Studies of conformational preferences of derivatives fragments of protein G (1IGD) using temperature dependent potentiometric titration methodology

• β structures of peptides could be induced by the presence of ionizable groups.
• Results showed the influence of temperature on peptide conformations.
• Computed ΔH, ΔS indicate that major transition is associated with turn conformation.

The pKa values of charged amino-acid groups present in the sequences of the peptide excised from the C terminal hairpin of B3 domain of the immunoglobulin binding protein G from Streptococcus (1IGD 51-56): Ac-DDATKT-NH2 (Dag1) and its variants: Ac-DVATKT-NH2 (Dag2) and Ac-OVATKT-NH2 (Dag3), where D, A, T, K, V, and O denote aspartic acid, alanine, threonine, lysine, valine, and ornithine, respectively, were determined by potentiometric titration measurements at three different temperatures. Our previous studies have shown that the presence of charged amino acid residues at opposite ends of a short peptide chain can stabilize the structure. The values of ΔH and ΔS and the folding-unfolding transition temperatures calculated from the CD are in good agreement with those obtained from the DSC measurements, which indicates that the major conformational transition is associated with turn formation. The measured pKa values of Dag1, Dag2 and Dag3, except for pKa1 of Dag2, decrease with increasing temperature as expected, because deprotonation in water of these groups is an endothermic process. For Dag1 and Dag2, the pKa1 values are lower than that of the reference compound (acetic acid), which suggests the formation of a salt bridge in both peptides. The folding-unfolding transition enthalpy of Dag1 is remarkably greater than those of Dag2 and Dag3, which suggests that this compound has the most stable structure in water.