The conformation of end-groups is one determinant of carotenoid topology suitable for high fidelity molecular recognition: A study of β- and ε-end-groups

Conformation affects a carotenoid's ability to bind selectively to proteins. We calculated adiabatic energy profiles for rotating the ring end-groups around the C6C7 bond and for flexing of the ring with respect to the polyene chain. The choice of computational methods is important. A low, 4.2 kcal/mol barrier to rotation exists for a β-ring. An 8.3 kcal/mol barrier exists for rotation of an ε-ring. Rotation of the ε-ring is sensitive to substitution at C3. In the absence of external forces neither β- nor ε-rings are rotationally constrained. The nearly parallel alignment of the β-ring to the C6C7 bond axis contrasts to the more perpendicular orientation of the ε-ring. Flexion of a β-ring to the minimized ε-ring conformation requires ∼23 kcal/mol; extension of the ε-ring to the minimized β-ring conformation requires ∼8 kcal/mol. Selectivity associated with β- versus ε-rings is dominated by the inability of the β-ring to flex to minimize protein/ring steric interactions and maximize van der Waal's attractions with the binding site.