Breakthrough in the precision engineering of four-stranded beta-sheets

In addition to the natural sequence of amino acids that makes up a protein, their three-dimensional arrangement in space is also critical to their function. For example, β-sheets, which are sheet-like structures formed via hydrogen bonds between adjacent peptide strands, play a crucial role in protein stability and folding. They are also implicated in various neurodegenerative diseases, including Alzheimer's disease. On the flip side, the engineering of β-sheets has potential applications in biotechnology, medicine, and nanomaterials science.

Unfortunately, producing β-sheet assemblies with a precisely controlled number of strands is quite challenging for two reasons. First, multi-stranded β-sheets tend to clump up into aggregates called fibrils, which can easily become insoluble and alter or negate their biological function. Second, when peptide strands are combined during β-sheet synthesis, many structural isomers are possible. This means that the resulting assemblies often have unpredictable strand orientations, alignments, or numbers, making it difficult to produce only a specific target compound. Because of these reasons, a new method for creating custom β-sheets is needed.

In a recent study published in Angewandte Chemie International Edition on22 October 2024, a team of researchers led by Associate Professor Tomohisa Sawada from Institute of Science Tokyo (Science Tokyo), Japan, set out to find a solution to these issues. As reported in their study, they developed a promising approach to produce four-stranded β-sheets using silver atoms as metal-peptide coordination centers.

The researchers engineered a pentapeptide, referred to simply as '1,' in which the second and fourth residues were 3-pyridyl-substituted alanine residues. The pyridyl groups, which were introduced on opposite sides of the main chain, served as metal complexation sites for the silver atoms. Upon addition of silver (Ag), two '1' molecules would combine to form an Ag₂(1)₂ ring as a hypothetical intermediate. Interestingly, because of the reversibility of metal coordination during the reaction, pairs of Ag₂(1)₂ rings end up in an interlocked state, with hydrogen bonds between adjacent pentapeptides holding the overall β-sheet structure together.

The team confirmed the successful synthesis of these interlocked structures, [Ag₂(1)₂]₂, via nuclear magnetic resonance and X-ray crystallographic measurements. Most remarkably, the four-stranded β-sheets produced converged to a single type of isomeric structure without aggregation. Simply put, all β-strands were made of two interlocked rings, with the first and third strands pointing in one direction and the second and fourth strands pointing in the other. The relative positions of the interlocking metal-complexation sites were also equal across all produced β-strands. "Our results demonstrated that the combination of β-sheet amide hydrogen bond formation and metal cross-linking of side chains limits the number of possible isomers. In other words, we showed that non-covalent side chain cross-linking can induce highly selective single structure β-sheets in a discrete form," remarks Sawada.

The findings presented in this study could make the study of β-sheets easier, thereby unlocking their potential in next-generation biotechnology and nanotechnology. "To the best of our knowledge, this is the first example of the precise construction of a four-stranded β-sheet assembled from non-covalent interactions only. We believe that our efforts pave the way for the rational construction of β-sheet structures and functions in the future," concludes Sawada, excited about the possibilities.