Length-dependent stability and strand length limits in antiparallel beta-sheet secondary structure

Designed peptides that fold autonomously to specific conformations in aqueo us solution are useful for elucidating protein secondary structural prefere nces. For example, autonomously folding model systems have been essential f or establishing the relationship between alpha -helix length and alpha -hel ix stability, which would be impossible to probe with alpha -helices embedd ed in folded proteins. Here, we use designed peptides to examine the effect of strand length on antiparallel beta -sheet stability. alpha -Helices bec ome more stable as they grow longer. Our data show that a two-stranded beta -sheet ("beta -hairpin"), becomes more stable when the strands are lengthe ned from five to seven residues, but that further strand lengthening to nin e residues does not lead to further beta -hairpin stabilization for several extension sequences examined. (In one case, all-threonine extension, there may be an additional stabilization on strand lengthening from seven to nin e residues.) These results suggest that there may be an intrinsic limit to strand length for most sequences in antiparallel beta -sheet secondary stru cture.