(A) The β-sheet surface of GB1 showing possible orientations for side chains Arg6, Glu53, and Arg44. In the positions shown, nitrogen–oxygen distances are 2.92 Å and 2.85 Å for residue pairs 6–53 and 44–53, respectively. Side chains were positioned with a dead-end elimination algorithm (Voigt et al. 2000) and the figure was created with MOLSCRIPT (Kraulis 1991). (B) Thermal denaturation curves for GB1 variants. From left to right (at 50% unfolded): R6A53R44, R6A53A44,I6A53R44, I6A53A44,R6E53A44, I6E53A44,R6E53R44, and I6E53R44.