Bond, bridge covalent

Although the formula for beryllium chloride is BeCl2, the compound exists in chains in the solid state. The bonding is covalent, and the environment around each Be is essentially tetrahedral with each Cl bridging between two Be atoms separated by 263 pm. 

When two cysteine residues are close together, a covalent disulfide bond can be formed between them as a result of oxidation. A covalent bonded bridge is thus formed between two different parts of the protein chain (Fig. 3.8). 

Chemically, keratin is formed from polypeptide chains formed from 20 different amino acids. The relative proportion of which vary for different keratins. Some possess an acid side group (12 Z of which are glutamic acid) others a amine function (30 Z of which are lysine), others a hydroxyl (10 Z of which are serine). But the keratin is above all characterized by an important quantity of sulfur due to the presence of cystine units which form a disulfide bridge between two chains greatly contributing to the stability of the proteins. In conclusion, the chains are connected by a nuid>er of diverse interactions including hydrogen bonds, salt bonds and covalent bonds. 

The individual DNA molcule which localized in eukaryotic chromosomes are large polymers and they contain a linear backbone of alternating sugar and phosphate residues. DNA molecule includes the five carbon sugar "deoxyribose," and consecutive sugar structures are linked by covalent phosphodiester bridge. Covalently bonded to carbon atom number V (one prime) of each sugar is a... 

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