DIPOLE-MOMENT CHANGE OF NBD GROUP UPON EXCITATION STUDIED USING SOLVATOCHROMIC AND QUANTUM-CHEMICAL APPROACHES - IMPLICATIONS IN MEMBRANE RESEARCH

Lipids that are covalently labeled with the 7-nitrobenz-2-oxa-1,3-diaz ol-4-yl (NBD) group are widely used as fluorescent analogues of native lipids in model and biological membranes to study a variety of proces ses. We have recently shown that one such NBD labeled lipid, NBD-PE, i n which the NBD label is covalently attached to the headgroup of a pho sphatidylethanolamine molecule, exhibits the red edge excitation shift (REES) effect when incorporated into vesicles of dioleoyl-sn-glycero- 3-phosphocholine (DOPC) [Chattopadhyay, A.; Mukherjee, S. Biochemistry 1993, 32, 3804]. One of the necessary conditions for a fluorophore to be able to exhibit REES is that the fluorophore must be polar and, mo re importantly, there should be a change in its dipole moment upon exc itation. In this paper, we have determined the actual change in dipole moment of the NBD group upon excitation using the solvatochromic shif t approach. Our results show that the dipole moment of the NBD group c hanges by 3.9 D upon excitation. We have complemented this experimenta l observation by semiempirical quantum chemical calculations of dipole moment changes of various NBD derivatives. These calculated dipole mo ment changes (3.5-3.6 D) agree very well with our experimental value. These calculations also point out that the process of charge separatio n is mainly limited to the NBD ring system and is independent of the l ength of the alkyl chain. These results are relevant to ongoing and fu ture studies that utilize photophysical properties of the NBD group, e specially in microheterogeneous media such as membranes and micelles.