The exploitation of biological macromolecules, such as nucleic acids, for the fabrication of advanced materials is a promising area of research. Although a greater variety of structural and functional uses can be envisioned for protein-based materials, systematic approaches for their construction have yet to emerge. Consistent with theoretical models of polymer macrocyclization, we have demonstrated that, in the presence of dimeric methotrexate (bisMTX), wild-type Escherichia coli dihydrofolate reductase (DHFR) molecules tethered together by a flexible peptide linker (ecDHFR(2)) are capable of spontaneously forming highly stable cyclic structures with diameters ranging from 8 to 20 nm. The nanoring size is dependent on the length and composition of the peptide linker, on the affinity and conformational state of the dimerizer, and on induced protein-protein interactions. Delineation of these and other rules for the control of protein oligomer assembly by chemical induction provides an avenue to the future design of protein-based materials and nanostructures.

The sol–gel transition in dilute gels of gelatin are studied by differential scanning calorimetry (DSC) and static light scattering techniques. The sol–gel phase diagram clearly shows the existence of an upper critical solution temperature in this system. In addition, the DSC data conclusively exhibits two more phase curves in the sol state—one pertaining to the coexistence of monomers and aggregates and the second one separating the random coil and helix domains in the solution phase. The Ferry–Eldridge equation has been used to determine the enthalpy of the melting of the gel structure (ΔHg) which is equal to (30±2.0) kcal mol−1. The gelation temperature Tg is correlated to Flory’s statistical model of gelation, which gives the enthalpy of melting of pure gelatin crystallites (ΔHv) as ΔHv=(35±2.0) kcal cm−3 and crystallite melting temperature (Tm) as Tm=(588±20.0) K. The Flory–Huggins interaction parameter (χ) has been determined as χ=0.49±0.05. Interesting scaling behavior has been observed through light scattering measurements. The intensity of scattered light, Is∼ε−γ and Is∼ε0−β; ε=(T/Tg−1); T≥Tg and ε0=(t/tg−1); t≥tg, where t is the time and tg is the gelation time for ε and ε0≤0.5 with γ≂β≂0.02±0.005. The initial phase of cooling of the sol has been analyzed within the framework of Smoluchowski aggregation kinetics. On the basis of the above results it is being proposed that the sol–gel transition path in this polymer has three distinct steps—monomer aggregation, coil–single-helix transition, and single-helix–triple-helix transition followed by gelation.

Aqueous gelatin solutions have been studied in a systematic and exhaustive manner in a neutral buffer medium, in the temperature range of 30–60 °C, by static (SLS) and dynamic laser light scattering. It has been clearly established that upon cooling the sol from 60 °C, the gelatin chains form thermoreversible aggregates until the gelation is encountered below 30 °C. The radius of gyration of the chains (Rg) at any temperature was found to scale with corresponding molecular weight as Rg∼M0.57±0.03w. The ratio of Rg to hydrodynamic radius RH has been found to be Rg/RH=1.82±0.03 as expected for random coils. The translational diffusion coefficient of the chains Dz(c) exhibited linear concentration dependence; Dz(c)=D0(1+KDc) and D0∼M−0.57±0.03. The second virial coefficient (A2) of osmotic pressure obtained through SLS could be excellently related to KD through KD≂2A2Mw for all temperatures. The Flory–Huggins interaction parameter χ was determined to be χ≂0.48 and it showed negligible temperature dependence. The temperature dependence of sol viscosity η(T) could be fitted to Doolittle equation ln η(T)=A+B/T. At any given temperature, the concentration dependence of specific viscosity ηsp(c) followed ηsp(c)/c=1/([η]+k√c). The intrinsic viscosity [η] was found to scale with Mw as [η]∼M0.69±0.08w also yielding a typical overlap concentration of c*≂2.5% (w/w). The Flory–Fox constant Φ was found out to vary between 2.66×1021 at 35 °C and 1.27×1021 at 60 °C, against the standard value of 2.21×1021. The density values of the sol have been measured by a simple but novel method. Both the sol density and refractive index show monotonous increase as the gel point is approached from the hot sol state of gelatin.

Hydrogel formation triggered by a change in temperature is an attractive mechanism for in situ gelling biomaterials for pharmaceutical applications such as the delivery of therapeutic proteins. In this study, hydrogels were prepared from ABA triblock polymers having thermosensitive poly(N-(2-hydroxypropyl) methacrylamide lactate) flanking A-blocks and hydrophilic poly(ethylene glycol) B-blocks. Polymers with fixed length A blocks (~22 kDA) but differing PEG-midblock lengths (2, 4 and 10 kDa) were synthesized and dissolved in water with dilute fluorescein isothiocyanate (FITC)-labeled dextrans (70 and 500 kDA). Hydrogels encapsulating the dextrans were formed by raising the temperature. Fluorescence recovery after photobleaching (FRAP) studies showed that diffusion coefficients and mobile fractions of the dextran dyes decreased upon elevating temperatures above 25 °C. Confocal laser scanning microscopy and cryo-SEM demonstrated that hydrogel structure depended on PEG block length. Phase separation into polymer-rich and water-rich domains occurred to a larger extent for polymers with small PEG blocks compared to polymers with a larger PEG block. By changing the PEG block length and thereby the hydrogel structure, mobility of FITC-dextran could be tailored. At physiological pH the hydrogels degraded over time by ester hydrolysis, resulting in increased mobility of the encapsulated dye. Since diffusion can be controlled according to polymer design and concentration, plus temperature, these biocompatible hydrogels are attractive as potential in situ gelling biodegradable materials for macromolecular drug delivery.