Thermodynamical Studies of an Example Peptide Containing Metaaminobenzoic Acid (MABA) that Promotes Bends in Proteins

The pK a values of the peptide derived from the immunoglobulin binding protein G, Ac-Asp-Asp-Ala-Thr-Lys-Thr-NH2 (Dag1), and its derivative Ac-Asp-Asp-(meta-aminobenzoic acid)-Lys-Thr-NH2 (Dag1_M), were measured using potentiometric titrations in the temperature range from 25 to 50 °C. The heat capacity profiles for both systems were determined by differential scanning calorimetry in the temperature range 10–90 °C and CD measurements were made at nine different temperatures.To decrease the mobility of the peptide under study (Dag1), and to ensure a better stability of its structure (better stiffness), the amino acids from the turn region of Dag1 (Ala3 and Thr4) in the original sequences were replaced by the meta-aminobenzoic acid (MABA) residue, which acts as a β-mimetic. The results obtained in this work suggest that the charged residues present in both sequences (Asp1, Asp2 and Lys5) are in similar environments in both peptides. Therefore, the shapes of the dominant conformations of Dag1 and Dag1_M should be similar to each other; specifically, both peptides probably have a bent structure. Negative bands in the CD spectra are present at 195 and 215 nm for Dag1 and Dag1_M, respectively, which are characteristic of a statistical coil conformation. The pK a values for charged amino acids (two Asp and one Lys) at t = 25 °C in both sequences are very similar, which suggests that the environments for those ionizable groups are similar.