Analyzing the dynamic behavior of amphiphilic peptides on the surface of lipid-coated microbubbles

Pierre Desir1,*, Joseph Badami, and Raymond Tu

Department of Chemical Engineering, The City College of New York, New York, NY 10031

Abstract

Microbubbles are gaseous particles usually encapsulated by a lipid monolayer and dispersed in an aqueous solution. These particles have been engineered to implement various types of targeting molecules onto their surface for drug delivery. We investigated how the partitioning of alpha-helical peptides onto the bubble interface influenced the overall structure of the system in terms of stability and size of the microbubbles. It follows that implementing such molecules on the microbubble encapsulation creates an anchor on the lipid monolayer, which can prevent its collapse and the rapid disintegration of the bubble structure. Two fluorescent peptides, Peptide 2.5 and Peptide 6.0, were synthesized to have the same amino acid composition and three positive charge residues with different charge separations. The encapsulation of the microbubbles was made of two lipids, DPPC and DPPA, in the respective mass ratios of 5:1 and 5:2 for the two samples prepared. The relative amount of peptide bound to the bubble interface was determined by spectroscopic measurements of its molecular constituents using a spectrophotometer. For the two compositions of the bubble encapsulation used, the relative amount of Peptide 2.5 partitioned changed from
16.42% to 16.24% and that of Peptide 6.0 from 22.27% to 28.89% as the concentration of DPPA increased. Moreover, Circular Dichroism was used to analyze the secondary structure of the peptides once integrated on the liposome surface. Thereon, the secondary structure of Peptide 2.5 changed from an alpha-helical to a beta sheet configuration while Peptide 6.0 conserved its alpha- helical structure after binding. It followed that the charge separation on the peptides influenced their relative partitioning to the bubble interface and their configuration once partitioned. Furthermore, the influence of peptide partitioning on microbubble size was assessed by measuring the mean diameter of these structures under laser scattering for a period of 20 minutes. The control bubbles had an average size of 579 nm. Peptide 2.5 partitioned onto microbubbles with an average size of 492 nm while Peptide 6.0 produced microbubbles of 545 nm in size. Overall, the microbubbles displayed a relatively constant size with the peptides partitioned onto the interface, which denoted that the peptides stabilized these structures.