Click chemistry offers highly selective and orthogonal reactions that proceed rapidly and under a variety of mild conditions with the opportunity to create highly defined and multifunctional materials. This work illustrates a strategy where step-growth networks are formed rapidly via a copper-free, azide−alkyne click chemistry between tetrafunctional poly(ethylene glycol) molecules and difunctionalized synthetic polypeptides. The molecular weight of the polymer precursors (10, 15, or 20 kDa PEG) and the stoichiometry of reactive end group functionalities (1.5:1 to 1:1.5) provide control over the material cross-linking density, enabling elastic materials with tunable moduli (G′ = 1000−6000 Pa). A sequential photochemically activated thiol-ene chemistry allows subsequent functionalization of the network through reaction with pendant alkene moieties on the peptide. Because the thiol-ene reaction is light-driven, the degree of modification is directly related to the dosage of light delivered to the system (0−6 J cm−2). We exploit this feature to create complex biochemical gradients of multiple peptides with well-defined magnitude and slope throughout the three-dimensional (3D) network. Since both reactions can occur in the presence of cells, this material ultimately enables independent and in situ tuning of biochemical and biomechanical properties of biomaterial networks, suggesting an avenue to direct cell function throughout specific regions within a 3D material.

Because of its utility in network polymerization, dendrimer synthesis, and monomer development, the photoinitiated addition of thiols to alkynes has rapidly become an important tool for polymer scientists. Yet, because this chemistry has only recently been applied to cross-linked polymer development, understanding of the nature of how the yne structure affects the reactions and information on the relative reactivities of alkynes bearing various substituents is unavailable as is the relative addition rate of the thiol to the yne as compared to the vinyl sulfide. Herein, the photoinitiated addition of octanethiol to various alkynes is explored. The most rapid addition of thiols to alkynes is that to cyclooctyne, although the resulting vinyl sulfide does not permit subsequent thiol addition. Furthermore, in the absence of radical initiators and light, thiols add spontaneously to cyclooctynes, suggesting limitations to the orthogonality of the strain-promoted copper-less azide, alkyne cycloadditions. In order of decreasing reaction rates, the consecutive addition of two thiols occurs with the aliphatic 1-octyne > propargyl acetate > methyl propargyl ether > 2-octyne. Ethyl propiolate and methyl propargylamine exhibit very small reaction rates with thiols, and no consecutive addition is observed.

This study aimed to identify signaling pathways that oppose connective tissue fibrosis in the aortic valve. Using valvular interstitial cells (VICs) isolated from porcine aortic valve leaflets, we show that basic fibroblast growth factor (FGF-2) effectively blocks transforming growth factor-β1 (TGF-β1)-mediated myofibroblast activation. FGF-2 prevents the induction of α-smooth muscle actin (αSMA) expression and the exit of VICs from the cell cycle, both of which are hallmarks of myofibroblast activation. By blocking the activity of the Smad transcription factors that serve as the downstream nuclear effectors of TGF-β1, FGF-2 treatment inhibits fibrosis in VICs. Using an exogenous Smad-responsive transcriptional promoter reporter, we show that Smad activity is repressed by FGF-2, likely an effect of the fact that FGF-2 treatment prevents the nuclear localization of Smads in these cells. This appears to be a direct effect of FGF signaling through mitogen-activated protein kinase (MAPK) cascades as the treatment of VICs with the MAPK/extracellular regulated kinase (MEK) inhibitor U0126 acted to induce fibrosis and blocked the ability of FGF-2 to inhibit TGF-β1 signaling. Furthermore, FGF-2 treatment of VICs blocks the development of pathological contractile and calcifying phenotypes, suggesting that these pathways may be utilized in the engineering of effective treatments for valvular disease.—Cushing, M. C., Mariner, P. D., Liao, J. T., Sims, E. A., Anseth, K. S. Fibroblast growth factor represses Smad-mediated myofibroblast activation in aortic valvular interstitial cells.

We report the large-scale synthesis of 1,3-cyclooctanedione in five steps with 29% yield. This molecule is a synthetic precurser to difluorinated cyclooctyne, which participates in a bioorthogonal copper-free click reaction with azides. The final step demonstrates the first successful application of the Wacker-Tsuji oxidation to form a cyclic 1,3-dione.

A Mild, Large-Scale Synthesis of 1,3-Cyclooctanedione: Expanding Access to Difluorinated Cyclooctyne for Copper-Free Click Chemistry. -A large-scale five-step synthesis of target compound (VII) is developed. The latter is a known synthetic precursor for difluorinated cyclooctyne (VIII) (DIFO3), which participates in a bioorthogonal copper-free 1.3-dipolar cycloaddition with azides in the end-functionalization of synthetic polypeptides. -(SIMS, E. A.; DEFOREST, C. A.; ANSETH*, K. S.; Tetrahedron Lett. 52 (2011) 16, 1871-1873, http://dx.