Molecular Sieving with PEGylated Dendron-Protein Conjugates.

A series of generation 3-5 dendrons based on a bis(2,2-hydroxymethylpropionic acid) (bis-MPA) scaffold bearing three respective lengths of linear poly(ethylene glycol) at their periphery and a dibenzocyclooctyne unit at their core was prepared. These dendrons were appended to the surface of azide-decorated α-chymotrypsin (α-CT) via strain-promoted azide-alkyne cycloaddition to yield a library of dendron-protein conjugates. These conjugates were characterized by FT-IR and NMR spectroscopy and were imaged using cryo-electron microscopy. The activity of the PEGylated α-CT-dendron conjugates was investigated using a small molecule (benzoyl-l-tyrosine p-nitroanilide) as well as different proteins of different sizes and crystallinities (casein and bovine serum albumin) as substrates. It was found that the activity of the conjugates toward the small molecule was largely retained, while the activity toward the proteins was significantly diminished. Furthermore, the results indicate that for most of the conjugates the PEG length had a more pronounced impact on enzyme activity than the dendron generation. Overall, the highest sieving ratios were found for α-CT-dendron conjugates decorated with G3-PEG2000, G4-PEG2000, and G5-PEG1000, with the latter two structures offering the best combination of sieving ratio and small molecule activity.

Dearomatizing and Derivatizing a Mesityl Group on Boron by One-Pot Photoisomerization and [4+2] Diels-Alder Addition.

Boron compounds having a conjugated chelate backbone (N,C-chelate or C,C-chelate) and two mesityl substituents on boron have been found to undergo a facile one-pot transformation/reaction with dienophiles, which leads to the dearomatization of one mesityl ring and its [4+2] Diels-Alder addition with the dienophile. Photochemical activation is the key in this transformation of the aryl ring.