Charge density-dependent modifications of hydration shell waters by Hofmeister ions

Abstract

Gadolinium (Gd(3+)) vibronic sideband luminescence spectroscopy (GVSBLS) is used to probe, as a function of added Hofmeister series salts, changes in the OH stretching frequency derived from first-shell waters of aqueous Gd(3+) and of Gd(3+) coordinated to three different types of molecules: (i) a chelate (EDTA), (ii) structured peptides (mSE3/SE2) of the lanthanide-binding tags (LBTs) family with a single high-affinity binding site, and (iii) a calcium-binding protein (calmodulin) with four binding sites. The vibronic sideband (VSB) corresponding to the OH stretching mode of waters coordinated to Gd(3+), whose frequency is inversely correlated with the strength of the hydrogen bonding to neighboring waters, exhibits an increase in frequency when Gd(3+) becomes coordinated to either EDTA, calmodulin, or mSE3 peptide. In all of these cases, the addition of cation chloride or acetate salts to the solution increases the frequency of the vibronic band originating from the OH stretching mode of the coordinated waters in a cation- and concentration-dependent fashion. The cation dependence of the frequency increase scales with charge density of the cations, giving rise to an ordering consistent with the Hofmeister ordering. On the other hand, water Raman spectroscopy shows no significant change upon addition of these salts. Additionally, it is shown that the cation effect is modulated by the specific anion used. The results indicate a mechanism of action for Hofmeister series ions in which hydrogen bonding among hydration shell waters is modulated by several factors. High charge density cations sequester waters in a configuration that precludes strong hydrogen bonding to neighboring waters. Under such conditions, anion effects emerge as anions compete for hydrogen-bonding sites with the remaining free waters on the surface of the hydration shell. The magnitude of the anion effect is both cation and Gd(3+)-binding site specific.

Figure 1

a) Luminescence spectrum of 1 M aqueous gadolinium solution, showing the vibronic side bands (VSBs) originating from the water bending and OH stretching modes of waters in the first hydration shell of the Gd³⁺. The excitation wavelength was 273 nm. X-axis is calibrated to represent the energy difference from the pure electronic transition centered at 311nm (3,2154 cm⁻¹). b) Diagram showing the first and second hydration shell waters surrounding Gd³⁺. The OH stretch frequency is inversely correlated with the strength of the hydrogen bond to the second layer water. Therefore the OH stretching frequency monitored in VSB spectrum is a specific probe of the local hydrogen bonding in the hydration shell of Gd³⁺. c) Simplified energy level diagram of hydrated Gd³⁺.

Figure 2

Plot of the frequency shift of –OH stretching mode vibration derived from: i) GVSBLS of 0.5 M Gd³⁺ in aqueous solution with: MgCl₂ () and NaCl (); and ii) Raman from a solution of 0.5 M Gd³⁺ in aqueous solution with: MgCl₂ () and NaCl ().

Figure 3

VSBs for samples of 100 mM EDTA and 80 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0 in the absence of MgCl₂ () and presence of 4.0 M MgCl₂ (). Inset: normalized –OH stretching mode VSB

Figure 4

Bar graph of OH stretch vibration frequency shift of first hydration shell waters of: 0.5 M GdCl₃ with chloride salts; 0.5 M GdCl₃ with acetate salts and 80mM EDTA-Gd³⁺ with chloride salts.

Figure 5

a) Bar graph of–OH stretching mode vibration frequencies derived from GVSBLS as function of added MgCl₂ for the following samples: 0.5 M Gd³⁺ ; 100 mM EDTA with 80 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0; 1 mM Calmodulin and 1 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0 and 1 mM mSE3 and 1 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0. Bar graph of –OH stretching mode vibration frequency derived from GVSBLS of hydrated GdCl₃ powder heated for 5 hours. b). Bar graph of –OH stretching mode vibration frequency derived from GVSBLS as a function of added NaCl for the following samples: 0.5 M Gd³⁺; 100 mM EDTA with 80 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0; 1 mM Calmodulin and 1 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0 and 1 mM mSE3 and 1 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0 with varied concentration of NaCl.

Figure 6

Bar graph of –OH stretching mode vibration frequency derived from GVSBLS of 0.5 M Gd³⁺ (empty bar) ; and 100 mM EDTA with 80 mM Gd³⁺ in 10 mM HEPES buffer at pH 7.0 (bar with lines) in the a) absence of salts and presence of Saturated MgF₂, 1 M MgCl₂ and 1 M MgBr₂, b) absence of salts and presence of 1 M KF, 1 M KCl, 1 M KBr and 1 M KI amd c) absence of salts and presence of 1 M NaF, 1 M NaCl, 1 M NaBr and 1 M NaI. d) Plot of the frequency shift of –OH stretching mode vibration derived from GVSBLS of 0.25 M GdCl₃ in trehalose glassy matrices with absence and presence of MgCl₂, NaCl, NaF. (The original concentration of salts is 0.5 M.)

Figure 7

Schematic showing the interactions that affect H: the hydrogen bonding between the first and second hydration shell waters around Gd³⁺. Cation R₊ can exert its effect indirectly either by disrupting nearby water clusters (influence 3) which in turn results in a weakening of the hydrogen bonding between the free waters and the hydration waters of the Gd₃₊ or by sequestering free waters thus enhancing anion X⁻ effects (influence 2). See details in the text.