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Pauling suggested two kinds of secondary conformation

•    Alpha helix

•    Beta pleated sheet

The α-Helix

Some structural proteins are found to be in a coiled form. The alpha helix is a spiral formed by coiling of the polypeptide chain around the fibre axis. Each turn of the helix is 5.4 Å long and contains 3.6 amino acid residues, length of each residue being 1.5 Å. The helical structure is stabilized by noncovalent hydrogen bonds linking the amide nitrogen with carbonyl oxygen.

                            R        O    H
                            I          I I     I    
                       –––– N – (C H  – C – N )  – C – –    
                     I                                   II
                                             ________ H   –––– – – – – – – –  O    

                                                                                    H – bond

The side chains of the resides project outwards. The alpha helix may be either right handed or left handed . The α-keratin of hair and wool exists in right handed helical form.

β-Pleated Sheet    

Pauling and Corey proposed yet another arrangement of protein-β-pleated sheet form which occurs in an extended form. Some fibrous proteins, like β-keratin, and collagens exist in sheet like structure where maximum number of hydrogen bonds are formed perpendicular to the long axis of the polypeptide chains.

ball of stick model of alpha helix

showing the dimension of alpha helical turns

left handed helix with hydrogen bonds

streched keratin and silk

The bonds exist between peptide bonds so that parallel or antiparallel sheet like structures are formed such proteins are fibrous and insoluble in aqueous solvents.

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