Acidity of Sulfonic and Phosphonic Acid-Functionalized SBA-15 under Almost Water-Free Conditions

Abstract

The hydrogen bond interaction of pyridine with sulfonic and phosphonic acid moieties at the surface of SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The composition of the materials is characterized by 29Si MAS NMR, the residual water content is inspected by 1H MAS NMR, and the hydrogen bond interactions are characterized by 15N CPMAS NMR at 130 K using pyridine-15 N as a probe molecule. It is shown that (i) all acid moieties at the surface are accessible for pyridine; (ii) each sulfonic acid moiety interacts with one pyridine molecule; (iii) each phosphonic acid moiety can interact simultaneously with two pyridine molecules; (iv) for both materials the interaction of the acid moieties with the base results in proton transfer to pyridine. The observed proton-donating ability of the acid moieties depends on the presence of residual water. In contrast to nonfunctionalized SBA-15, the sulfonic acid-functionalized SBA-15 material contains about six water molecules per acid moiety after drying at 420 K in high vacuum. From the 15N chemical shift of pyridine in the hydrogen-bonded complex, it is estimated that the proton-donating ability of the acidic functional groups solvated by such small water clusters is equivalent to that of acids in water exhibiting a pK a of about 0.6 and 1.3, respectively, for the sulfonic and phosphonic acid moieties. The O···H and H···N distances in the hydrogen bond of the pyridine complex are r OH ≈ 1.69 Å and r HN ≈ 1.05 Å for sulfonic acid, as compared to r OH ≈ 1.53 Å and r HN ≈ 1.09 Å for phosphonic acid.

Crystal structures of the apo form and a complex of human LMW-PTP with a phosphonic acid provide new evidence of a secondary site potentially related to the anchorage of natural substrates

“…Conversely, compound 2 is expected to be 100% monodissociated at pH 5, but 50% monodissociated and 50% fully dissociated at crystallization pH and may have charge À1 or À2 depending on pH value. 46 Thus, considering that P-loop is composed of protonated and positively charged Arg18 and also several N-H peptidic bonds oriented toward to center of the active site (residues Cys12 to Arg18), it is expected that better hydrogen bond acceptors could interact with this environment. Since sulfonate moiety has no bonded hydrogen, it is able to accept a higher number of acidic hydrogen bonds from the active site (P-loop).…”

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates

“…Hydrogen bonding (H-bonding) and steric effects are important tools of molecular engineering. Under certain conditions, their interplay can stabilize species that otherwise exhibit high chemical reactivity [ 1 , 2 , 3 , 4 ]. The structural complexity increases when there are either other noncovalent interactions or competing H-bonds.…”

NMR Study of Solvation Effect on the Geometry of Proton-Bound Homodimers of Increasing Size

“…[2][3][4][5] The presence of competing H-bonds and solvent complicates the analysis. [6][7][8] Sophisticated theoretical tools are required in order to simulate enzymatic reactions and transporters and receptors activity at the atomic level. [9][10][11] At the current stage of knowledge, it is possible to predict the effect of a mutation on the fate of a particular biosystem, to prove this prediction experimentally, and to explain it theoretically.…”