Dispersion and Interaction of Charged Fluorescent Dyes in Protein-Polymer Surfactant-based Non-Aqueous Liquid.

Viscous, non-aqueous liquid comprising stoichiometric conjugates of polymer surfactant-bovine serum albumin (PSpBSA) is used as a host matrix for the dispersion of chemically distinct hydrophilic dyes. Using a combination of bright field polarized optical microscopy and fluorescence spectroscopy, we investigate the dispersion of dry and powdered cationic (Rhodamine 6G; Rh6G) and anionic (Fluorescein; FL) dyes in the PSpBSA liquid at room temperature. As the dyes disperse and dissolve in the PSpBSA liquid, it results in a pronounced increase in emission intensity of the former. Interestingly, a shift from 571 to 582 nm is observed in the emission maxima of Rh6G as it disperses in the PSpBSA solvent. Whilst no such red shift is found for the Rh6G dispersion in the aqueous solutions of either native BSA or polymer-surfactant conjugated BSA, a similar shift occurs when Rh6G is dispersed in neat polymer-surfactant (PS), suggesting the interaction of the dye with the PS chains. In the case of anionic FL, no shift is observed in its emission maximum as it disperses in the PSpBSA liquid. Furthermore, within 120 minutes of FL dispersion in the PSpBSA liquid, we observe a ≈26 % decrease in the tryptophan emission intensity (λexc. =285 nm; λemi. =330 nm) of BSA, which could be attributed to both static and dynamic quenching. Our findings provide a proof of concept of an alternative non-aqueous solvent matrix which can dissolve and disperse charged fluorescent dyes, provide suitable binding sites, and show substantial photoluminescence. Thus, it can be envisaged for utilization as an alternative solvent medium for lasing dyes and related applications.

α-Synuclein Spontaneously Adopts Stable and Reversible α-Helical Structure in Water-Less Environment.

A highly stable, spontaneous, and reversible α-helical-structure formation in recombinant and chemically modified α-synuclein protein is demonstrated for the first time in a water-less (1.5 % w/w H2 O) polymer surfactant environment. Using a combination of circular dichroism and ATR-FTIR spectroscopy, we show that whilst native α-synuclein in aqueous solution shows a predominant unordered conformation (≈64 %), mixing with polyethylene glycol based anionic polymer surfactant (PS) and removing water reveals a 25 % unordered, 25 % α-helical, and 27 % ß-sheet structure. Interestingly, bioconjugation of native α-synuclein with a diamine molecule, to increase the positive charge on the protein chain, and subsequent electrostatic coupling with the PS forms a conjugate with a retained unordered structure. Removal of water from this system provides a highly stable α-helical (≈74 %) water-less liquid system. Surprisingly, the α-helical-to-unordered state transition is completely reversible and is achieved at ≈25-30 w/w% of water in the system. Moreover, the α-helix shows an extraordinary temporal stability (>6 months) in a waterless environment.

Neat Protein-Polymer Surfactant Bioconjugates as Universal Solvents.

Solvents, particularly those having low volatility, are of great interest for the biocatalytic synthesis of utility chemicals and fuels. We show novel and universal solvent-like properties of a neat and water-less polymer surfactant-bovine serum albumin (BSA) conjugated material (WL-PS pBSA). This highly viscous, nonvolatile material behaves as a liquid above its solid-liquid transition temperature (∼25-27 °C) and can be used as a solvent for variety of completely dry solutes of different sizes and surface chemistries. We show using a combination of optical microscopy and steady -state and time-resolved fluorescence spectroscopy that dry and powdered dyes (hydrophobic Coumarin 153 (C153)), enzymes (α-chymotrypsin (α-Chy)), or even 1 µm microparticles (diffusion coefficient ca. three orders slower than C153), can be solubilized and completely dispersed in the WL-PS pBSA solvent above 25 °C. While C153, irrespective of its mode of addition to WL-PS pBSA, binds similarly to BSA, α-Chy can be stabilized and activated to perform its hydrolysis function, even at 100 °C. This work therefore provides insights into the form of universal solvent characteristic property for this new class of nonaqueous, nonvolatile, biodegradable protein-polymer surfactant-based conjugated materials and suggests potential new avenues that can have a huge impact on biocatalysis, bionanotechnology, drug delivery, and other applications.

Ultraslow Biological Water-Like Dynamics in Waterless Liquid Protein.

Fluorescence correlation spectroscopy and time-dependent fluorescence Stokes shift have been employed to elucidate dynamics in different time scales, ranging from picoseconds to nanoseconds, for human serum albumin, in its native and cationized forms as well as in the self-assembled complex of the cationized protein with the polymer surfactant (PS) glycolic acid ethoxylate lauryl ether. The effect of crowding in this complex, especially in the waterless condition, is of prime importance in this context. Excellent correlation of the dynamics with the structures, obtained by circular dichroism and Fourier transform infrared spectroscopy, has been observed. Slow solvation, associated classically with biological water, has been observed in these systems, even in the waterless condition. This apparently intriguing observation has been rationalized by the relaxation of segments of the protein and the PS in the microenvironment of the fluorescent probe.

Bulk rheology of sticky DNA-functionalized emulsions.

We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian droplet emulsions functionalized with surface-bound single-stranded (ss), "sticky," DNA. In the absence of ssDNA, the emulsion viscosity increases with the dispersed phase volume fraction ϕ, before passing through a liquid-solid transition at a critical ϕ_{c} related to random close packing. Introducing ssDNA leads to a liquid-solid transition at ϕ<ϕ_{c}, the onset being set by the droplet valency N and the ssDNA concentration (or simulated binding strength ε). Using insight from simulation, we identify three key behaviors: (i) jammed suspensions (ϕ>ϕ_{c}≈0.64) show weak effects of functionalization, with elastic rheology instead governed by droplet stiffness; (ii) suspensions with ϕ<ϕ_{c} and N=1, 2 always exhibit viscous rheology, regardless of functionalization; and (iii) for ϕ<ϕ_{c} and N>3, functionalization leads to a controllable viscous-elastic transition. We present state diagrams showing the range of rheological tuning attainable by these means.