Versatile Site-Selective Protein Reaction Guided by WW Domain–Peptide Motif Interaction

A short, flexible, and unstructured peptide tag that has versatile and facile use in protein labeling applications is highly desirable. Here, we report an 11-residue peptide tag with an internal cysteine (a W-tag, derived from a Comm PY peptide motif that is known to bind with Nedd4 WW3* domain) that can be installed at different regions of the target protein without compromising its covalent reactivity with the reactive label (a 35-residue synthetic Nedd4 WW3* domain derivative). This versatility is explained by the unique structural features of the reaction. NMR analysis reveals that both the W-tag peptide and reactive Nedd4 WW3* protein are unstructured before they encounter each other. The binding interaction of the two induces noticeable structural changes and promotes global folding. Consequently, the reactive cysteine residue at W-tag and the electrophilic chloroacetyl group at Nedd4 WW3* domain are positioned to be in close proximity, inducing an intermolecular covalent cross-linking. The covalent linkage in turn stabilizes the folding of the protein complex. This unique multistep mechanism renders this labeling reaction amenable to different sites of the proteins of interest: installation of the tag at N- and C-termini, in the flexible linker region, in the loop region, and the extracellular terminus of target proteins exhibited comparable reactivity. This work therefore represents the first proximity-induced cysteine reaction based on the unique binding features of WW domains that demonstrates unprecedented versatility

Anion-Assisted Synthesis of TiO₂ Nanocrystals with Tunable Crystal Forms and Crystal Facets and Their Photocatalytic Redox Activities in Organic Reactions

In this work, we develop a facile and general strategy to control the crystal forms and crystal facets of TiO₂ nanocrystals. Ti­(OH)₄ was used as the precursor and different anions were used as capping agents without any other organic surfactants. These anions can selectively adsorb on the specific crystal facets of anatase, inducing the transformation of conventional {101} facets to unconventional {001} facets and {100} facets or even phase transformation to rutile and brookite. Rutile and brookite TiO₂ nanocrystals as well as anatase TiO₂ nanocrystals with different facets ({101}, {001}, and {100}) exposed are obtained. Photocatalytic selective reduction of nitrobenzene and selective oxidation of benzyl alcohol are employed as a probe reaction to test the redox properties of the as-prepared TiO₂ nanocrystals. The results show that the photocatalytic redox properties of TiO₂ NCs are dependent on their crystal forms and crystal facets. Specially the photocatalytic activities of different anatase crystal facets show different orders in reduction and oxidation reactions, respectively. The reduction ability of different anatase crystal facets can be ranked as {101} > {001} > {100}. While the oxidation ability of different facets can be ranked as {101} ≈ {001} ≈ {100}. Surface and electronic structures should be the origin that account for their different activity orders in different reactions. Based on the results in the two model reactions, one important principle should be pointed out. When we discuss the crystal-facet-dependent catalytic activities of TiO₂ nanocrystals, we should analyze the results based on specific reactions