Complex Coacervation and Overcharging during Interaction between Hydrophobic Zein and Hydrophilic Laponite in Aqueous Ethanol Solution.

In this paper, for the first time, we have reported the formation of complex coacervate during interaction between hydrophobic protein, zein, and hydrophilic nanoclay, Laponite, in a 60% v/v ethanol solution at pH 4. Dynamic light scattering and viscosity measurements revealed the formation of zein-Laponite complexes during the interaction between zein at fixed concentration, C Z = 1 mg/mL, and varying concentrations of Laponite, C L (7.8 × 10-4 - 0.25% w/v). Further investigation of the zein-Laponite complexes using turbidity and zeta potential data showed that these complexes could be demarcated in three different regions: Region I, below the charge neutralization region (C Z = 1 mg/mL, C L ≤ 0.00625% w/v) where soluble complexes was formed during interaction between oppositely charged zein and Laponite; Region II, the charge neutralization region (C Z = 1 mg/mL, 0.00625 < C L ≤ 0.05% w/v) where zein-Laponite complexes form neutral coacervates; and Region III, the interesting overcharged coacervates region (C Z = 1 mg/mL, C L > 0.05% w/v). Investigation of coacervates using a fluorescence imaging technique showed that the size of neutral coacervates in region II was large (mean size = 1223.7 nm) owing to aggregation as compared to the small size of coacervates (mean size = 464.7 nm) in region III owing to repulsion between overcharged coacervates. Differential scanning calorimeter, DSC, revealed the presence of an ample amount of bound water in region III. The presence of bound water was evident from the presence of an additional peak at 107 °C in region III apart from normal enthalpy of evaporation of water from coacervates.

Spontaneous Fluctuations Can Guide Drug Design Strategies for Structurally Disordered Proteins.

Structure-based "rational" drug design strategies fail for diseases associated with intrinsically disordered proteins (IDPs). However, structural disorder allows large-amplitude spontaneous intramolecular dynamics in a protein. We demonstrate a method that exploits this dynamics to provide quantitative information about the degree of interaction of an IDP with other molecules. A candidate ligand molecule may not bind strongly, but even momentary interactions can be expected to perturb the fluctuations. We measure the amplitude and frequency of the equilibrium fluctuations of fluorescently labeled small oligomers of hIAPP (an IDP associated with type II diabetes) in a physiological solution, using nanosecond fluorescence cross-correlation spectroscopy. We show that the interterminal distance fluctuates at a characteristic time scale of 134 ± 10 ns, and 6.4 ± 0.2% of the population is in the "closed" (quenched) state at equilibrium. These fluctuations are affected in a dose-dependent manner by a series of small molecules known to reduce the toxicity of various amyloid peptides. The degree of interaction increases in the following order: resveratrol < epicatechin ∼ quercetin < Congo red < epigallocatechin 3-gallate. Such ordering can provide a direction for exploring the chemical space for finding stronger-binding ligands. We test the biological relevance of these measurements by measuring the effect of these molecules on the affinity of hIAPP for lipid vesicles and cell membranes. We find that the ability of a molecule to modulate intramolecular fluctuations correlates well with its ability to lower membrane affinity. We conclude that structural disorder may provide new avenues for rational drug design for IDPs.

Bioinspired pH and magnetic responsive catechol-functionalized chitosan hydrogels with tunable elastic properties.

We have developed pH- and magnetic-responsive hydrogels that are stabilized by both covalent bonding and catechol/Fe(3+) ligands. The viscoelastic properties of the gels are regulated by the complexation valence and can be used to tune drug release profiles. The stable incorporation of magnetic nanoparticles further expands control over the mechanical response and drug release, in addition to providing magnetic stimuli-responsivity to the gels.

Ergodic-to-nonergodic phase inversion and reentrant ergodicity transition in DNA-nanoclay dispersions.

We have observed DNA concentration and hydration dependent inversion from ergodic to non-ergodic phase followed by reentry into the ergodic phase in DNA-nanoclay (laponite) dispersions at room temperature (25 °C), using results obtained from dynamic light scattering (DLS) and rheology data. The interaction between the DNA strand and the anisotropically charged discotic platelets of laponite (L) was found to be strongly hierarchical in DNA concentration. For a fixed laponite concentration (CL = 1% (w/v)) and varying DNA concentration (CDNA) from 0.3-2.3% (w/v), we observed three distinct phase regions characterized by the following: region (i): CDNA < 1.0% (w/v), ergodic region with weak DNA-L attractive interaction, region (ii): 1.0% < CDNA < 1.6% (w/v), non-ergodic regime having strong DNA-L associative interaction and region (iii): CDNA > 1.6% (w/v), showing phase reentry into the ergodic regime due to repulsion between DNA strands. Hydration study in these three regions revealed that a loss in the abundance of amorphous water, signified by Raman frequency 3460 cm(-1), caused the ergodic to nonergodic phase transition. In summary, it is shown that maximum stability and interaction between DNA and nanoclay platelets occurred at an intermediate concentration of DNA where the hydration was at its minimum. The present system is qualitatively different from the hard-sphere/polymer systems for which reentrant phase transition has been reported in the literature. However, some similarity between the two classes of systems is not ruled out.

Condensation, complex coacervation, and overcharging during DNA-gelatin interactions in aqueous solutions.

Interaction between DNA (effective hydrodynamic radius, R(DNA) ≈ 140 nm) and Gelatin A (GA) (effective hydrodynamic radius, R(GA) ≈ 55 nm) with charge ratio (DNA:GA = 16:1) and persistence length ratio (5:1) was studied by using fixed DNA concentration (5 × 10(-3) % (w/v)) and varying GA concentration (C(GA) = 0-0.25% (w/v)). Experimentally, three interesting regions of interaction were observed from dynamic light scattering, turbidity, zeta potential, and viscosity data: (i) C(GA) < 0.05% (w/v), GA binds to DNA forming soluble complexes of size R(complex) ≈ 60 nm < R(DNA) (primary binding causing condensation); (ii) 0.05% < C(GA) < 0.1% (w/v), R(complex) ≈ 60 to 180 nm was observed up to charge-neutralization point (secondary binding); and (iii) C(GA) > 0.1% (w/v) showed interesting overcharging behavior of DNA-GA complexes, followed by liquid-liquid phase separation (complex coacervation). Aforesaid regions of interaction were further examined theoretically by modeling the problem using electrostatic and van der Waals interaction potentials treating GA molecules as counterions to DNA macroion. Region (i) was explained on the basis of electrostatic screening, followed by reduction in persistence length, which resulted in condensation of DNA-GA complex. In region (ii), the dominance of van der Waals forces led to the formation of large soluble complexes through selective binding. This was possible due to closer proximity between GA and DNA-GA complexes and the absence of strong electrostatic forces. In region (iii), these oversized and overcharged complexes coarsened, leading to complex coacervation. Here the interaction energy profile showed that a greater number of counterions was required over and above the usual charge neutralization requirement for low-energy configurations to be achieved.

Concentration selective hydration and phase states of hydroxyethyl cellulose (HEC) in aqueous solutions.

Solution behaviour of hydroxyethyl cellulose (HEC) is reported in the polymer concentration range spanning over two decades (c=0.002-5% (w/v)). The results conclude the following: (i) dilute solution regime prevailed for c<0.2% (w/v), flexible HEC fibres of typical length ≈ 1 µm and persistence length ≈ 10 nm were found here, (ii) for 0.2