The Influence of Proximal Charged Groups on the Hydrophobic Interactions of a Nanoscopic Domain

C. Derek Ma, Claribel Acevedo-Vélez, Samuel H. Gellman, and Nicholas L. Abbott

Water-mediated interactions involving hydrophobic domains are known to play a central role in a broad range of colloidal and interfacial phenomena, including molecular self-assembly and macromolecular (e.g., protein) folding processes. In the majority of these systems, although polar and charged groups are present in close proximity to the hydrophobic domains, how these proximal groups modulate hydrophobic interactions is not understood. Theories and simulations have predicted these proximity effects to be significant, but the lack of suitable experimental systems has prevented such ideas from being tested. Thrust 3 has recently discovered that the placement of cationic functional groups (e.g., ammonium and guanidinium ions) adjacent to nanoscopic domains of comprised of non-polar amino acids can have a dramatic influence on hydrophobic interactions mediated by the non-polar domains.

Specifically, whereas ammonium strengthens the hydrophobic interaction, Thrust 2 researchers have measured guanidinium ions to diminish the strength of the hydrophobic interaction. These results, obtained by performing single molecules force measurements using organic nanorods synthesized from b-amino acids (see figure), suggest new nano-scale design rules for directed assembly processes.