Secondary Protein - Structure
Introduction:
The secondary protein structure is the specific geometric shape caused by intramolecular and intermolecular hydrogen bonding of amide groups. The geometry assumed by the protein chain is directly related to molecular geometry concepts of hybridization theory. Experimental evidence shows that the amide unit is a rigid planar structure. This is derived from the planar triangle geometry of the carbonyl unit ( C = O ). See the graphic on the left.
The geometry around the nitrogen is derived from an unusual situation with a planar triangle geometry. Apparently, the double bond on oxygen can alternate to make a double bond between carbon and nitrogen. Rotation around bonds C-C and N-C does take place. The C=O and NH are always in a rigid plane. Notice that the carbonyl group and the hydrogen on nitrogen are almost always trans to each other. The result is that chains of amino acids as peptides with amide bonds reflect this geometry.
As a result of studying X-ray photographs and constructing molecular models, Linus Pauling and Robert Cory, in 1951, proposed that the protein structures were either in the form of an alpha helix or the beta pleated sheet.