The conformation of end-groups is one determinant of carotenoid topology suitable for high fidelity molecular recognition: a study of beta- and epsilon-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 beta-ring. An 8.3 kcal/mol barrier exists for rotation of an epsilon-ring. Rotation of the epsilon-ring is sensitive to substitution at C3. In the absence of external forces neither beta- nor epsilon-rings are rotationally constrained. The nearly parallel alignment of the beta-ring to the C6C7 bond axis contrasts to the more perpendicular orientation of the epsilon-ring. Flexion of a beta-ring to the minimized epsilon-ring conformation requires approximately 23 kcal/mol; extension of the epsilon-ring to the minimized beta-ring conformation requires approximately 8 kcal/mol. Selectivity associated with beta- versus epsilon-rings is dominated by the inability of the beta-ring to flex to minimize protein/ring steric interactions and maximize van der Waal's attractions with the binding site.
Florida International University, Department of Chemistry and Biochemistry, Miami, FL 33199, USA. landrumj@fiu.edu
John T
JT
Landrum
David C
DC
Chatfield
Alex M
AM
Mebel
Francesca
F
Alvarez-Calderon
Melissa V
MV
Fernandez
D002338:Carotenoids / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D013816:Thermodynamics
D002338:Carotenoids