Figure 5: The histogram represents the distribution of (N– –C) angles for residues located in -sheets derived from 5139 protein chains extracted from the PDB. These chains were selected by applying the following criteria: resolution better than 2.2 Å, lower than 0.20, and mutual sequence identities lower than 25%. The black points represent energies derived from Maxwell-Boltzmann relationship exp(− ), where is the number of observations in state , is the number of observations in the most populated state, is the Boltzmann factor, is the temperature of the system, and is the energy difference between two energy states.