Cations bind only weakly to amides in aqueous solutions.

We investigated salt interactions with butyramide as a simple mimic of cation interactions with protein backbones. The experiments were performed in aqueous metal chloride solutions using two spectroscopic techniques. In the first, which provided information about contact pair formation, the response of the amide I band to the nature and concentration of salt was monitored in bulk aqueous solutions via attenuated total reflection Fourier transform infrared spectroscopy. It was found that molar concentrations of well-hydrated metal cations (Ca(2+), Mg(2+), Li(+)) led to the rise of a peak assigned to metal cation-bound amides (1645 cm(-1)) and a decrease in the peak associated with purely water-bound amides (1620 cm(-1)). In a complementary set of experiments, the effect of cation identity and concentration was investigated at the air/butyramide/water interface via vibrational sum frequency spectroscopy. In these studies, metal ion-amide binding led to the ordering of the adjacent water layer. Such experiments were sensitive to the interfacial partitioning of cations in either a contact pair with the amide or as a solvent separated pair. In both experiments, the ordering of the interactions of the cations was: Ca(2+) > Mg(2+) > Li(+) > Na(+) ≈ K(+). This is a direct cationic Hofmeister series. Even for Ca(2+), however, the apparent equilibrium dissociation constant of the cation with the amide carbonyl oxygen was no tighter than ∼8.5 M. For Na(+) and K(+), no evidence was found for any binding. As such, the interactions of metal cations with amides are far weaker than the analogous binding of weakly hydrated anions.

Specific anion effects on water structure adjacent to protein monolayers.

Vibrational sum frequency spectroscopy (VSFS) was used to explore specific ion effects on interfacial water structure adjacent to a bovine serum albumin (BSA) monolayer adsorbed at the air/water interface. The subphase conditions were varied by the use of six different sodium salts and four different pH values. At pH 2 and 3, the protein layer was positively charged and it was found that the most chaotropic anions caused the greatest attenuation of water structure. The order of the salts followed an inverse Hofmeister series. On the other hand, when the protein layer was near its isoelectric point (pH 5), the most chaotropic anions caused the greatest increase in water structure, although the effect was weak. In this case, a direct Hofmeister series was obeyed. Finally, virtually no effect was observed when the protein layer was negatively charged with a subphase pH of 9. For comparison, similar experiments were run with positively charged, negatively charged, and zwitterionic surfactant monolayers. These experiments gave rise to nearly the identical results as the protein monolayer which suggested that specific anion effects are dominated by the charge state of the interfacial layer rather than its detailed chemical structure. In a final set of experiments, salt effects were examined with a monolayer made from an elastin-like polypeptide (ELP). The peptide consisted of 120 pentameric repeats of the sequence Val-Pro-Gly-Val-Gly. Data from this net neutral biopolymer followed a very weak, but direct Hofmeister series. This suggested that direct anion binding to the amide groups in the backbone of a polypeptide is quite weak in agreement with the BSA data. The results from the variously charged protein, surfactant, and polymer monolayers were compared with a modified Gouy-Chapman-Stern model. The agreement with this simple model was quite good.

Hydrogen bonding of beta-turn structure is stabilized in D(2)O.

The lower critical solution temperature (LCST) of elastin-like polypeptides (ELPs) was investigated as a function of ELP chain length and guest residue chemistry. These measurements were made in both D(2)O and H(2)O. Differences in the LCST values with heavy and light water were correlated with secondary structure formation of the polypeptide chains. Such structural information was obtained by circular dichroism and infrared measurements. Additional thermodynamic data were obtained by differential scanning calorimetry. It was found that there is a greater change in the LCST value between H(2)O and D(2)O for those polypeptides which form the greatest amount of beta-turn/beta-aggregate structure. Moreover, these same molecules were the least hydrophobic ELPs. Therefore, hydrogen bonding rather than hydrophobicity was the key factor in the stabilization of the collapsed state of ELPs in D(2)O compared with H(2)O.

Role of carboxylate side chains in the cation Hofmeister series.

Thermodynamic and surface-specific spectroscopic investigations were carried with an elastin-like polypeptide (ELP) containing 16 aspartic acid residues. The goal was to explore the role of the carboxylate moieties in hydrophobic collapse and related Hofmeister effects. Experiments were conducted with a series of monovalent and divalent metal chloride salts. Both phase transition temperature and spectroscopic data demonstrated that the divalent cations showed relatively strong association to the carboxylate sites on the biopolymer with K(d) values in the range of 1 to 10 mM. The ordering of the divalent series was: Zn(2+) > Ca(2+) > Ba(2+) > Sr(2+) > Mg(2+). Monovalent cations displayed weaker binding which ranged from 78 mM for NH(4)(+) to 345 mM for Cs(+). The order for this series was: NH(4)(+) > Li(+) > Na(+) > NMe(4)(+) > K(+) > Rb(+) ≥ Cs(+). These results are in general agreement with the notion that strongly hydrated cations bind more tightly to carboxylate groups than do weakly hydrated cations. Moreover, the data for the monovalent series was partially consistent with the law of matching water affinity, although Li(+) and NH(4)(+) did not follow the model. The series for the divalent cations did not appear to obey the law of matching water affinity at all.