Stable spherical hollow particles composed of bola-form amidesvia non-covalent interactions

Abstract

Dipeptide-based bola-form amides, which self-assemble into fibrous structures under usual conditions, formed stable micrometer-sized hollow spheres directed by hydrophilic interfaces in aqueous solution. The chemical structure of the bola-form amide and the surface properties of the substrate proved to significantly affect the self-assembly process. Bis(N-α-amide-L-valyl-L-valine)1,n-alkane dicarboxylate (n = 10: (Val)₂C₁₀, 12: (Val)₂C₁₂) and bis(N-α-amide-L-isoleucyl-L-isoleucine)1,n-alkane dicarboxylate (n = 10: (i-Leu)₂C₁₀) produced hollow spheres, whereas (Val)₂C_n (n = 7–9, 11) and bis(N-α-amide-L-valyl-L-methionine)C₁₀ ((Val/Met)₂C₁₀) formed no spheres. Static light scattering measurements revealed that the rod-like micelles of (Val)₂C₁₀, (Val)₂C₁₂ and (i-Leu)₂C₁₀ were converted to the hollow spheres via vesicle-like intermediates. The vesicle-like intermediates gathered together to form the spherical hollow particles with the aid of the surface of hydrophilic glass substrates. On the other hand, (Val)₂C_n (n = 7–9, 11) and (Val/Met)₂C₁₀ directly self-assembled into fibrous structures from rod-like micelles without passing through the vesicle-like intermediates. The carbon number of the spacer, bulkiness of the head groups and surface properties of the substrate played critical roles in determining the self-assembly. FT-IR, XRD and DSC measurements revealed that the packing of the bola-form amides in the hollow spheres differed from that in the fibrous assembly. Molecules in the spheres were more tightly packed, as in the crystalline state, than those in fibrous structures.