Cw. Hollars et Rc. Dunn, Probing single molecule orientations in model lipid membranes with near-field scanning optical microscopy, J CHEM PHYS, 112(18), 2000, pp. 7822-7830
Single molecule near-field fluorescence measurements are utilized to charac
terize the molecular level structure in Langmuir-Blodgett monolayers of L-a
lpha-dipalmitoylphosphatidylcholine (DPPC). Monolayers incorporating 3x10(-
4) mol % of the fluorescent lipid analog N-(6-tetramethylrhodaminethiocarba
moyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium
salt (TRITC-DHPE) are transferred onto a freshly cleaved mica surface at l
ow (pi=8 mN/m) and high (pi=30 mN/m) surface pressures. The near-field fluo
rescence images exhibit shapes in the single molecule images that are indic
ative of the lipid analog probe orientation within the films. Modeling the
fluorescence patterns yields the single molecule tilt angle distribution in
the monolayers which indicates that the majority of the molecules are alig
ned with their absorption dipole moment pointed approximately normal to the
membrane plane. Histograms of the data indicate that the average orientati
on of the absorption dipole moment is 2.2 degrees (sigma=4.8 degrees) in mo
nolayers transferred at pi=8 mN/m and 2.4 degrees (sigma=5.0 degrees) for m
onolayers transferred at pi=30 mN/m. There is no statistical difference in
the mean tilt angle or distribution for the two monolayer conditions studie
d. The insensitivity of tilt angle to film surface pressure may arise from
small chromophore doped domains of trapped liquid-expanded lipid phase rema
ining at high surface pressure. There is no evidence in the near-field fluo
rescence images for probe molecules oriented with their dipole moment align
ed parallel with the membrane plane. We do, however, find a small but signi
ficant population of probe molecules (similar to 13%) with tilt angles grea
ter than 16 degrees. Comparison of the simultaneously collected near-field
fluorescence and force images suggests that these large angle orientations
are not the result of significant defects in the films. Instead, this small
population may represent a secondary insertion geometry for the probe mole
cule into the lipid monolayer. (C) 2000 American Institute of Physics. [S00
21-9606(00)70616-7].