Aqueous Self-Assembly within the Homologous Peptide Series A(n)K
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We compare the aqueous self-assembly behavior within the homologous peptide series A(n)K, where A is alanine, K is lysine, and n = 4, 6, 8, and 10. The aqueous peptide solubility, phi(s) (volume fraction), depends strongly on the number of hydrophobic alanine residues and decreases approximately as phi(s) approximate to 10(-n). Also the self-assembly structure depends on n. A(4)K is highly water-soluble and shows no relevant self-assembly. A(6)K, which has been extensively studied previously, forms hollow nanotubes in water. A(8)K and A(10)K self-assembly is characterized here using a combination of small- and wide-angle X-ray scattering, static and dynamic light scattering, cryo transmission electron microscopy, and circular dichroism spectroscopy. They both form similar thin rodlike aggregates with lengths on the order of 100 nm and a biaxial cross-section with dimensions of 4 nrn x 8 nm. We show that different sample preparation protocols result in different lengths of the A(10)K rodlike aggregates. On the basis of these findings, the question of thermodynamic equilibrium of peptide self-assembly is discussed.