Weak Bonds in a Biomimetic Adhesive Enhance Toughness and Performance.

Developing future high performance adhesives is predicated upon achieving properties including strength and ductility. However, designing tough materials that are simultaneously strong and soft is usually contradictory in nature. Biological materials including shells and wood achieve impressive toughness by using weak bonds to connect larger structures at several length scales. Here, we show that this toughness design approach can be applied to synthetic adhesives. A biomimetic adhesive polymer, poly(catechol-acrylic acid), was examined in conjunction with several compounds containing two organic functional groups. In a typical example, the diol ethylene glycol decreased the overall system modulus. Performance was seen to increase significantly. Spectroscopic and physical methods indicated that these bifunctional additives created an interpolymeric network of weak hydrogen bonds. Material toughness was enhanced when breakable bonds were available to dissipate mechanical stresses while leaving the surrounding matrix intact. These discoveries illustrate how a biological materials strategy of interplay between strength and ductility can be achieved with sacrificial bonds in an adhesive. Such an approach may be a general principle applicable to designing higher performance electronics, transportation, and aerospace systems.

High Strength Underwater Bonding with Polymer Mimics of Mussel Adhesive Proteins.

When it comes to underwater adhesion, shellfish are the true experts. Mussels, barnacles, and oysters attach to rocks with apparent ease. Yet our man-made glues often fail when trying to stick in wet environments. Results described herein focus on a copolymer mimic of mussel adhesive proteins, poly(catechol-styrene). Underwater bonding was examined as a function of parameters including polymer molecular weight and composition. In doing so, several surprising results emerged. Poly(catechol-styrene) may be the strongest underwater adhesive found to date. Bonding even exceeded that of the reference biological system, live mussels. Adhesion was also found to be stronger under salt water than deionized water. Such unexpected findings may contradict an earlier proposal in which charged amino acids were suggested to be key for mussel adhesive function. Taken together, these discoveries are helping us to both understand biological adhesion as well as develop new materials with properties not accessed previously.

Interplay of flavin's redox states and protein dynamics: an insight from QM/MM simulations of dihydronicotinamide riboside quinone oxidoreductase 2.

Dihydronicotinamide riboside quinone oxidoreductase 2 is known to catalyze a two-electron reduction of quinone to hydroquinone using its cofactor, flavin adenine dinucleotide. Using quantum mechanical/molecular mechanical simulations, we have computed the reorganization free energies of the electron and proton transfer processes of flavin in the free state as well as when it is bound in the active site of the enzyme. The calculated energetics for electron transfer processes demonstrate that the enzyme active site lowers the reorganization energy for the redox process as compared to the enzyme-free aqueous state. This is most apparent in the two electron reduction step, which eliminates the possibility of flavosemiquinone generation. In addition, essential dynamics study of the simulated motions revealed spectacular changes in the principal components of atomic fluctuations upon reduction of flavin. This alteration of active site dynamics provides an insight into the "ping-pong" kinetics exhibited by the enzyme upon a change in the redox state of the enzyme-bound flavin. A charge perturbation analysis provides further support that the observed change in dynamics is correlated with the change in energetics due to the altered electrostatic interactions between the flavin ring and the active site residues. This study shows that the effect of electrostatic preorganization goes beyond the chemical catalysis as it strongly impacts the postcatalytic intrinsic protein dynamics.

Improved density functional description of the electrochemistry and structure-property descriptors of substituted flavins.

The energetics of electrochemical changes have been investigated for several substituted flavins with the M06-L density functional. The reduction potentials for one- and two-electron reductions of these molecules have been determined and the results are consistent with experimental findings with a mean unsigned error of only 42 mV. It is especially noteworthy that the M06-L density functional makes a significant difference in the computed free energy of the first reduction of lumiflavin, which produces a neutral semiquinone. We also investigate the effects of flavin ring substituents on the geometries, charge distributions, reduction potentials, pK(a)'s, ionization potentials, electron affinities, hardnesses, softnesses, electrophilic powers, and nucleophilicities.