Bicarbonate gradient

To HEPES buffer (100 mL, 200 mM, pH 7.5) were added ManNAc 15 (1.44 g, 6 mmol), PEP sodium salt (1.88 g, 8 mmol), pyruvic acid sodium salt (1.32 g, 12 mmol), CMP (0.64 g, 2 mmol), ATP (11 mg, 0.02 mmol), pyruvate kinase (300 U), myokinase (750 U), inorganic pyrophosphatase (3 U), /V-acctylneuraminic acid aldolase (100 U), and CMP-sialic acid synthetase (1.6 U). The reaction mixture was stirred at room temperature for 2 days under argon, until CMP was consumed. The reaction mixture was concentrated by lyophilization and directly applied to a Bio-Gel P-2 column (200-400 mesh, 3 x 90 cm), and eluted with water at a flow rate of 9 mL/h at 4°C. The CMP-NeuAc fractions were pooled, applied to Dowex-1 (formate form), and eluted with an ammonium bicarbonate gradient (0.1-0.5 M). The CMP-NeuAc fractions free of the nucleotides were pooled and lyophilized. Excess ammonium bicarbonate was removed by addition of Dowex 50W-X8 (H+ form) to the stirred solution of the residual powder until pH 7.5. The resin was filtered off and the filtrate was lyophilized to yield the ammonium salt of CMP-NeuAc 17 (1.28 g, 88%). 

Adenosine 5"-[P-thio]diphosphate tri-lithium salt [73536-95-5] M 461.1. Purified by ion-exchange chromatography on DEAE-Sephadex A-25 using gradient elution with 0.1-0.5M triethylammonium bicarbonate. [Biochem Biophys Acta 276 155 7972.]... 

The carbon dioxide produced during cellular metabolism diffuses out of the cells and into the plasma. It then continues to diffuse down its concentration gradient into the red blood cells. Within these cells, the enzyme carbonic anhydrase (CA) facilitates combination of carbon dioxide and water to form carbonic acid (H2C03). The carbonic acid then dissociates into hydrogen ion (H+) and bicarbonate ion (HC03). 

This entire reaction is reversed when the blood reaches the lungs. Because carbon dioxide is eliminated by ventilation, the reaction is pulled to the left. Bicarbonate ions diffuse back into the red blood cells. The hemoglobin releases the hydrogen ions and is now available to load up with oxygen. The bicarbonate ions combine with the hydrogen ions to form carbonic acid, which then dissociates into carbon dioxide and water. The carbon dioxide diffuses down its concentration gradient from the blood into the alveoli and is exhaled. A summary of the three mechanisms by which carbon dioxide is transported in the blood is illustrated in Figure 17.8. 

There is also a gradient from the low luminal pH through the mucus layer, under which gastric bicarbonate secretion maintains neutral conditions. Mechanically, this is explained by the acid secretion occurring like small finger-like ejections penetrating the thick gel-like mucus layer into the gastric lumen [27]. 

Proximal tubule Cells of the PCT are responsible for bulk transport of solutes, with approximately 70-80% of the filtered load of sodium chloride (active processes) and water (passive, down the osmotic gradient established by sodium reabsorption) and essentially all of the amino acids, bicarbonate, glucose and potassium being reabsorbed in this region. 

Because of their metastabiUty preparation of hydrolyzed iron (III) solutions by base addition is a tricky problem. Most bases cause immediate and irreversible precipitation due to local concentration gradients in mixing. Hedstrom and Biedermann avoided the problem by using bicarbonate, a base sufficiently weak that local precipitates are not produced. Since the solutions are quite acid (pH 2—3) all of the bicarbonate is rapidly converted to CO2 which escapes from the system. In their spectro-photometric work, Milburn and Voshurgh (16) avoided base addition altogether, using simple dilution to adjust the pH. At the low concentrations used in their study adequate degrees of hydrolysis could be obtained by this technique. 

Nucleotide thiophosphate analogues. The preparation and purification of [ HJATPyS, pHJGTPyS, s ITPyS (6-thioinosine), cl ITPyS (6-chloroinosine) and [ HJATPyS are described and the general purification was achieved by chromatography of the nucleotide thiophosphates in the minimum volume of H2O placed onto a DEAE-Sephadex A25 column and eluting with a linear gradient of triethylammonium bicarbonate (0.1 to 0.6M for G and I nucleotides and 0.2 to 0.5M for A nucleotides). [Biochim Biophys Acta 276 155 7972]. 

The limited solubility of membrane proteins and related polypeptides in aqueous mobile phases can also cause problems. These could be solved, e.g., by adding guanidine hydrochloride (6 M) or urea (8 M) to the portion of initial eluent used for sample preparation 69). The urea was always eluted in the breakthrough volume of the column. Thus, the retained hydrophobic polypeptides might have been temporarily precipitated upon the column. Collagen chains, dissolved in 0.5 M acetic acid, were successfully separated by RP-HPLC through gradients of 0.1 M TFA/acetonitrile 70> or (0.05 M ammonium bicarbonate + TFA)/ tetrahydrofuran 57>. 

The remaining solution was diluted with DCM (2 ml), and m-chloroperbenzoic acid (mCPBA) (0.30 g, 2.0 mmol) was added portionwise. After stirring at RT for 40 h, the mixture was extracted with DCM and washed successively with aqueous sodium sulfite solution, aqueous sodium bicarbonate solution and water, dried (MgS04) and evaporated in vacuum. Column chromatography (45-70% ether-petrol, gradient elution) of the residue afforded the title epoxides (3aa,6a,7a,7aa)-6,7-epoxy-3a,6,7,7a-tetrahydrobenzo[d]-l,3-dioxol-2-one (73 mg, 47%) mp 87°-89°C and (3aa,6p,7p,7aa)-6,7-epoxy-3a,6,7,7a-... 

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