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The rational design of interfacially confined biomolecules offers a unique opportunity to explore the cooperative relationship among self-assembly, nucleation, and growth processes. This article highlights the role of electrostatics in the self-assembly of β-sheet-forming peptides at the air-water interface. We characterize the phase behavior of a periodically sequenced sheet-forming peptide by using Langmuir techniques, Brewster angle microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and circular dichroism spectroscopy. We find that peptides with an alternating binary sequence transition at high pressures from discrete circular domains to fibrous domains. The qualitative behavior is independent of surface pressure but dependent on molecular areas. In addition, thermodynamic models are employed to specifically quantify differences in electrostatics by obtaining parameters for the critical aggregation area, the limiting molecular area, and the dimensionless ratio of line tension/dipole density. Using these parameters, we are able to relate localized charge distribution to phase transitions, which will allow us to apply these molecules to examine how the dynamics of self-assembly can be directly coupled to the formation of composite nanostructures in biology.

The phenomenon of counterion-mediated DNA-condensation is fundamental to most DNA related activity in the cell, from chromosome packaging to control over translational mechanisms. Developing synthetic systems to manipulate DNA-condensation is essential for the development of biotechnologies for gene encapsulation and DNA-separation. We investigate the dynamics of the DNA-condensation by using our model peptide where the interaction between DNA and peptide is non-specific. We have designed a peptide that shows switchable surface activity, where the folded form of the peptide is amphiphilic and the unfolded form is not amphiphilic. The peptide is α-helical, containing 23 amino acids with variation in the number and distribution of hydrophobic and charged amino acids. The designs incorporate hydrophobic residues on one side (leucines and alanines) and hydrophilic residues on the opposite side so that helix is surface active. The secondary structure has been characterized by using circular dichroism spectropolarimetry, and we show that the peptide has a transient secondary structure as a function of monovalent salt concentration. The behavior of the peptide at air-water interface is characterized by pendant drop/bubble method and modeled accordingly. Our hypothesis is that the unfolded peptide is in equilibrium with the folded peptide in the bulk solution but in presence of DNA, the unfolded peptide folds and then binds to DNA. Critical Aggregate concentration of the peptide for DNA condensation is determined by using multi angle light scattering, which is also used to calculate the radius of gyration and molecular weight of these condensates. We investigate the kinetics of the condensation process by using Circular dichroism in Stop-flow mode and also by isothermal titration calorimetry.