Search for: All records

RationaleThe function of a protein or the binding affinity of an antibody can be substantially altered by the replacement of leucine (Leu) with isoleucine (Ile), and vice versa, so the ability to identify the correct isomer using mass spectrometry can help resolve important biological questions. Tandem mass spectrometry approaches for Leu/Ile (Xle) discrimination have been developed, but they all have certain limitations. MethodsFour model peptides and two wild‐type peptide sequences containing either Leu or Ile residues were subjected to charge transfer dissociation (CTD) mass spectrometry on a modified three‐dimensional ion trap. The peptides were analyzed in both the 1+ and 2+ charge states, and the results were compared to conventional collision‐induced dissociation spectra of the same peptides obtained using the same instrument. ResultsCTD resulted in 100% sequence coverage for each of the studied peptides and provided a variety of side‐chain cleavages, includingd,wandvions. Using CTD, reliabledandwions of Xle residues were observed more than 80% of the time. When present,dions are typically greater than 10% of the abundance of the correspondingaions from which they derive, andwions are typically more abundant than thezions from which they derive. ConclusionsCTD has the benefit of being applicable to both 1+ and 2+ precursor ions, and the overall performance is comparable to that of other high‐energy activation techniques like hot electron capture dissociation and UV photodissociation. CTD does not require chemical modifications of the precursor peptides, nor does it require additional levels of isolation and fragmentation. 

Abstract Spacious M₄L₆tetrahedra can act as catalytic inhibitors for base‐mediated reactions. Upon adding only 5 % of a self‐assembled Fe₄L₆cage complex, the conversion of the conjugate addition between ethylcyanoacetate and β‐nitrostyrene catalyzed by proton sponge can be reduced from 83 % after 75 mins at ambient temperature to <1 % under identical conditions. The mechanism of the catalytic inhibition is unusual: the octacationic Fe₄L₆cage increases the acidity of exogenous water in the acetonitrile reaction solvent by favorably binding the conjugate acid of the basic catalyst. The inhibition only occurs for Fe₄L₆hosts with spacious internal cavities: minimal inhibition is seen with smaller tetrahedra or Fe₂L₃helicates. The surprising tendency of the cationic cage to preferentially bind protonated, cationic ammonium guests is quantified via the comprehensive modeling of spectrophotometric titration datasets. 

Abstract Appending functional groups to the exterior of Zn₄L₄self‐assembled cages allows gated control of anion binding. While the unfunctionalized cages contain aryl groups in the ligand that can freely rotate, attaching inert functional groups creates a “doorstop”, preventing rotation and slowing the guest exchange rate, even though the interiors of the host cavities are identically structured. The effects on anion exchange are subtle and depend on multiple factors, including anion size, the nature of the leaving anion, and the electron‐withdrawing ability and steric bulk of the pendant groups. Multiple exchange mechanisms occur, and the nature of the external groups controls associative and dissociative exchange processes: these bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF₆⁻and SbF₆⁻can vary by as much as 400‐fold between identically sized cavities.