Partially folded conformational ensembles of bovine pancreatic trypsin inhibitor (BPTI) are accessed by replacing Cys 5, 30, 51, and 55 by α-amino-n-butyric acid (Abu) while retaining the disulfide between Cys 14 and 38; the resultant variant is termed [14-38]Abu. Two new analogues with modifications in the β-turn, P26D27[14-38]Abu and N26G27K28[14-38]Abu, are compared to partially folded [14-38]Abu, as well as to [R]Abu, the unfolded protein with all six Cys residues replaced by Abu. Structural features of the new analogues of [14-38]Abu have been determined by circular dichroism (CD), one-dimensional 1H NMR, and 8-anilino-1-naphthalenesulfonic acid (ANS) fluorescence experiments. Both analogues are more disordered than the parent [14-38]Abu, but while P26D27[14-38]Abu has a small population of native-like conformations observed by NMR, no ordered structure is detected for N26G27K28[14-38]Abu. Trypsin inhibition assays were carried out using a modified rat trypsin, C191A/C220A, that minimizes cleavage of unfolded peptides. Both [14-38]Abu and P26D27[14-38]Abu significantly inhibit modified trypsin. N26G27K28[14-38]Abu has low but measurable inhibitor activity, while [R]Abu has no activity even when in very high molar excess relative to trypsin. ANS fluorescence is enhanced by [14-38]Abu and by both variants but not by [R]Abu. We conclude that partially folded ensembles of BPTI, even those with little or no CD- or NMR-detectable structure, contain minor populations of native-like conformations. Partially folded [14-38]Abu and both variants, as well as [R]Abu, have enhanced negative ellipticity in CD spectra acquired in the presence of the osmolyte trimethylamine N-oxide (TMAO). TMAO-induced structure is formed cooperatively, as indicated by thermal unfolding curves. Inhibitor activity as a function of TMAO concentration implies that the osmolyte-induced structure is native-like for [14-38]Abu and P26D27[14-38]Abu and is probably native-like for N26G27K28[14-38]Abu. [R]Abu also shows increased CD-detected structure in the presence of TMAO, but such structure is likely to be collapsed and non-native.