Impact of Strand Number on Parallel β-Sheet Stability.

We have examined whether parallel β-sheet secondary structure becomes more stable as the number of β-strands increases, via comparisons among peptides designed to adopt two- or three-stranded parallel β-sheet conformations in aqueous solution. Our three-strand design is the first experimental model of a triple-stranded parallel β-sheet. Analysis of the designed peptides by nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy supports the hypothesis that increasing the number of β-strands, from two to three, increases the stability of the parallel β-sheet. We present the first experimental evidence for cooperativity in the folding of a triple-stranded parallel β-sheet, and we show how minimal model systems may enable experimental documentation of characteristic properties, such as CD spectra, of parallel β-sheets.