In a recent article published in Organic and Biomolecular Chemistry, graduate student Taysir Bader, in the research group of Professor Mark Distefano, in collaboration with graduate student Alicia E. Mangubat-Medina, and undergraduates Hallie O. Trial, Reyner D. Vargas, and Mekedlawit T. Setegne in the research group of Dr. Zachary Ball at Rice University, described the synthesis and characterization of two new nitrodibenzofuran (NDBF) based boronic acid reagents for peptide backbone N–H caging and subsequent photocleavage of the resulting C(sp2)-X bond.
The boronic acid reagents were synthesized using an elegant strategy that started with 4-fluoro-2-nitrobenzaldehyde, which was used to construct the dibenzofuran moiety via a C–H activation/cyclization and late-stage introduction of a propargyl group, followed by hydroboration. This strategy was used to produce two boronic acid analogues; one containing the parent NDBF, and the other with a dimethylamino-substituted analogue with a red-shifted absorbance profile that gives further access to two-photon mediated cleavage. With the boronic acids in hand, they were used to protect the backbones of three different peptides: pep1, a collagen-type sequence that exhibits triple-helix folding behavior known to be disrupted by backbone N– H alteration (seq: Ac-(POG)3POGHOG(POG)3-NH2); pep2, a hormone releasing peptide (LHRH) that contains a pyroglutamate–histidine motif (seq: pE– HWSYGLRPG-NH2); and pep3, a manufactured hormone (leuprolide) used to treat several types of cancer (seq: pE-HWSY-DLeu-LRP-NHEt). HPLC and MALDI were used to show clean Cu(NO3)2 mediated caging of these peptides in NMM buffer, and subsequent uncaging upon irradiation with a blue LED light in isoamylamine buffer. Analysis of the uncaging kinetics using pep3 showed that NDBF uncaging (t90% = 37 s, Φ·σ = 4.7 × 106 cm2 mol−1) was 7-fold faster than previous generation nitroveratrole protecting group (t90% = 257 s, Φ·σ = 6.7 × 105 cm2 mol−1), and 23-fold faster than the dimethylamino analogue (t90% = 907 s, Φ·σ = 1.9 × 105 cm2 mol−1), indicating that it disfavors one photon uncaging. Two photon uncaging at 800 nmon the other hand was noticeably faster for the dimethylamino analogue compared to the parent NDBF, with a cross section on the order of 0.13 GM.