Generalization of the Packing Parameter for Quantifying the Morphology of Peptide Amphiphile Micelles

We present a quantitative means for classifying the shape of molecular dynamics simulated peptide amphiphile micelles (PAMs) that is both consistent with existing metrics and extendable to estimating shape-dependent free energy contributions. The presented framework not only outlines an approach for characterizing the shape of simulated PAMs but also presents expressions that can readily be applied to quantify the shape of particles from experimental techniques where aspect ratios are measured. The generalization of the packing parameter introduces a characteristic length that, when applied to simulated PAMs, functions intuitively as an effective radius for a PAM whose core is a perfect sphere or an infinite cylinder. The presented shape assignment scheme is used to develop a model for the free energy penalty associated with packing the tails of the amphiphiles into a core whose shape is modeled by an ellipsoid. Good agreement with previous models and scaling behaviors is observed and the importance of accounting for the shape and size dependence of the core is illustrated. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=111 SRC="FIGDIR/small/598326v1_ufig1.gif" ALT="Figure 1"> View larger version (16K): org.highwire.dtl.DTLVardef@1b326d4org.highwire.dtl.DTLVardef@ba5e82org.highwire.dtl.DTLVardef@192e29forg.highwire.dtl.DTLVardef@5c63b1_HPS_FORMAT_FIGEXP M_FIG C_FIG