Fluorescence spectroscopy of all-trans-lycopene:: comparison of the energy and the potential displacements of its 2Ag- state with those of neurosporene and spheroidene

The fluorescence spectra of all-trans-lycopene were recorded, and the 2A(g)(-) energy was determined to be 13300 +/- 300 cm(-1) at both 295 and 170 K. The 2A(g)(-) energies of open-chain carotenoids having the number of conjugated double bonds, n = 7-11, so far determined by fluorescence spectroscopy, showed a linear relation, E (2A(g)(-))= 220 550/(2n + 1) + 3700 cm(-1). The displacements of the 2A(g)(-) potential minima for lycopene, spheroidene and neurosporene were determined by spectral simulation of the 2A(g)(-) fluorescence to be Delta (1) = 1.2 and Delta (2) = 1.9 along the nu (1) and nu (2) dimensionless normal coordinates. Based on the assumptions, i.e., (1) the 2A(g)(-) energy follows the above linear relation, (2) only the nu (1) mode plays a major role as a promoting and an accepting mode, and (3) the above potential displacements are the same among carotenoids with n = 9-13, the Englman-Jortner equation predicted the relative rate constants of the 2A(g)(-)-to-1A(g)(-) internal conversion (the 2A(g)(-) lifetime) to be 4.1 : 1.9: 1.0: 0.54: 0.35, which is in agreement with those determined by time-resolved absorption spectroscopy, i.e., 4.5: 2.0:1.0:0.47:0.28. An empirical equation predicting the 2A(g)(-) lifetimes of those carotenoids is also proposed. (C) 2001 Elsevier Science B.V. All rights reserved.