Thermodynamic and structural characterization of Asn and Ala residues in the disallowed II' region of the Ramachandran plot.

Residue Asn47 at position L1 of a type II' beta-turn of the alpha-spectrin SH3 domain is located in a disallowed region of the Ramachandran plot (phi = 56 +/- 12, psi = -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 (approximately 0.7 kcal/mol; approximately 3 kJ/mol) and comparable to that found when mutating a Gly to Ala in a alpha-helix or beta-sheet. X-ray structural analysis of these mutants shows that the conformation of the beta-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 (phi = 60 +/- 30, psi = -115 +/- 15) should be revised for at least Ala and Asn in structure validation and protein design.