title: Conformational Analysis of Retinoic Acids: Effects of Steric Interactions on Nonplanar Conjugated Polyenes.
abstract: Retinoic acids and other vitamin A analogs contain a trimethylcyclohexenyl ring in conjugation with a polyene chain joined at carbon-6 (C6) and carbon-7 (C7). A MP2-SCS/cc-pVDZ// B3LYP/6-31G(d) 2-D potential energy surface was computed for all-transretinoic acid, which had 6 minima (3 enantiomeric pairs). The global minima were distorteds-gaucheenantiomers (6-7= 53°) with half-chair conformations of the ring. Distorteds-gaucheenantiomers (6-7= 55°) with inverted half-chair ring conformations were 1.7 kJ/mol above the global minima. Thes-transenantiomers (6-7= 164°) were 11.3 kJ/mol above the global minima. Steric energies were computed by the method of Guo and Karplus to identify key structural elements in retinoic acids which determines their conformation. Small molecule crystal structures in the CCDC database with trimethylcyclohexenyl ring and exocyclic double bonds have ring-chain geometries near to one of the 6 energy minima of retinoic acids, except for retinaldehyde iminium cations.
authors: Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294.
authors: Bryan D
authors: BD
authors: Cox
authors: Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294.
authors: Donald D
authors: DD
authors: Muccio
authors: Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294.
authors: Tracy P
authors: TP
authors: Hamilton
keywords: Retinoic acid; conformational analysis; retinaldehyde iminium cations; steric energy; substituted cyclohexene ring inversion; β ionone