Conversion of Azides into Diazo Compounds in Water

Diazo compounds are in widespread use in synthetic organic chemistry but have untapped potential in chemical biology. We report on the design and optimization of a phosphinoester that mediates the efficient conversion of azides into diazo compounds in phosphate buffer at neutral pH and room temperature. High yields are maintained in the presence of common nucleophilic or electrophilic functional groups, and reaction progress can be monitored by colorimetry. As azido groups are easy to install and maintain in biopolymers or their ligands, this new mode of azide reactivity could have substantial utility in chemical biology

Dimethyl Labeling Coupled with Mass Spectrometry for Topographical Characterization of Primary Amines on Monoclonal Antibodies

Site-specific solvent accessibility of the primary amines (mainly lysine or the N-termini) on proteins is of great interest in many research areas because amines are an important functional group for protein conjugation. In this study, we coupled dimethyl labeling via reductive amination with liquid chromatography–mass spectrometry (LC-MS) to fully characterize the solvent accessibility of lysine residues and the N-termini on human immunoglobulin G (IgG). Circular dichroism (CD) and fluorescence spectroscopy revealed that dimethyl labeling did not alter the conformation of the native IgG molecule. Based on intact protein measurements, up to 28 (light chain) and 66 (heavy chain) dimethyl tags, covering all lysine residues and the N-termini, were sequentially incorporated into IgG molecules in 1000 s. All labeled sites were identified and quantified by a bottom-up proteomics approach. Some highly exposed hot-spots (for example, the N-termini of both the heavy and the light chains) and some buried sites (for example, K415 in the heavy chain and K39 in the light chain) were unambiguously revealed. This method was also used to characterize aggregation-induced structural changes in IgGs by increasing the temperature. Substantial changes in the labeling percentage of many lysine sites were observed, indicating a non-native aggregation triggered by thermal stress. Due to high labeling yields and the van der Waals surface of the labeling reagents being comparable to that of water, dimethyl labeling is a highly promising technique for probing the amine’s surface topography of proteins. It can also be used as a complementary approach to other methods for resolving the higher-order structure of proteins by LC-MS