STRUCTURE DEPENDENT PKA SHIFTS

Cornell University Ithaca, Ithaca NY

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Abstract

Recently Urry, et.al., reported large pKa shifts of charged glutamic acid groups associated with changes of composition in polypentapeptides (PPP) of the family poly[fv(IPGVG), fe(IPGEG)] (where the mole fractions fv and fe satisfied the relation fv + fe = 1). The experimentally determined titration curve clearly shows two regimes with respect to fe : fe > 0.65 and fE< 0.65. The first regime is referred to by the authors as the electrostatic regime and the second one as the hydrophobic regime. In particular, they found that the electrostatic-induced pKa shift is not as large as the hydrophobic-induced pKa in both pure water and aqueous 0.15N NaCl. This familyof PPPs has been proposed to adopt a folded beta-spiral conformation with approximately three pentamers per turn, wi th beta-turns acting as spacers between turns of the spiral. According to the authors, this folded structure leads to an arrangement of hydrophobic residues in close proximity to the side chains of Glu residues at position 4th in the pentapeptides. These structure-dependent intramolecular interactions were assumed by Urry, et. al., to be mainly responsible for the experimentally observed pKa shift of Glu in PPP and related peptides. In order to test their assumption, we carried out a series of calculations in which the PPP was modeled as a collection of 9 pentamers with fe = 0.11; i.e., with the sequence (IPGVG)4 (IPGEG) (IPGVG)4. Our theoretical result for the pKa shift is in good agreement with the experimental value and provides support for the proposed molecular basis for the observed pKa shift in this type of polypeptide.