Minimal Increments of Hydrophobic Collapse within the N-Terminus of the Neuropeptide Galanin.

The neuropeptide galanin has a 35-year history as an intriguing target in drug design owing to its implication as a potential anticonvulsant and neuronal trophic factor among many other therapeutically interesting functions including analgesia and mood alteration. In this study, we report the structural characterization of three synthetic fragments of the galanin N-terminus in buffered aqueous solution: hGal(2-12)KK, hGal(1-12)KK, and hGal(1-17)KK. High-field two-dimensional 1H-1H nuclear magnetic resonance (NMR) data were acquired for these fragments and used to derive distance restraints. We further utilized modified hydrogen bonding and dihedral restraints to reflect chemical shift patterns in the data, which revealed the signature of a weakly folded helix. Together, these sets of restraints were used to generate NMR structures of all three fragments, which depict a core of hydrophobic residues that cluster together regardless of the presence of a helical structure, and correspond to residues in the N-terminus of galanin that have been previously shown to be critical for binding its receptors. The helical structure only appears following the inclusion of Gly(1) in the sequence, and at longer sequence lengths, unlike many other peptides, the helix does not propagate. Rather, a few turns of poorly ordered helix appear to be a secondary consequence of clusters of hydrophobic sidechains that are conserved across all of the peptides in this study; the helices themselves appear ordered as a consequence of this clustering, and these clusters compare directly to those observed recently to make contacts between galanin and two of its receptor subtypes. Collapsed hydrophobic residues therefore organize and compose the functional core of human galanin and raise interesting questions about the nature of the conformational order in ligands that bind cell surface receptors.

Switchable Regioselective 6-endo or 5-exo Radical Cyclization via Photoredox Catalysis.

Controlling the regioselectivity of radical cyclizations to favor the 6-endo mode over its kinetically preferred 5-exo counterpart is difficult without introducing substrate prefunctionalization. To address this challenge, we have developed a simple method for reagent controlled regioselective radical cyclization of halogenated N-heterocycles onto pendant olefins. Radical generation occurs under mild photoredox conditions with control of the regioselectivity governed by the rate of hydrogen atom transfer (HAT). Utilizing a polarity-matched thiol-based HAT agent promotes the highly selective formation of the 5-exo cyclization product. Conversely, limiting the solubility of the HAT reagent Hantzsch ester (HEH) leads to selective formation of the thermodynamically favored 6-endo product. This occurs through an initial 5-exo cyclization, with the resulting alkyl radical intermediate undergoing neophyl rearrangement to form the 6-endo product. Development of this switchable catalysis strategy allows for two modes of divergent reactivity to form either the 6-endo or 5-exo product, generating fused N-heteroaromatic/saturated ring systems.

The neuropeptide galanin adopts an irregular secondary structure.

Human galanin is a 30-residue neuropeptide targeted for development of analgesics, antidepressants, and anticonvulsants. While previous work from our group and others has already produced significant insights into galanin's N-terminal region, no extant structures of galanin in databases include its full-length sequence and the function of its C-terminus remains ambiguous. We report the NMR solution structure of full-length human galanin C-terminal amide, determined from 2D 1H-1H COSY, TOCSY, and ROESY NMR data. Galanin adopts an irregular helical structure across its N-terminus, likely the average of several coiling states. We present the NMR structure of a peptide encompassing the C-terminus of galanin as a stand-alone fragment. The C-terminus of full-length galanin appears to indirectly assist the intramolecular association of hydrophobic sidechains within its N-terminus, remotely rigidifying their position when compared to previously studied N-terminal galanin fragments. By contrast, there is flexibility in the C-terminus of galanin, characterized by two i to i + 2 hydrogen-bonded turns within an otherwise dynamic backbone. The C-terminal portion of the peptide renders it soluble, and plays a hitherto undescribed biophysical role in pre-organizing the galanin receptor binding epitope. We speculate that hydrophilic microdomains of signaling peptides, hormones, and perhaps intrinsically disordered proteins may also function similarly.

Radical Chain Reduction via Carbon Dioxide Radical Anion (CO2•-).

We developed an effective method for reductive radical formation that utilizes the radical anion of carbon dioxide (CO2•-) as a powerful single electron reductant. Through a polarity matched hydrogen atom transfer (HAT) between an electrophilic radical and a formate salt, CO2•- formation occurs as a key element in a new radical chain reaction. Here, radical chain initiation can be performed through photochemical or thermal means, and we illustrate the ability of this approach to accomplish reductive activation of a range of substrate classes. Specifically, we employed this strategy in the intermolecular hydroarylation of unactivated alkenes with (hetero)aryl chlorides/bromides, radical deamination of arylammonium salts, aliphatic ketyl radical formation, and sulfonamide cleavage. We show that the reactivity of CO2•- with electron-poor olefins results in either single electron reduction or alkene hydrocarboxylation, where substrate reduction potentials can be utilized to predict reaction outcome.

Defluoroalkylation of Trifluoromethylarenes with Hydrazones: Rapid Access to Benzylic Difluoroarylethylamines.

Here, we report an efficient and modular approach toward the formation of difluorinated arylethylamines from simple aldehyde-derived N,N-dialkylhydrazones and trifluoromethylarenes (CF3-arenes). This method relies on selective C-F bond cleavage via reduction of the CF3-arene. We show that a diverse set of CF3-arenes and CF3-heteroarenes react smoothly with a range of aryl and alkyl hydrazones. The ß-difluorobenzylic hydrazine product can be selectively cleaved to form the corresponding benzylic difluoroarylethylamines.