The Secondary structure of a protein is characterized by regular elements such as alpha helices (α helices), Beta sheets (β sheets) and irregular elements such as Beta bulges, tight turns and random coils. Alpha helices, Beta sheets and Turns are the three common secondary structures in proteins and the segment of polypeptide which cannot be classified into these three are grouped into the category of Loops. Tight Turns are a subset of loops and they can be classified as β -turns, γ -turns, α -turns, δ -turns.
Among Tight Turns Beta Turns represent the largest category of non-repetitive protein secondary structure and play important roles in protein and peptides. They consist of 4 consecutive residues defined by positions i,i+1,i+2,i+3 and the distance between Cα(i) and Cα(i+3) should be less than 7Å.
They have been classified into nine different types I,II,VIII,I',II',VIa1 ,VIa2,VIb,IV . These types are based on dihedral angle values of the i+1 and i+2 positions of the turn which is now widely accepted. These dihedral angles can have a maximum deviation of ±30o from their ideal values for three angles, while the fourth can deviate ±45o. If this criterion is not respected, the beta turns is classified as type IV.
Gamma turns is the second most characterized and commonly found tight turn in proteins after β-turns. γ-turn comprise of 3.4% of amino acids and are defined as 3 residue turns with H-bond between the carbonyl oxygen of residue (i) and the hydrogen of the amide group of residue (i+2). There are 2 types of γ-turn in proteins: inverse and classic. They differ in that the main-chain atoms of the two forms are related by mirror symmetry (just as type I and I' or type II and II' beta-turns are). Of these classic ones are far less common but responsible for the reversal of polypeptide chain, and those that do exist are found at the loop ends of beta hairpins. On the other hand inverse gamma turns tend to give rise to kink in the peptide chain and account for nearly 91% of total gamma turns.
The dihedral angle intervals for classic and inverse gamma-turns are: