title: Autochelation in dipeptide boronic acids: pH-dependent structures and equilibria of Asp-boroPro and His-boroPro by NMR spectroscopy.
abstract: Many dipeptide boronic acids of the type H(2)N-X-Y-B(OH)(2) are potent protease inhibitors. Interest in these compounds as drugs for cancer, diabetes, and other diseases is growing. Because of the great mutual B-N affinity, cyclization through the N- and B-termini, forming six-membered rings, is a common occurrence at neutral pH and higher where the terminal amino group is unprotonated. Here we report the discovery that when X, the N-terminal amino acid, contains a side chain having a functional group with boron affinity and suitable geometry, additional cyclization in the form of bidentate intramolecular chelation or "autochelation" may occur, predominantly at mid pH. NMR studies of two compounds, l-Aspartyl-l-boroProline (Asp-boroPro) and l-Histidyl-l-boroProline (His-boroPro), are reported here from pH 0.5 to pH 12 by (1)H, (15)N, (13)C, and (11)B NMR. Both of these previously unreported autochelates contain two fused six-membered rings, cis-proline, chiral boron, and -NH(2)(+) protons in slow exchange with water, even at 25 degrees C and pH as high as 4. Using microscopic acid-base equilibrium constants, we show that at high pH (>8 for Asp-boroPro and >10 for His-boroPro) hydroxide competes with the side chains for boron, reducing the chelates from bidentate to monodentate. At low pH (<0.5), proton competition for N-terminal nitrogens causes both compounds to become noncyclic. High chelate stability causes a reduction of the apparent acidic dissociation constant of the protonated N-terminal amino group greater than eight units. In the His-boroPro autochelate, imidazolate anion is produced at the extraordinarily low pH value of approximately 9.
authors.affiliation: Department of Biochemistry, Tufts University School of Medicine, 136 Harrison Avenue, Boston, Massachusetts 02111, USA.
authors.forename: James L
authors.initials: JL
authors.lastname: Sudmeier
authors.forename: Yuhong
authors.initials: Y
authors.lastname: Zhou
authors.forename: Jack H
authors.initials: JH
authors.lastname: Lai
authors.forename: Hlaing H
authors.initials: HH
authors.lastname: Maw
authors.forename: Wengen
authors.initials: W
authors.lastname: Wu
authors.forename: William W
authors.initials: WW
authors.lastname: Bachovchin
mesh_terms: D001224:Aspartic Acid / Q000031:analogs & derivatives; D001897:Boronic Acids / Q000737:chemistry*; D002614:Chelating Agents / Q000737:chemistry; D004151:Dipeptides / Q000737:chemistry*; D006639:Histidine / Q000031:analogs & derivatives; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods; D011392:Proline / Q000031:analogs & derivatives*; D011480:Protease Inhibitors / Q000737:chemistry; D011487:Protein Conformation
chemical_list: D001897:Boronic Acids; D002614:Chelating Agents; D004151:Dipeptides; D011480:Protease Inhibitors; D001224:Aspartic Acid; D006639:Histidine; D011392:Proline