Numerous studies have shown that a surface can direct and regulate molecular assembly. In this study, the nanofiber growth of an ionic-complementary peptide, EAK16-II, on a mica surface was investigated under various solution conditions via in situ atomic force microscopy. In comparison to the assembly in bulk solution, nanofiber growth of EAK16-II on mica is surface-assisted and involves two steps: (1) adsorption of nanofibers and fiber clusters (from the bulk solution) on the surface, serving as the "seeds"; (2) fiber elongation of the "seeds" from their active ends. The nanofiber growth can be controlled by adjusting the solution pH since it modulates the adsorption of the "seeds" on mica and their growth rates. The amount of the adsorbed "seeds" decreases with increasing solution pH, while the growth rate under different solution conditions is found to follow the order pure water > 1 mM HCl > 1 mM NaOH > 10 mM HCl approximately 10 mM NaOH approximately 0. The pH-dependent nanofiber growth is due to the surface charge of the peptides and peptide assemblies in various solutions as indicated by zeta-potential measurements. A simple model was proposed to describe surface-assisted nanofiber growth. This study provides insights into the assembly of peptide/protein on a surface, which is essential to understand such physiological protein aggregation systems as amyloid fibrillogenesis. In addition, the potential of this finding to construct biocompatible electrodes for biomolecular sensing is also discussed.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry B, copyright 2007 © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see DOI: 10.1021/jp065851tThe surface tension of polymers in a supercritical fluid is one of the most important physicochemical parameters in many engineering processes, such as microcellular foaming where the surface tension between a polymer melt and a fluid is a principal factor in determining cell nucleation and growth. This paper presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape AnalysisProfile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendant drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and good qualitative agreement with experiment is obtained. The physical mechanisms for three main experimental trends are explained using SCFT, and none of the explanations quantitatively depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the slope of surface tension change with temperature is dependent on pressure.

Most drug delivery systems have been developed for efficient delivery to tumor sites via targeting and on-demand strategies, but the carriers rarely execute synergistic therapeutic actions. In this work, C8, a cationic, pH-triggered anticancer peptide, is developed by incorporating histidine-mediated pH-sensitivity, amphipathic helix, and amino acid pairing self-assembly design. We designed C8 to function as a pH-responsive nanostructure whose cytotoxicity can be switched on and off by its self-assembly: Noncytotoxic β-sheet fibers at high pH with neutral histidines, and positively charged monomers with membrane lytic activity at low pH. The selective activity of C8, tested for three different cancer cell lines and two noncancerous cell lines, is shown. Based on liposome leakage assays and multiscale computer simulations, its physical mechanisms of pore-forming action and selectivity are proposed, which originate from differences in the lipid composition of the cellular membrane and changes in hydrogen bonding. C8 is then investigated for its potential as a drug carrier. C8 forms a nanocomplex with ellipticine, a nonselective model anticancer drug. It selectively targets cancer cells in a pH-responsive manner, demonstrating enhanced efficacy and selectivity. This study provides a novel powerful strategy for the design and development of multifunctional self-assembling peptides for therapeutic and drug delivery applications.

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