Hydrogen ionized forms

To realize the reason for this result from a simple intuitive point of view it is important to recognize that the ionized form of Aspc is more stable in the protein-active site than in water, due to its stabilization by three hydrogen bonds (Fig. 7.7). This point is clear from the fact that the observed pKa of the acid is around 3 in chymotrypsin, while it is around 4 in solution. As the stability of the negative charge on Aspc increases, the propensity for a proton transfer from Hisc to Aspc decreases. 

In the phosphoric acid fuel cell as currently practiced, a premium (hydrogen rich) hydrocarbon (e.g. methane) fuel is steam reformed to produce a hydrogen feedstock to the cell stack for direct (electrochemical) conversion to electrical energy. At the fuel electrode, hydrogen ionization is accomplished by use of a catalytic material (e.g. Pt, Pd, or Ru) to form solvated protons. 

The gas component, which is the most important form in which matter is found in outer space (up to 98-99%), is dominated by the element hydrogen, which makes up 70% of the mass and 90% of the particles. In ionized form (the H-II regions) the gas can be recognized by its recombination and fluorescence light emission. The hydrogen is mainly present in neutral form (H-I regions), at a mean density of 2x 107 particles per cubic metre and a mean temperature of about 80 K. 

Absorption. Absorption of cyanide across the gastrointestinal mucosa depends on the pH of the gut and the pKa and lipid solubility of the particular cyanide compound. Hydrogen cyanide is a weak acid with a pKa of 9.2 at 25 °C. The acidic environment in the stomach favors the non-ionized form of hydrogen cyanide and facilitates absorption. Information regarding the rapid lethal effects following oral intake of cyanide in humans (Gosselin et al. 1976) indicates that cyanide is rapidly absorbed from the gastrointestinal tract. 

Weak acids are able to donate hydrogen ions to bases, but they are less determined to do so than strong acids. So when a weak acid dissolves in water, it establishes a dynamic equilibrium between the molecular form of the acid and the ionized form. For example, acetic acid is a very common weak acid. In water, acetic acid establishes the following equilibrium ... 

In this situation, if the hydrogen ion concentration increases, the proportion of drug in the ionized form (the original condition) increases. Therefore, consequences of a shift in pH away from equilibrium conditions are opposite for weak acids and weak bases. For example, a solution of the weak base dextromethorphan (a drug present in cough preparations with a pfCa of 9.2) in the stomach (pH 1) will have approximately 1 of every 160,000,000 molecules in the unionized form. Obviously, gastric absorption will be significantly curtailed. 

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