Non-enzymatic oligonucleotide ligation in photoswitchable coacervate protocells sustains compartment-content coupling

Modern cells are adaptive chemical compartments tightly regulated by an underlying DNA-encoded program. Reproducing such a coupling between information content and chassis in synthetic compartments represents a key step to the assembly of evolvable protocells, but remains challenging. Here, we rationally exploit complexation between end-reactive oligonucleotides able to stack into long physical polymers and a cationic azobenzene photoswitch to produce three different phases – soft solids, liquid crystalline or isotropic coacervates droplets – that promote non-enzymatic oligonucleotide ligation, with a marked phase-dependent reaction efficiency. Changes in the population of polynucleotides during polymerization induce in turn phase transitions that dramatically alter the physical properties of the compartments. Dynamical modulation of coacervate assembly and dissolution via trans-cis azobenzene photoisomerisation is last used to demonstrate cycles of light-actuated oligonucleotide ligation. Overall, by combining a tight reaction-structure coupling and environmental responsiveness, our light-responsive reactive coacervates provide a novel general route to the non-enzymatic synthesis of polynucleotides, and pave the way to the emergence of a primitive genotype-phenotype coupling in membrane-free protocells.

Adhesive and normal stress-dependent dynamic friction of a gelatin hydrogel

Adhesion and friction of soft solids on hard surfaces are the important properties for a variety of practical applications. In the present study, Coulomb's law of friction is used for characterizing adhesive friction as well as normal stress-dependent dynamic friction of a gelatin hydrogel on a fixed glass surface. The experimental data, concerning normal stress-dependent dynamic friction of different shear velocity, are obtained from literature. It is observed that both components of friction increase with shear velocity. More importantly, the scaling law shows that adhesive stress varies almost linearly with corresponding coefficient of friction of the hydrogel. A dynamic friction model is also used to analyze the same experimental data to predict a negative normal stress at which dynamic friction reduces to zero, and this result matches closely with the experimental value.

Dynamic fracture of expanding cavities in nonlinear soft solids

Recent experimental observation [Milner and Hutchens, Mech. of Mat., 2021] suggests that crack formation during rapid cavity expansion in low modulus, highly-deformable solids depends on the ratio of the rate of expansion and the acoustoelastic wavespeed, similar to observations in rock and metal [Grady and Kipp, Frac. Mech. of Rock, 1987]. Here, we explore the effect of material non-linearity on predictions of the number of cracks formed at the cavity surface. We find that non-linearity influences crack formation only when the cavity-size normalized elasto-fracture length is greater than one and the cavity's rate of expansion is greater than the acoustoelastic wavespeed. The sensitivity of these predictions on the assumed fracture geometry, either a spherical damaged zone or a discrete cracks, suggests a direction for further experimentation that may illuminate crack formation mechanism in soft solids under dynamic loading.