Structure Preference of Amino Acids

Notes:

Different amino acids have different structural tendencies. This is due to size, branching, stabilization factors, etc. 

Helix Formers: The twisted helix structure (like DNA!) strongly favors alanine. This is due to its small size, the fact that it is unbranched, and that it stabilizes helices. It also favors leucine and glutamate as they contain bulky groups that from the backbone of the helix. 

Helix Breakers: These are factors that would disrupt the formation of a helix. If you think about it intuitively, large rigid structures will not allow the flexibility that is needed to form a helix. For this reason, proline is a prominent helix breaker. Its ridgid ring disrupts the hydrogen bonding patterns essential for its formation. It also adds bulk which breaks up the structure. On a different note glycine is also a helix disrupter due to the fact that it is too flexible. To a certain extent, helices need structure to stabilize their formation and if a bond is able to be rotated around too much, it will not hold shape. 

Turn/Loop Promoters: For the same reason it was a helix breaker, glycine is a turn and loop promoter. It is very flexible, allowing there to be bends and twists to make a turn or loop in a structure. Additionally proline is a loop promoter as its large rigid ring introduces a kink to the structure. This bends the linear molecule and allows the structure to turn or loop. 

Beta Sheet Promoters: Valine, Isoleucine, and Phenylalanine are all beta sheet promoters. This is because they are bulky and have branched side chains. Due to the linear, branched structure of a beta sheet, these types of amino acids work well.