 Primary structure is also called the covalent structure of proteins. And the primary structure refers to the amino acid sequence of its polypeptide chain. There are thousands of proteins present in this nature. All the proteins are different from each other. So how they are different from each other? They are different from each other only on the basis of their amino acid sequence. Every protein has a unique amino acid sequence. And if this amino sequence is changed, the protein is entirely changed. As you know in case of sickle cell, only one amino acid change can change the structure of protein and in turn can make its function changed. So if a particular sequence of protein is changed, its structure is changed and its function is also changed. So a protein function is due to its specific structure. And that specific structure of protein is mainly due to its amino acid sequence. Peptide bond is rigid and planar. This was said by Pauling and Coray. Linus Pauling and Robert Coray, they carefully analyzed the peptide bond present in the protein molecules. They also knew the importance of hydrogen bonding and other weak interactions which are present in the biomolecules. And they made number of important conclusions. Actually their findings laid the foundation of our present understanding of protein structure. They demonstrated that the peptide CN bond is somewhat shorter than a normal CN bond present in a simple amine. Here you can see in this picture, this is peptide bond between C and N. This peptide bond is covalent in nature. But as they said, this covalent bond is little shorter than normal CN bond present in other biomolecules. As you might be knowing, one CN bond is present in the DNA structure in a nucleotide when ribose sugar is attached with a minor group of a nitrogenous base. So that is a mine. So if this bond is present between a simple amine, this is a bit larger than the bond present between C and N in a protein molecule. Why this bond is shorter than normal bond? Here you can see this nitrogen. This nitrogen has a lone pair of electrons and this lone pair is shared with this oxygen. Now this lone pair is transferred to this bond and from here it is transferred to oxygen. When this lone pair is transferred to oxygen, its double bond is shifted from here to here. Here you can see this bond becomes double while this co bond becomes single. Again, when this lone pair goes back to carbon and then carbon to nitrogen, this double bond will be shifted back to here like this. So this peptide bond becomes single and C double bond O becomes double. So in this way, we can say that this peptide bond is resonance hybrid. It is a partial double bond. That's why it is shorter than the normal CN bond. The second important finding of Pauling and Coray was that the six atoms of a peptide group, they are coplanar. It means these six atoms, they lie in a single plan. So a polypeptide or a protein contains different peptide groups and each peptide group has six atoms. So six atoms lie in a single plan. The other six atoms lie in another plan. The other six atoms and other peptide group lie in another plan. So in this way, these peptide groups are coplanar. And the oxygen atom of the carbonyl group in this peptide group and the hydrogen atom of the amide nitrogen, they are trans in position. As we can see in this picture, this is peptide group, this whole, and it contains six atoms. One, two, three, four, five and six. This group containing six atoms, they lie in a single plan. So another peptide group will also contain six atoms, but it will lie in another plan. Similarly, here is another peptide group. So they lie in a single plan. And in this peptide group, the CO and NH, they are always in trans configuration. They can never be in cis configuration. This is not present in the protein molecules. Pauling and Coray concluded that the peptide CN bonds are unable to rotate freely because of their partial double bond character. Single bonds can rotate, but double bonds, they cannot rotate. And as you know, peptide bond is partially double bond. So it is a rigid bond and it cannot rotate. If it cannot rotate, then how different peptide groups, they become angular to each other. They lie in different angles, how? That means rotation is allowed on some other bonds and rotation is permitted about N, alpha-carbon linkage bond and alpha-carbon and C bond. So at these two bonds, rotation is allowed while peptide bond, which is between C and N, that is rigid bond. Here you can see this is alpha-carbon, this black ball and this is N. So this angle is between N and alpha-carbon. Here rotation is allowed and this angle is called phi. Again, this is alpha-carbon and this one is carbon. So at this bond, rotation is also allowed and this rotation is called psi. So as you have seen, the rotations are allowed at C and alpha-C and at a bond between N and alpha-C. So the bond angles resulting from rotations at carbon atom are labeled as phi. If this rotation is between N and alpha-carbon and it is called psi, if this rotation is between alpha-carbon and C bond. In principle, phi and psi can have any value between plus 180 and minus 180. So it is wide range. Any value can be adopted by a phi or psi but some values are still prohibited due to steric interference.