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

Abstract

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.

Photocatalytic Modification of Amino Acids, Peptides, and Proteins

“…Other light‐induced selective modifications of methionine have been described in the field of benzophenone photochemistry . Specifically, benzophenone‐mediated photolabeling at methionine in peptides and proteins plays an important role in chemical biology .…”

Robust High‐Capacitance Polymer Gate Dielectrics for Stable Low‐Voltage Organic Field‐Effect Transistor Sensors

“…Using this approach, a bilayer dielectric with excellent chemical resistance and a large C i (169 nF cm −2 at 1 kHz) was obtained (see Figure 1e). 20 In this system, illumination at 365 nm leads to covalent bond formation between neighboring macroradical polymer chains that are produced by hydrogen abstraction with a photo‐excited MK (see Figure S1, Supporting Information, for the absorption spectra of MK and plausible crosslinking mechanism) 35,36. The polymer chain thermal property (i.e., whether it is the glassy or rubbery state) has a substantial effect on the intermixing with the photoinitiator and the addition of butyl side chains increases the number of reactive sites for hydrogen abstraction.…”

Proton‐Coupled Electron Transfer from Hydrogen‐Bonded Phenols to Benzophenone Triplets

“…The relative position of the pendant base and the phenolic OH is the key structural feature controlling the “switch” of the reaction mechanism. This type of reaction control must be taken into account, particularly in biological systems such as radical enzymes featuring tyrosyl radicals, in which virtually the same reactions can occur either in polar or non‐polar environments, and in the applications of benzophenone in the photo‐labeling and photo‐grafting of peptides, proteins, and oligonucleotides …”