Optimization of a β-sheet-cap for long loop closure.

Protein loops make up a large portion of the secondary structure in nature. But very little is known concerning loop closure dynamics and the effects of loop composition on fold stability. We have designed a small system with stable β-sheet structures, including features that allow us to probe these questions. Using paired Trp residues that form aromatic clusters on folding, we are able to stabilize two β-strands connected by varying loop lengths and composition (an example sequence: RWITVTI - loop - KKIRVWE). Using NMR and CD, both fold stability and folding dynamics can be investigated for these systems. With the 16 residue loop peptide (sequence: RWITVTI-(GGGGKK)2 GGGG-KKIRVWE) remaining folded (ΔGU  = 1.6 kJ/mol at 295K). To increase stability and extend the series to longer loops, we added an additional Trp/Trp pair in the loop flanking position. With this addition to the strands, the 16 residue loop (sequence: RWITVRIW-(GGGGKK)2 GGGG-WKTIRVWE) supports a remarkably stable β-sheet (ΔGU  = 6.3 kJ/mol at 295 K, Tm  = ∼55°C). Given the abundance of loops in binding motifs and between secondary structures, these constructs can be powerful tools for peptide chemists to study loop effects; with the Trp/Trp pair providing spectroscopic probes for assessing both stability and dynamics by NMR.