Photochemical Methods for Peptide Macrocyclisation

Photochemical reactions have been the subject of renewed interest over the last two decades, leading to the development of many new, diverse and powerful chemical transformations. More recently, these developments have been expanded to enable the photochemical macrocyclisation of peptides and small proteins. These constructs benefit from increased stability, structural rigidity and biological potency over their linear counterparts, providing opportunities for improved therapeutic agents. In this review, an overview of both the established and emerging methods for photochemical peptide macrocyclisation is presented, highlighting both the limitations and opportunities for further innovation in the field.

The Life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore

The widespread applications of benzophenone (BP) photochemistry in biological chemistry, bioorganic chemistry, and material science have been prominent in both academic and industrial research. BP photophores have unique photochemical properties: upon n-π* excitation at 365 nm, a biradicaloid triplet state is formed reversibly, which can abstract a hydrogen atom from accessible C-H bonds; the radicals subsequently recombine, creating a stable covalent C-C bond. This light-directed covalent attachment process is exploited in many different ways: (i) binding/contact site mapping of ligand (or protein)-protein interactions; (ii) identification of molecular targets and interactome mapping; (iii) proteome profiling; (iv) bioconjugation and site-directed modification of biopolymers; (v) surface grafting and immobilization. BP photochemistry also has many practical advantages, including low reactivity toward water, stability in ambient light, and the convenient excitation at 365 nm. In addition, several BP-containing building blocks and reagents are commercially available. In this review, we explore the "forbidden" (transitions) and excitation-activated world of photoinduced covalent attachment of BP photophores by touring a colorful palette of recent examples. In this exploration, we will see the pros and cons of using BP photophores, and we hope that both novice and expert photolabelers will enjoy and be inspired by the breadth and depth of possibilities.

Photosensitized oxidation of methionine-containing dipeptides. From the transients to the final products

The Met residue oxidation has been studied for decades. Although many efforts have been made on the identification of free radicals, some doubts remain about their final fates, i.e., the nature of stable oxidation products. The photosensitized oxidation processes of two peptides, methionyl lysine (Met-Lys) and lysyl methionine (Lys-Met), were investigated using 3-carboxybenzophenone (3CB) as a sensitizer. Therefore, not only the transients were characterized but also the final products (by high-performance liquid chromatography and mass spectrometry) together with the quantum yields. As for the transients, the sulfur radical cations stabilized by a two-center three electron bonds with a nitrogen (S.·.N)(+) were identified in the case of Met-Lys. On the other hand, in Lys-Met, the intermolecular (S.·.S)(+) radical cations were found. The peptide-3CB adduct was the only stable product detected and was accompanied neither by sulfoxide formation nor by decarboxylation. It shows that both (S.·.N)(+) and (S.·.S)(+) radicals are converted into the relatively long-lived α-(alkylthio)alkyl radicals, which add to the 3CB-derived radicals. This addition reaction prevented all other oxidation processes such as formation of sulfoxide. The lysine residue was totally protected, which may also be of importance in biological processes.

Manipulating Y receptor subtype activation of short neuropeptide Y analogs by introducing carbaboranes

Short selective neuropeptide Y (NPY) analogs are highly attractive because of their facile synthesis. Based on the reduced-size NPY analog [Pro(30), Nle(31), Bpa(32), Leu(34)]NPY 28-36 position 32 was identified as a key position to alter the preferential activation pattern of the human neuropeptide Y receptors (hYRs). By replacing benzoylphenylalanine (Bpa) by a biphenylalanine (Bip) the photostability was first improved while the biological activity was maintained. SAR-studies showed that both aromatic rings have a high influence on the preferential hYR subtype activation. Interestingly, replacement of Bpa(32) by a strongly hydrophobic moiety changed the hYR subtype preference of the analog. Whereas the parent compound is able to activate the human neuropeptide Y1 receptor (hY1R) subtype, the introduction of an N(ε)-ortho-carbaboranyl propionic acid modified lysine resulted in a loss of activity at the hY1R but in an increased activity at both the hY2R and the hY4R. However, subsequent receptor internalization studies with this novel analog revealed that receptor internalization can neither be triggered at the hY2R nor at the hY4R suggesting a biased ligand. Surprisingly, investigations by (1)H NMR spectroscopy revealed structural changes in the side chains of residues Pro(30) and Leu(34) which nicely correlates with the shift from hY1R/hY4R to hY2R/hY4R activation preference. Thus, position 32 has been identified to switch the bioactive conformation and subsequently influences receptor subtype activation behavior.

Building a bridge between peptide chemistry and organic chemistry: intramolecular macrocyclization reactions and supramolecular chemistry with helical peptide substrates

In our ongoing efforts to build a bridge between peptide chemistry and organic chemistry, we are currently investigating: (1) two types of intramolecular macrocyclization reactions in 3(10)-helical peptides, and (2) a peptido[2]rotaxane molecular machine as a supramolecular tool using a 3(10)-helical peptide as the axle. More specifically, we studied the following two reactions: (a) the intramolecular ring-closing olefin metathesis between two amino acid residues with side chains bearing an allyl group, and (b) the intramolecular Paternò-Yang photoreaction, using a benzophenone-based amino acid as a photoaffinity reagent for a Met residue. Both reactions involve formation of a new C--C bond. As for the supramolecular system examined, we were able to identify the two stations of a new peptido[2]rotaxane characterized by an -(Aib)(6)- axle and to reversibly switch the aromatic tetramide macrocyclic wheel from one station to the next. This article summarizes the information available in the literature from other groups and the published/unpublished data originated from our laboratory on these research areas.