Proton donors and acceptors

Interestingly, the sulfanes H2S are both proton acceptors and donors. In the first case sulfonium ions H3S are formed, in the second case hydrogen polysulfide anions HS are the result. While the latter have never been isolated in salts, several salts with sulfonium cations derived from the sulfanes with n = 1, 2, and 4 have been published. However, none of these salts has been structurally characterized by a diffraction technique. Therefore, the structures of the HsSn cations and HS anions are known from theoretical calculations only. 

All chemical reactions in a biological cell take place with the direct participation of enzymes as catalysts. Enzymes are proteins, which are macromolecules composed of a combination of the 20 amino acids. Enzymes, as catalysts, are highly efficient and selective in binding small molecular species called ligands. A ligand that is acted upon by an enzyme to cause a chemical reaction is called a substrate. Only a certain, small portion of the amino acids that comprise an enzyme is involved in the catalytic reaction. This region is called the active site, and is directly involved in the formation of product(s). For example, the amino acid residues of proteins are greatly influenced by their local pH values, and the activity of proton acceptors and donors occurs in the active site. 

Table 3.43 Calculated frequencies (in cm ) and intensities (km mol ) of HF and (HF)j. A and D refer to proton acceptor and donor, respectively .

Figure 14 also shows the amino acid residues playing a key role in the proton transport Asp 85, Asp 212, and Tyr 185 with negatively charged terminal groups that are counterions with respect to the positively charged protonated Schiff base Asp 85 and Asp 96 are inner proton acceptor and donor, respectively Arg 82 favors the deprotonation of the retinal Schiff base. The data... 

A meticulous experimenter, Thomas Lowry is best known for his conceptualization of acid-base chemistry. Studies of nitrogenous compounds led Lowry to question fundamental aspects of the role of hydrogen during acid-base reactions. Three months before Br0nsted published his theory, Lowry released his own similar thoughts on proton acceptors and donors In print. 

In these systems Dha can be controlled by the concentration of X and S and the nature of these species. Lipophilic alcohols, as proton acceptors and donors, are... 

Table 6. Frequency shifts (cm caused by dimerization for proton acceptor and donor molecules in (HF)2...

This resonance stabilization is related to that in amides (Sections 20-1 and 26-4). Imidazole is significantly protonated at physiological pH (p7 = 7.6). It can therefore function as a proton acceptor and donor at the active site of a variety of enzymes (see, e.g., chymotrypsin. Section 26-4). 

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