Configurations of allosteric disulfide bonds. (A) Classification of disulfide bonds based on their geometry.^95,96  The geometry is defined by the 5 bond angles (χ angles) linking the 2 α-carbons of the cystine residue. C_α, main chain carbon atom; C_β, side chain carbon atom of each cysteine residue. The χ angles are recorded as being either positive or negative. The 3 basic types of bond configurations (spirals, hooks, and staples) are based on the signs of the central 3 angles, and they can be either RH or LH depending on whether the sign of the χ³ angle is positive or negative, respectively. These 6 bond types expand to 20 when the χ¹ and χ¹′ angles are taken into account. (B) Examples of the structures of the emerging allosteric configurations: –RHStaple, –LHHook, and −/+RHHook.