Phosphorus acid derivatives definition

When a Br nsted base functions catalytically by sharing an electron pair with a proton, it is acting as a general base catalyst, but when it shares the electron with an atom other than the proton it is (by definition) acting as a nucleophile. This other atom (electrophilic site) is usually carbon, but in organic chemistry it might also be, for example, phosphorus or silicon, whereas in inorganic chemistry it could be the central metal ion in a coordination complex. Here we consider nucleophilic reactions at unsaturated carbon, primarily at carbonyl carbon. Nucleophilic reactions of carboxylic acid derivatives have been well studied. These acyl transfer reactions can be represented by... 

Phosphoproteins.—Proteins of this class when subjected to the action of pepsin and hydrochloric acid, undergo hydrolysis, and yield proteoses and an acid containing phosphorus, which is called paranucleic acid. This acid is sharply distinguished from the nucleic acids, as neither xanthine bases, pyrimidine derivatives, nor pentoses occur among its dissociation-products. Paranucleic acid has not been obtained in the pure condition, and its decomposition-products are not definitely known. The phosphoproteins are distinctly acid, are insoluble in water, and form soluble salts which contain the alkali metals and ammonium. 

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