1H and 11B NMR evidence for specific binding of borate ion to cytochrome c

Interactions of boron released from surface pre-reacted glass ionomer with enamel/dentin and its effect on pH

This study investigated the interaction of borate ions released from surface pre-reacted glass ionomer (S-PRG) fillers with the biological apatites of enamel and dentin using solid-state (SS) magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy analysis. We further evaluated the effect of borate ions on the pH change. Bovine enamel and dentin powder were submerged in S-PRG eluate (with borate ion concentration of 100 mM) for 3 h, then washed with deionized water (DW). The dried enamel and dentin specimens were used for ¹¹B SS-NMR and Fourier transform infrared spectroscopy (FT-IR) analysis. Enamel and dentin blocks were treated with three solutions: DW (control), S-PRG eluent and borate solution (100 mM). The treated samples were submerged in the demineralization solution and the pH was measured using a pH meter daily for 6 days. The surfaces of the enamel and dentin blocks were then observed using field emission scanning electron microscopy (FE-SEM). SS-NMR analysis revealed the presence of adsorbed borate ions in the enamel and dentin samples in a tetra-coordinated form. The pH results demonstrated an increase in pH values in the S-PRG and borate groups. SEM images showed that the surfaces of the control group were demineralized, whereas the surfaces of the S-PRG and borate groups were intact. These results concluded that borate ions could be adsorbed to enamel and dentin in the tetra-coordinated form. Borate ions possess a buffer capacity which may help to protect the tooth structure against acid attacks and promote remineralization.

Applications of heteronuclear NMR spectroscopy in biological and medicinal inorganic chemistry

There is a wide range of potential applications of inorganic compounds, and metal coordination complexes in particular, in medicine but progress is hampered by a lack of methods to study their speciation. The biological activity of metal complexes is determined by the metal itself, its oxidation state, the types and number of coordinated ligands and their strength of binding, the geometry of the complex, redox potential and ligand exchange rates. For organic drugs a variety of readily observed spin I = 1/2 nuclei can be used (1H, 13C, 15N, 19F, 31P), but only a few metals fall into this category. Most are quadrupolar nuclei giving rise to broad lines with low detection sensitivity (for biological systems). However we show that, in some cases, heteronuclear NMR studies can provide new insights into the biological and medicinal chemistry of a range of elements and these data will stimulate further advances in this area.

Reversible stiffening transition in beta-hairpin hydrogels induced by ion complexation

We have previously shown that properly designed lysine and valine-rich peptides undergo a random coil to beta-hairpin transition followed by intermolecular self-assembly into a fibrillar hydrogel network only after the peptide solutions are heated above the intramolecular folding transition temperature. Here we report that these hydrogels also undergo a stiffening transition as they are cooled below a critical temperature only when boric acid is used to buffer the peptide solution. This stiffening transition is characterized by rheology, dynamic light scattering, and small angle neutron scattering. Rheological measurements show that the stiffening transition causes an increase in the hydrogel storage modulus (G') by as much as 1 order of magnitude and is completely reversible on subsequently raising the temperature. Although this reversible transition exhibits rheological properties that are similar to polyol/borax solutions, the underlying mechanism does not involve hydroxyl-borate complexation. The stiffening transition is mainly caused by the interactions between lysine and boric acid/borate anion and is not driven by the changes in the secondary structure of the beta-hairpin peptide. Addition of glucose to boric acid and peptide solution disrupts the stiffening transition due to competitive glucose-borate complexation.

NMR and Crystallographic Characterization of Adventitious Borate Binding by Trypsin

Recent 11B NMR studies of the formation of ternary complexes of trypsin, borate, and S1-binding alcohols revealed evidence for an additional binding interaction external to the enzyme active site. We have explored this binding interaction as a prototypical interaction of borate and boronate ligands with residues on the protein surface. NMR studies of trypsin in which the active site is blocked with leupeptin or with the irreversible inhibitor 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF) indicate the existence of a low-affinity borate binding site with an apparent dissociation constant of 97 mM, measured at pH 8.0. Observation of a field-dependent dynamic frequency shift of the (11)B resonance indicates that it corresponds to a complex for which omegatau >> 1. The 0.12 ppm shift difference of the borate resonances measured at 11.75 and 7.05 T, corresponds to a quadrupole coupling constant of 260 kHz. A much larger 2.0 ppm shift is observed in the 11B NMR spectra of trypsin complexed with benzene boronic acid (BBA), leading to a calculated quadrupole coupling constant of 1.1 MHz for this complex. Crystallographic studies identify the second borate binding site as a serine-rich region on the surface of the molecule. Specifically, a complex obtained at pH 10.6 shows a borate ion covalently bonded to the hydroxyl oxygen atoms of Ser164 and Ser167, with additional stabilization coming from two hydrogen-bonding interactions. A similar structure, although with low occupancy (30%), is observed for a trypsin-BBA complex. In this case, the BBA is also observed in the active site, covalently bound in two different conformations to both His57 Nepsilon and Ser195 Ogamma. An analysis of pairwise hydroxyl oxygen distances was able to predict the secondary borate binding site in porcine trypsin, and this approach is potentially useful for prediction of borate binding sites on the surfaces of other proteins. However, the distances between the Ser164/Ser167 Ogamma atoms in all of the reported trypsin crystal structures is significantly greater than the Ogamma distances of 2.2 and 1.9 angstroms observed in the trypsin complexes with borate and BBA, respectively. Thus, the ability of the hydroxyl oxygens to adopt a sufficiently close orientation to allow bidentate ligation is a critical limit on the borate binding affinity of surface-accessible serine/threonine/tyrosine residues.

Biomedical applications of 10B and 11B NMR

This review focuses mainly on the detection and investigation of molecules used for boron neutron capture therapy (BNCT) by 10B and 11B NMR. In this binary radiation treatment, boron-containing molecules (also called 'BNCT agents') enriched in the 10B isotope, are targeted to the tumor, and irradiated with thermal or epithermal neutrons. Capture of these neutrons by 10B nuclei generates cell-damaging radiation, confined to single cell dimensions. NMR research efforts have primarily been applied in two directions: first, to investigate the metabolism and pharmaco-kinetics of BNCT agents in-vivo, and second, to use localized NMR spectroscopy and/or MRI for non-invasive mapping of the administered molecules in treated animals or patients. While the first goal can be pursued using 11B NMR for natural-abundance samples (80% 11B / 20% 10B), molecules used in the actual treatment are > 95% enriched in 10B, and must therefore be detected by 10B NMR. Both 10B (spin 3) and 11B (spin 3/2) are quadrupolar nuclei, and their typical relaxation times, in common BNCT agents in biological environments, are rather short. This poses some technical challenges, particularly for MRI, which will be reviewed, along with possible solutions. The first attempts at 11B NMR and MRI detection of BNCT agents in biological tissue were conducted over a decade ago. Since then, results from 11B MRI in laboratory animals and in humans have been reported, and 11B NMR spectroscopy provided interesting and unique information about the metabolism of some BNCT agents in cultured cells. 10B NMR was applied either 'indirectly' (in double-resonance experiments involving coupled protons), but also by direct 10B MRI in mice. However, no results involving the NMR detection of 10B-enriched compounds in treated patients have been reported yet.

Detection and characterization of boric acid and borate ion binding to cytochrome c using multiple quantum filtered NMR

The application of multiple quantum filtered (MQF) NMR to the identification and characterization of the binding of ligands containing quadrupolar nuclei to proteins is demonstrated. Using relaxation times measured by MQF NMR multiple binding of boric acid and borate ion to ferri and ferrocytochrome c was detected. Borate ion was found to have two different binding sites. One of them was in slow exchange, k(diss) = 20 +/- 3 s(-1) at 5 degrees C and D(2)O solution, in agreement with previous findings by (1)H NMR (G. Taler et al., 1998, Inorg. Chim. Acta 273, 388-392). The triple quantum relaxation of the borate in this site was found to be governed by dipolar interaction corresponding to an average B-H distance of 2.06 +/- 0.07 A. Other, fast exchanging sites for borate and boric acid could be detected only by MQF NMR. The binding equilibrium constants at these sites at pH 9.7 were found to be 1800 +/- 200 M(-1) and 2.6 +/- 1.5 M(-1) for the borate ion and boric acid, respectively. Thus, detection of binding by MQF NMR proved to be sensitive to fast exchanging ligands as well as to very weak binding that could not be detected using conventional methods.

The interaction of borate ions with cytochrome c surface sites: a molecular dynamics study

Ionic interactions of cytochrome c play an important role in the electron transfer process. Molecular dynamics simulations of the binding of borate ion, which serves as a model ion, at three different cytochrome c surface sites are performed. This work is motivated by previous NMR studies of cytochrome c in borate solution, which indicate the existence of two types of binding sites, a slow exchange site and a fast exchange site. These two types of binding behavior were observed in the dynamic simulations, offering a molecular interpretation of "loose" and "tight" binding. At the "loose" binding sites (near Lys25/Lys27 and Lys55/Lys73) the ion forms two to three hydrogen bonds to the nearest lysine residue. This binding is transient on the time scale of the simulation, demonstrating the feasibility of fast exchange. At the "tight" binding site (near Lys13/Lys86), on the other hand, the ion becomes integrated into the protein hydrogen bond network and remains there for the duration of the simulation (exemplifying slow exchange). Binding simulations of the ion at the "tight" site of H26Q mutant cytochrome c also showed integration of the ion into the protein's hydrogen bond network. However, this integration differs in details from the binding of the ion to the native protein, in agreement with previous NMR observations.