Helical parameters of regular -helices in proteins (part 2)

The α-helix, 310-helix, π-helix and -helix have been observed in protein structures. They account for 32% of residues, 4%, 0.3% and 0.2%, respectively. However, these per- centages depend on resolution of solved structures and method for assignment of secondary structures. May 2016, culled Protein Data Bank (PDB) data set, containing 2901 protein chains with less than 25% sequence identity and  1.6Å resolution (R-value  0.25), was used in this analysis. Secondary structure assignments are performed by DSSP, STRIDE and SECSTR for π-helices. Helical parameters-pitch, residues per turn, radius, handedness and p = rmsd/(N-1)1/2 for π-helices are determined by HELFIT program. p-Value, esti- mates helical regularity and all π-helices with p  0.10Å, were identified as regular. Heli- cal parameters of protein π-helices are compared with those of canonical π-helices and other types of protein helices.