Consider first those that aren't branched. Amino acids can be divided into two kinds, those with branches at the beta C and those with none. Some amino acids are more commonly found in alpha helices than other.All the R-groups extend backward and away from the helix axis.There are not holes or pores in the helix. the core of the helix is packed tightly. the left-handed alpha helix, although allowed from inspections of a Ramachandran plot, is never observed, since the side chains are too close to the backbone.in proteins, the average number of amino acids in a helix is 11, which gives 3 turns.the alpha helix is more compact than the fully extended polypeptide chain with phi/psi angles of 180o.It can also be characterized by n (the number of amino acid units/turn = 3.6) and pitch (the helix rise/turn = 5.4 angstroms). The phi/psi angles for those amino acids in the alpha helix are - 57,-47, which emphasizes the regular repeating nature of the structure. These helices are formed when the carbonyl O of the i th amino acid H bonds to the amide H of the i th +4 aa (4 amino acids away). Figure: Right Handed Alpha helices - image made with VMD
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