*4.1. Nb23 Expression and Labeling*

Nb23 was previously obtained by immunization of a llama with a truncated version of β2-microglobulin, ∆N6β2-m (a β2-m variant devoid of the first six residues), as reported by Domanska et al. [11]. Nb23 was obtained uniformly doubly labeled with <sup>13</sup>C and <sup>15</sup>N by growing the transgenic *E. coli* strain containing the expression vector previously described [11] on <sup>13</sup>C and <sup>15</sup>N enriched medium. Expression and purification were performed by ASLA Biotech AB (Riga, Latvia), that also provided the unlabeled Nb23. Nb23 consists of 136 amino acids, including an initial Met introduced as a start codon for expression in

*E. coli*, and a His6 tag at the C-terminus of the protein for purification purposes, amounting to a molecular weight of 15.1 kDa.
