pK A3 Research Articles

Design of a Water Soluble Fluorescent 3-Hydroxy-4-Pyridinone Ligand Active at Physiological pH Values.

In the present work we report the structure and the spectroscopic characterization of a new fluorescent 3-hydroxy-4-pyridinone ligand D-3,4-HPO. The synthesis of the compound was performed in two steps, which involve the reaction of the commercially available fluorophore dansyl chloride with a 3-hydroxy-4-pyridinone chelating unit and further deprotection. The new fluorescent chelator was characterized in the solid state by single-crystal X-ray diffraction and in solution by NMR, MS, absorption and fluorescence spectroscopies. The analysis of the variation of the absorption spectrum with pH allowed the determination of four pK a values (pK a1 =3.50, pK a2 =4.50, pK a3 =9.60, pK a4 =10.20) and establishment of the corresponding distribution diagram. The study of the fluorescence properties of the ligand show that in the pH range between 4 and 9 the fluorescence intensity is constant and has its maximum value thus allowing its further use at physiological pH values. The interaction of the ligand with copper(II) was accessed by fluorescence spectroscopy in MOPS buffer and the results show that the presence of copper(II) quenches the fluorescence of the ligand in ca 94% at a ligand: metal ratio of 2:1. The latter result is consistent with the formation of a copper(II) complex with the bidentate ligand, as confirmed by the EPR spectroscopy. Graphical Abstract New water soluble fluorescent ligand active at physiological pH values.

The dynamic balance between organic acids and circumneutral groundwater in a large boreal peat basin

Large raised bogs and patterned fens cover 56% of the landscape in the Glacial Lake Agassiz region of northern Minnesota (USA). Organic acids supply most of the acidity in the surface water of the bogs, but groundwater upwelling from the underlying glacial deposits neutralizes these organic acids within deep peat. Substantial concentrations of organic acids also occur in the surface waters of fens mixed with variable amounts of inorganic solutes contributed by groundwater discharge. We used a triprotic analog model to determine the extent to which organic acids in fen and bog waters behave as strong or weak acids. The modeling approach optimized charge balance by calibrating estimates of mole site density in the DOC (dissolved organic carbon) of surface and pore waters with estimates of triprotic acid dissociation constants. Before the calibration process, all of the bog waters and 76% of the fen waters had more than +20% imbalance in charge balance. After calibration, more than 75% of all waters were electrochemically balanced within 20%. In the best calibration, the mole site denisty of bog DOC was estimated as ∼0.05 mmol/mmol C., approximately six times smaller than that estimated for fen DOC or the DOC in the fen deeper fen peats that underlie all bog landforms. The three modeled de-protonation constants were; pK a1=∼3.0, pK a2=∼4.5 and pK a3=∼7.0 for the bog DOC, and; pK a1=∼5.2, pK a2=∼ 6.5 and pK a3=∼7.0 for the fen DOC. Bog DOC, behaves as a strong acid despite its small mole site density. The DOC in bog runoff can therefore theoretically acidify the surface waters in adjacent fens wherever these waters do not receive sufficient buffering alkalinity from active groundwater seepage.

The effects of catenation on the acidity constants of porphyrin [2]catenanes

The acidity constants ( pK a3 and pK a4) of a series of porphyrins and their corresponding [2]catenanes where the porphyrin is over-arched with a hydroquinol or naphthoquinol polyether strap and mechanically linked to a bipyridinium macrocycle cyclobis-paraquat(4,4'-biphenylene), are calculated from acid-base titration data. As the length of the strap decreases from tetraethylene through triethylene to diethylene, the pK a values decrease, and the separation between pK a3 and pK a4 increases. In the diethylene glycol strapped derivatives, only the monocation is accessible above pH = 0. The trend in values reflects the ease of protonation which becomes increasingly difficult as the coulombic repulsions between the protonated porphyrin and the tetracationic macrocycle becomes more severe, as a consequence of enforced proximity of the two charged centres. On protonation, any electronic interactions between the interlinked subunits is replaced by strong electrostatic repulsions. This is alleviated by a 90° rotation of the tetracation and a movement away from the porphyrin; such a reorientation is possible only in the longer strapped derivatives, and is prevented in the “tight” analogues.

Characterization of natural organic matter from eight Norwegian surface waters: Proton and copper binding

As a part of the International NOM-Typing Project, proton and copper binding studies were conducted on ultrafiltered (>1000 dal.) and dialysed (>100 dal.) natural organic matter (NOM) samples from eight Norwegian Lakes. The NOM samples were similar with respect to proton and copper binding, although minor differences were observed among the samples of different origin. Slight variations occurred for seasonal changes and lime additions. The proton binding pK a values grouped in three distinct classes: pK a1 = 4.20–4.28; pK a2 = 6.61–6.87; and pK a3 = 9.27–9.8. The average total proton binding site concentration of the NOM samples was: 12.7 meq/g total organic carbon (TOC). Two binding sites were established for copper with average log K1 = 4.84 and log K2 = 7.17. The average site concentration for samples isolated by reverse osmosis (RO) was 0.56 μmol/mg C. Copper binding characteristics appear to be influenced by the method of sample isolation. For NOM isolated by low pressure evaporation (evap), binding site concentrations are smaller, conditional stability constants are higher, variability of the copper complexation capacity is smaller, and correlation with elemental composition (C, N, H) is better than in the case of NOM isolated by RO.

Highly Potent Bisphosphonate Ligands for Phosphoglycerate Kinase and Protein Binding Studies

Several novel α,ω-bisphosphonate analogs (1) of 1,3-bisphospho-D-glyceric acid, 1,3-BPG (2) have been made and their binding to phosphoglycerate kinase, PGK (EC 2.7.2.3) evaluated. Non-scissile methanephosphonic acids replace the two phosphate monoesters of 1,3-BPG. Fluorine substitution in the α-methylene groups of the phosphonic acid analogs (1) markedly improves their binding to PGK as determined by NMR analysis. The best ligands bind some 50-100 times more strongly than does 3-phospho-D-glyceric acid and show a requirement for pK a3 to be below 6.0 while the presence of a β-carbonyl group (1, Y = CO) seems to be of secondary importance. One analog, 1,1,5,5-tetrafluoropentanebisphosphonic acid, has been linked to adenosine 5′-phosphate and 5′-diphosphate to generate bisubstrate analogs (3) whose affinity for PGK and an X-ray crystal structure of a binary complex with PGK will be discussed.

Highly Potent Bisphosphonate Ligands for Phosphoglycerate Kinase and Protein Binding Studies

Several novel α,ω-bisphosphonate analogs (1) of 1,3-bisphospho-D-glyceric acid, 1,3-BPG (2) have been made and their binding to phosphoglycerate kinase, PGK (EC 2.7.2.3) evaluated. Non-scissile methanephosphonic acids replace the two phosphate monoesters of 1,3-BPG. Fluorine substitution in the α-methylene groups of the phosphonic acid analogs (1) markedly improves their binding to PGK as determined by NMR analysis. The best ligands bind some 50-100 times more strongly than does 3-phospho-D-glyceric acid and show a requirement for pK a3 to be below 6.0 while the presence of a β-carbonyl group (1, Y = CO) seems to be of secondary importance. One analog, 1,1,5,5-tetrafluoropentanebisphosphonic acid, has been linked to adenosine 5′-phosphate and 5′-diphosphate to generate bisubstrate analogs (3) whose affinity for PGK and an X-ray crystal structure of a binary complex with PGK will be discussed.