Helical parameters of regular -helices in proteins

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 percentages depend on resolution of solved structures and method for assignment of secondary structures. Culled 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 byDSSP, STRIDE, and SECSTR for π-helices.HELFIT program determines the helical parameters-pitch, residues per turn, radius, and handedness and p = rmsd/(N-1)1/2 for π-helices, where RMSD is the root mean squares deviation from the best fit helix and N is helix length. p-value, estimates helical regularity and all regular π-helices with p  0.10Å were identified. Helical parameters of π-helices are compared with the helical parameters of canonical π-helices and other types of protein helices.