Minimized, hairpins have provided additional data on the geometric preferences of Trp interactions in TW-loop-WT motifs. This motif imparts significant fold stability to peptides as short as 8 residues. High-resolution NMR structures of a 16-(KKWTWNPATGKWTWQE, Delta G(U)(298) >= + 7 kJ/ mol) and 12-residue (KTWNPATGKWTE, Delta G(U)(298) = + 5.05 kJ/mol) hairpin reveal a common turn geometry and edge-to-face (EtF) packing motif and a cation-pi interaction between Lys(1) and the Trp residue nearest the C-terminus. The magnitude of a CD exciton couplet ( due to the two Trp residues) and the chemical shifts of a Trp H is an element of 3 site ( shifted upfield by 2.4 ppm due to the EtF stacking geometry) provided near-identical measures of folding. CD melts of representative peptides with the -TW-loop-WT-motif provided the thermodynamic parameters for folding, which reflect enthalpically driven folding at laboratory temperatures with a small Delta C-p for unfolding (+ 420 J K-1/mol). In the case of Asx-Pro-Xaa-Thr-Gly-Xaa loops, mutations established that the two most important residues in this class of direction-reversing loops are Asx and Gly: mutation to alanine is destabilizing by about 6 and 2 kJ/ mol, respectively. All indicators of structuring are retained in a minimized 8-residue construct (Ac-WNPATGKW-NH2) with the fold stability reduced to Delta G(U)(278) = - 0.7 kJ/ mol. NMR and CD comparisons indicate that -TWXNGKWT-(X = S, I) sequences also form the same hairpin-stabilizing W/W interaction.