Theoretical and UV spectral study of isomeric 1-(quinolinyl)-β-carbolines conformations

On the basis of β-carboline (1) and 1-(quinolin-2′-yl)-β-carboline (3) α- and t-band energies differences (Δα,t) a equilibrium conformations of 1-(quinolin-4′(5′-8′)-yl)-β-carbolines (4–8) in solution have been estimated. Furthermore, as an example of model compounds 1-(α′-naphtyl)-β-carboline (MC1) and 1-(β′-naphtyl)-β-carboline (MC2) and also 5 and 6 by molecular mechanics (mm+), semi-empirical (AM1) and none empirical (RHF/6-31G(d)) methods a computations of internal rotation of quinoline fragment around single bond have been performed. It was found that the greater bathochromic shift of the long-wavelength band maxima of 1, in the case of 3 (Δλmax = 39 nm) relatively to 6, 7 (Δλmax = 17 ± 2 nm) and 4, 5, 8 (Δλmax = 9 ± 1 nm) caused by coplanarity of the molecule 3. Also, from experimental and theoretical investigations a less dihedral angle between β-carboline and quinoline nucleous for 6, and 7 than 4, 5 and 8 owing to steric and electronic interactions have been found.