ChemInform Abstract: Rate Constants for 1,2‐Hydrogen Migration in Cyclohexylidene and in Substituted Cyclohexylidenes.

Abstract

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Efficient Covalent Bond Formation in Gas-Phase Peptide–Peptide Ion Complexes with the Photoleucine Stapler

“…Although this result should not be over-interpreted because of the potentially uneven backbone fragmentation of the peptide complex ion, it nevertheless points to the realistic representation of the peptide dynamics in the complex. It is also consistent with the previously postulated extremely low activation energies for carbene insertion [9][10][11][12], justifying the assumption that covalent bond formation occurs with high probability upon close contact between the carbene and the X−H bond. The overall conclusion of this theoretical analysis Figure S8b, c), it appears that each laser pulse creates a reactive carbene at only one of the L* residues.…”

“…We selected 25 lowest-energy complex structures from DFT optimizations and ran trajectory calculations for 100 ps at 310 K corresponding to the experimental ion trap temperature. The 100 ps time frame is compatible with the half-life of a carbene intermediate, which is expected to be in the 0.1-5 ns range [9][10][11][12].…”

“…Photon absorption causes N 2 elimination, creating a highly reactive singlet carbene that undergoes insertion into proximate X−H bonds (X = C, N, O) to form new covalent H−C−X bonds [8]. Competing with insertion, the carbene can undergo fast intramolecular isomerization by 1,2-hydrogen shift to form a nonreactive olefin in the photoactive component [9][10][11][12], which can no longer covalently bind to the complex counterpart. This side reaction provides an internal clock that limits the time scale for carbene-X-H bond interactions to within 10 -7 s. The photochemical crosslinking (Bstitching^) can be readily detected by mass spectrometry, and the position of the new covalent link can be located by tandem mass spectrometry sequencing of gas-phase adduct ions [7].…”

Photoleucine Survives Backbone Cleavage by Electron Transfer Dissociation. A Near-UV Photodissociation and Infrared Multiphoton Dissociation Action Spectroscopy Study

“…9 s −1 range [32][33][34], and so the peptide carbene is too short-lived to be collisionally cooled. The dominant formation upon UVPD of small fragments, such as (a 2 -N 2 ) + , (b 2 − N 2 ) + and y 2 , is consistent with UVPD being more energetic than CID.…”

Position‐tunable diazirine tags for peptide–peptide ion cross‐linking in the gas phase

“…In contrast to other types of chemical cross‐linking, covalent insertions compete with carbene self‐destruction by 1,2‐hydrogen atoms shift yielding nonreactive olefins, with both reactions occurring on a nanosecond to microsecond timescale . Carbene‐based zero‐length cross‐linking has been made possible with diazirine‐tagged amino acid residues photoleucine (L‐2‐amino‐4,4‐azipentanoic acid, L*) and photomethionine (L‐2‐amino‐5,5‐azipentanoic acid, M*) that have been used as natural amino acid surrogates in synthetic peptides .…”