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The S protein–S peptide interaction is a model system to study binding thermodynamics in proteins. We substituted alanine at position 4 in S peptide by α-aminoisobutyric acid (Aib) to investigate the effect of this substitution on the conformation of free S peptide and on its binding to S protein. The thermodynamic consequences of this replacement were studied using isothermal titration calorimetry. The structures of the free and complexed peptides were studied using circular dichroic spectroscopy and X-ray crystallography, respectively. The alanine4Aib replacement stabilizes the free S peptide helix and does not perturb the tertiary structure of RNase S. Surprisingly, and in contrast to the wild-type S peptide, the Δ G ° of binding of peptide to S pro, over the temperature range 5–30°C, is virtually independent of temperature. At 25°C, the ΔΔ G °, ΔΔ H °, ΔΔ S  and ΔΔ C  p  of binding are 0.7 kcal/mol, 2.8 kcal/mol, 6 kcal/mol.K and –60 kcal/mol.K, respectively. The positive value of ΔΔ S  is probably due to a decrease in the entropy of uncomplexed alanine4Aib relative to the wild-type peptide. The positive value of ΔΔ H ° is unexpected and is probably due to favorable interactions formed in uncomplexed alanine4Aib. This study addresses the thermodynamic and structural consequences of a replacement of alanine by Aib both in the unfolded and complexed states in proteins.

Structural and thermodynamic consequences of introducing α-aminoisobutyric acid in the S peptide of ribonuclease S