Site‐Selective and Nondestructive Protein Labeling through Azaelectrocyclization‐Induced Cascade Reactions

Chemical Approach to a Whole Body Imaging of Sialo-N-Linked Glycans

“…Inspired by the notable electrocyclization reactivity during our enzyme inhibition study [72,73] and in synthetic applications [74][75][76][77][78][79][80][81], a new DOTA-labeling probe, DOTA-(E)-ester aldehyde 1a, was designed and synthesized (Scheme 2) [28][29][30]. Thus, THP-protected (Z )-vinyl bromide 2 and glycine linked (E)-stannane 3 were heated to 110 C in the presence of Pd 2 (dba) 3 , P(2-furyl) 3 , and LiCl in DMF to provide the Stille coupling product, which was subsequently treated with 3 M hydrochloric acid to give aminoalcohol 4 in 73% yield in two steps.…”

“…In this personal account, we want to describe our recent trials on the in vivo imaging of the sialo-N-glycans; we first describe (1) the new chemistry-based labeling of the amino groups [28][29][30], e.g., lysines or ethanol amines, which is quite useful not only for the labeling of the glycans and/or the glycoconjugates, but also generally for the peptides, proteins, as well as the cell surfaces [31]. Then, PET and the noninvasive fluorescence images of (2) sialo-and asialo-glycoproteins [8,9,[28][29][30], (3) dendrimer-type glycoclusters consisting of 16 molecules of N-glycans [32], and (4) chemically engineered lymphocytes by sialo-N-glycans [33] are discussed.…”

“…Then, PET and the noninvasive fluorescence images of (2) sialo-and asialo-glycoproteins [8,9,[28][29][30], (3) dendrimer-type glycoclusters consisting of 16 molecules of N-glycans [32], and (4) chemically engineered lymphocytes by sialo-N-glycans [33] are discussed. These imaging studies clearly visualize the remarkable dependence of in vivo dynamics and organ-specific accumulation on the partial structures of the non-reducing end of the N-glycans.…”

Electrocyclization‐Based Labeling Allows Efficient In Vivo Imaging of Cellular Trafficking

“…Interestingly, confocal microscopy detected specific localization of the TAMRA fluorescence on the cell membrane, such that the fluorescence surrounded the nucleus and then radiated outward (Figure 1 c). Based on previous results on the reactivity of 1 a and other unsaturated aldehyde probes, [12,13] azaelectrocyclization might proceed selectively at the protein assembly (high concentration of lysine) and/or the other ethanol amine functions on the cell surface under such a diluted concentration of 1 a. The preliminary SDS-PAGE experiments of the lysate of the 1 a-labeled C6 glioma cell gave a clear protein band at 40 KDa with the strong TAMRA fluorescence, which is currently being investigated.…”

“…Reactions with internal lysine residues of the tertiary protein structures, as well as the N-terminal amine (amine with a substitution on the a-carbon), are very slow (> 5 h at 24 8C), [11e] while lysines at protein surfaces react rapidly (10-30 min at 24 8C); therefore, labeling occurs preferentially at these positions. [11b, 12] Site-selective labeling of the target protein has also been achieved by directing reactive groups (unsaturated aldehydes) to a specific site using a smallmolecule ligand of the protein [13] so that the decrease in activity is suppressed to a minimum when lysines, which are critical for receptor binding, are situated at the most accessible site of a target protein. Dihydropyridines as the electrocyclization products, which retain the cationic charges as those of the inherent lysines, might also contribute to the retention of the protein activity.…”