Crystalline b-sheet Monolayers - A New Class of Ordered Molecular Templates

Hanna Rapaport*, Leslie Leiserowitz and David A. Tirrell*

*Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel

 

Natural proteins and peptides exhibit ubiquitous role in life as tissue building components and as catalytic agents. Protein chains can be represented in a rudiment level by a set of helices, b strands and coiled regions. Understanding the relation between amino acid sequence and the self-assembly topologies of these secondary structural motif will allow a better design of molecular templates composed on these natural polymers.

In this work b-sheet peptides were utilized for the generation of a new class of ordered molecular templates in the form of two-dimensional crystalline monolayers at interfaces. The peptides, seven to seventeen amino acids in length are composed of regularly alternating hydrophilic and hydrophobic amino acids that facilitates the pleated b-strand conformation at hydrophilic-hydrophobic interfaces. The films were characterized by surface pressure-area isotherms and by in situ grazing incidence X-ray diffraction (GIXD) using synchrotron radiation. The diffraction data confirmed the formation of two-dimensional crystalline domains of peptide monolayers. Bragg peaks indicated a lattice spacing that corresponds to the length of the peptide chain axis, signifying the registry between juxtaposed strands and the characteristic 4.7 Å repeat distance of peptide strands interlinked into a hydrogen bonded b-sheet assembly. Molecular modeling and diffraction pattern calculations suggest the existence of various elements of disorder within the crystalline domains, mostly associated with the conformation of the amino acid side chains decorating the peptide strands. On various salt solutions ordered arrays of ions were found to intercalate regularly between the b-sheet ribbon structures generating a composite superlattice.

These two-dimensional crystalline b-sheet architectures open new avenues in the area of reactive films providing a scaffold for regularly spaced side chains of natural or artificial amino acids with sub-nanometer precision.