Residue Asn47 at position L1 of a type II′ β-turn of
the α-spectrin SH3 domain is located in a disallowed region
of the Ramachandran plot (φ = 56 ± 12, ψ = −118
± 17). Therefore, it is expected that replacement of Asn47 by
Gly should result in a considerable stabilization of the protein.
Thermodynamic analysis of the N47G and N47A mutants shows that the
change in free energy is small (∼0.7 kcal/mol; ∼3 kJ/mol) and
comparable to that found when mutating a Gly to Ala in a α-helix or
β-sheet. X-ray structural analysis of these mutants shows that the
conformation of the β-turn does not change upon mutation and,
therefore, that there is no relaxation of the structure, nor is there
any gain or loss of interactions that could explain the small energy
change. Our results indicate that the energetic definition of II′
region of the Ramachandran plot (φ = 60 ± 30, ψ =
−115 ± 15) should be revised for at least Ala and Asn in
structure validation and protein design.