Acn d3

All NMR experiments were conducted at 298 K and pH 4.0 unless otherwise stated. HSTX-I (2 mg/mL) was dissolved in 33% deuterated ACN (ACN-d₃; Cambridge Isotope Laboratories, Cambridge, UK)/67% H₂O (v/v). One-dimensional (1D) ¹H spectra and two-dimensional (2D) total correlated spectroscopy (TOCSY) (80 ms mixing time) and nuclear Overhauser effect spectroscopy (NOESY) (200 ms mixing time) experiments were run and data collected on a Bruker Avance 600 MHz NMR spectrometer equipped with a cryoprobe (Bruker, Sydney, NSW, Australia). Additional NMR data collected for solution structure calculations included ¹H–¹⁵N HSQC in 33% ACN-d₃/67% H₂O (v/v) as well as 2D TOCSY at various temperatures (283–303 K) and ¹H–¹³C HSQC in 33% ACN-d₃/67% D₂O (v/v). Slow exchanging amide protons were identified from a series of 1D and TOCSY spectra collected following the dissolution in D₂O. Although D₂O exchange was monitored over 24 h, slow exchangers were identified as amide protons remaining beyond 4 h.