EFFECTS OF LIPID STRUCTURE ON ENERGY-TRANSFER FROM CARBAZOLYL TO ANTHRYL GROUPS IN A LIPID BILAYER

Two types of chromophoric amphiphiles were synthesized: one of them possesses a molecular structure of N,N-dialkyl aromatic amino acid (5X18 type, where X is A or Cz), and the other alpha,gamma-dialkylglutamate connected to aromatic amino acid (mXG12 type, where m is an integer). 5-N-Ethylcarbazolyl and 9-anthryl groups were chosen as the chromophore, and introduced to each amino acid derivative. All the amphiphiles formed assembly showing gel-liquid crystalline phase transition. The phase-transition temperature of the assembly composed of mXG21-type amphiphile was higher than that of 5X18-type amphiphile. Absorption and CD spectra of 6-(trimethylammonium)hexanoyl-L-3-(5-N-ethylcarbazolyl)alanine N,N-dioctadecylamide bromide (5Cz18) in the assembly indicated the absence of strong ground-state interactions between the carbazolyl groups, while those of 6-(trimethylammonium)hexanoyl-L-3-(5-N-ethylcarbazolyl)alanyl-L-glutamic acid alpha,gamma-didodecyl ester (5CzG12) or 11-(trimethylammonium)undecanoyl-L-3-(5-N-ethylcarbazolyl)alanyl-L-glutamic acid alpha,gamma-didodecyl ester (10CzG12) indicated the ground-state interactions based on dimer or higher aggregates. Fluorescence spectra of 5Cz18 showed very weak excimer emission, while excimer and/or excited dimer or higher aggregates were observed in the assembly of 5CzG12 or 10CzG12. Similar results were obtained for amphiphiles (mAG12) with anthryl and hydroxyethyldimethylammonium groups in places respectively of carbazolyl and trimethylammonium groups of 5CzG12 and 10CzG12. Taking these results together into consideration, the molecular packing of mXG12 in the assembly should be tighter than that of 5X18. In the binary assembly of 6-(trimethylammonium)hexanoyl-L-3-(9-anthryl)alanine N,N-dioctadecylamide bromide (5A18)/5Cz18 (1/99 mol/mol), about 60% of photoenergy absorbed by the carbazolyl groups was transferred to the anthryl groups, indicating an efficient energy migration along the two-dimensional array of carbazolyl chromophores of 5Cz18. On the other hand, in the mCzG12/mAG12 binary assembly, the energy-transfer efficiency was much lower due to the formation of dimer or the higher aggregates acting as energy-dissipating sites.