Linkage between the Basic Units

It has been previously noted that positions 1, 4 and 5 in the basic D-glucose unit of cellulosic materials are involved in some mode of chemical linkage. In order to determine the nature of this chemical linkage, degradation products of purified cellulosic materials were investigated. 

The synthesis of cellobiose octaacetate by the reaction of the sodium derivative of 1,2,3,6-tetraacetyl-D-glucopyranose with 2,3,4,6-tetraacetyl-a-D-glucopyranosyl bromide has recently been reported. Deacetylation resulted in the formation of cellobiose. 

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The application of this method of oxidation to the disaccharides without accompanying hydrolysis was successful and proved to be a valuable structural tool in showing the nature of the linkage between the two sugar units. Lactose was converted to lactobionic acid (33% yield) by Fischer and Meyer the sugar was treated with an equal weight of bromine at 20° for two days. Several salts were prepared, notably the insoluble basic lead salt, but the free acid would not crystallize. Ruff and Ollen-dorf reported a 38% jdeld, isolated as the calcium salt. Fischer and Meyer also reported the preparation of maltobionic acid, as the basic lead or calcium salt both were non-crystalline. 

Proteins are macromolecules consisting of 20 different amino acid residues arranged in a highly sophisticated three-dimensional structure. The basic structural units of proteins are amino acids. All proteins are constructed from the same set of 20 amino acids in different combinations. These 20 building blocks differ in size, shape, polarity, charge, and chemical reactivity. Amino acids are joined by peptide linkage between the a-carboxyl group of one amino acid... 

The twenty basic units of differing chemical properties, their random order and variable abundance in the polymer, and the variable length of proteins provide a tremendous diversity to proteins as a class. For example, there are 400 possible dipeptides (20x20) and 8000 (20 ) possible tripeptides, and the numbers become astronomical (20 ) when even moderately small proteins of as few as 100 amino acids are considered. Superimposed on this diversity are further modifications such as intrachain and interchain disulfide linkages, which are covalent linkages between cysteines, and other covalent modifications of the sidechains of individual amino acids (discussed below). The linear amino acid sequence together with further covalent modifications is known as the primary structure of a protein. 

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