Figure 1.

(a) Six standard backbone torsion angles of α, β, γ, δ, ε and ζ and (b) two backbone pseudo-torsion angles of η and θ for a nucleotide (denoted by n), where η is defined by the atoms C4^′_n−1, P_n, C4^′_n and P_n+1, while θ is defined by P_n, C4^′_n, P_n+1 and C4^′_n+1.