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Biophys J, June 2002, p. 3198-3206, Vol. 82, No. 6
*Laboratorio di Ultrastrutture, Istituto Superiore di Sanità,
00161 Rome, and Dipartimento di Fisica
Università "La Sapienza" and Istituto Nazionale per la
Fisica della Materia, Unità di Roma 1, 00185 Rome,
Italy
The aggregation of Gramicidin A (gA) in
dipalmitoylphosphatidylcoline (DPPC) monolayers is investigated by both
thermodynamic and structural methods. Compression isotherm analysis and
atomic force microscopy (AFM) observations are performed. Our
experimental results indicate that gA aggregation does occur in DPPC
monolayers even at very low gA concentration (about 8 × 10
4 mol%). At the low gA concentration limit, the
aggregation process seems to be mainly horizontal (i.e., side-by-side,
into the monolayer plane), following a fractal pattern growth producing
the formation of typical, flat (0.5 nm height) "doughnut"
structures, with a diameter of ~150 nm. These structures appear to be
composed of smaller subunits (about 70 nm diameter) showing the same
doughnut structure. At a molar fraction of ~3.8 mol%, the big
doughnuts start to disaggregate and only small doughnuts appear. Above
a gA concentration of ~4.4 mol%, all doughnuts (large and small) disappear, and the morphology assumes the appearance of a patchwork of
two distinct phases: one that, being very flat, can be associated with
a gA-free or gA-poor DPPC phase, and a second one, characterized by a
more corrugated surface, associated with a gA-rich DPPC phase. At gA
concentration of ~5 mol%, a percolation transition in the gA-rich
DPPC phase occurs. Thermodynamic data indicate that the maximum of
miscibility between gA and DPPC molecules occurs at ~28 mol%,
suggesting that gA could aggregate in hexamers that are, on average,
bound to 16 DPPC molecules. At the same concentration, AFM images show
a network of small gA aggregation units of a size compatible with gA hexamers.
Biophys J, June 2002, p. 3198-3206, Vol. 82, No. 6
© 2002 by the Biophysical Society 0006-3495/02/06/3198/09 $2.00
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