Chirality inversion in the bilirubin molecular exciton

The bichromophoric pigment bilirubin acts as a molecular exciton in its W-v isible and circular dichroism (CD) spectroscopy. In both polar and nonpolar solvents, an optically active analog, (betaR,beta 'R) -dimethylmesobilirub in-XIII alpha (1), exhibits intense bisignate CD Cotton effects in the regi on of its long wavelength UV-vis absorption near 400 nm: Delta is an elemen t of (max)(434) + 337, Delta is an element of (max)(389) - 186 (CHCl3), and Delta is an element of (max)(431) + 285, Delta is an element of (max)(386) - 177 (CH3OH). However, introduction of an amine into a CHCl3 solution of 1 causes the Cotton effect signs to become inverted, e.g., after addition o f NH3, Delta is an element of (max)(433) - 345, Delta is an element of (max )(389) + 243, and after addition of ethylene diamine, Delta is an element o f (max)(435) - 420, Delta is an element of (max)(390) + 299. The sign inver sion imply inversion of molecular chirality of the bilirubin and the phenom enon appears to be general for amines, including alpha,omega -diamines. 1,8 -Diaminooctane was found to be more effective than longer or shorter chain analogs in producing CD sign inversion. (C) 2001 Wiley-Liss, Inc.