Of the several types of βturns, the two (type I andtype II) are the most common. Type I turns occur more than twice as frequently as type II. All types of βturns are stable. All involve a four aminoacid sequence in which the carbonyl oxygen of AA-1 is H-bonded to the amino-H of AA-4 (rather than AA-5 as is found in the alpha helix). Type I and type IIturns differ in the bond linking residue 2 and residue 3. The two types differ in a 180 degree rotation around the bond linking residues 2 and 3. Although various amino acids can make up theturn, frequently AA-2 is a proline since it does introduce a sharp bend in the polypeptide chain. Type II βturns always have Gly as the third residue to avoid steric interactions between its R group and thecarbonyl oxygen residue of residue 2.