of small cell adhesive peptide sequences is useful in this respect as it provides the inherent nontoxic environment as well as the cross-linked polymeric network to the cells for adhesion and proliferation. A multiple cross-linking strategy is applied to create a peptide-based cross-linked polymer. Covalent linkage through disulfide bond formation, supramolecular linkage using homoternary complexation by CB[8], and enzymatic cross-linking by HRP-mediated dimerization of tyrosine are used to prepare the cross-linked, peptide-based polymer decorated with cell-adhesive RGDS sequence. The supramolecular cross-linking via CB[8] provided stability as well as brings the RGDS sequences at the surface of the polymer particles. The order of cross-linking allowed to fine-tune the particle size of the polymer and polymer particles of wide range (200-1000 nm) can be prepared by varying the order. The cross-linked polymer particles (P1 and P2) were found to be stable at wide range of temperature and pH. Moreover, as intended, the polymer was noncytotoxic in nature and showed efficient cell adhesion and proliferation property, which can be used for further biological applications.

Hydrogelation of a Naphthalene Diimide Appended Peptide Amphiphile and Its Application in Cell Imaging and Intracellular pH Sensing

This study reports the self-assembly and application of a naphthalene diimide (NDI)-appended peptide amphiphile (PA). H-bonding among the peptide moiety in conjunction with π-stacking between NDI and hydrophobic interactions within the alkyl chain are the major driving forces behind the stepwise aggregation of the PA to form hydrogels. The PA produced efficient self-assemblies in water, forming a nanofibrous network that further formed a self-supportive hydrogel. The molecule followed a three-step self-assembly mechanism. At a lower concentration (50 μM), it forms extremely small aggregates with a very low population of the molecules. With an increase in concentration, spherical aggregates are formed above 450 μM concentration. Importantly, this water-soluble conjugate was found to be nontoxic, cell permeable, and usable for cell imaging. Moreover, the aggregation process and consequently the emission behavior are highly responsive to the pH of the medium. Thus, the pH responsive aggregation and emission behavior has an extended biological application for assessing intracellular pH. The biocompatibility and intracellular pH determining capability suggest it is a promising candidate for use as a supramolecular material in biomedical applications.