Hofmeister ion effects on the solvation and thermal stability of model proteins lysozyme and myoglobin

Abstract

Circular dichroism (CD) and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopies were used to study how sucrose and salts, NaCl, NaClO4, Na2SO4, CaCl2, MgCl2, MgSO4, and NH4Cl alter lysozyme and myoglobin solvation and thermal stability. Both proteins follow the anionic Hofmeister series in which chaotropes (Cl−, ClO4−) decrease thermal stability, and kosmotropes (SO42−) increase thermal stability. CD spectra showed no significant secondary structural differences at 25 °C that correlate with changes in thermal stability. However, infrared spectra of lysozyme and myoglobin show a downshift in the Amide I′ (backbone CO) vibration and increased 1H/2H exchange in the presence of weakly hydrated, chaotropic anions (Cl−, ClO4−), both of which suggest increased protein solvation in the presence of chaotropes and correlate with decreased protein thermal stability. The presence of strongly hydrated, kosmotropic anions (SO42−) or sucrose prevent the downshift of the Amide I′ vibration in lysozyme and myoglobin and result in decreased 1H/2H exchange, suggesting decreased protein solvation in the presence of kosmotropes, that correlates with increased protein stability. The data show a strong correlation between the level of deuterium exchange and the relative thermal stability in the presence of kosmotropes or chaotropes that does not correlate with secondary structural changes and suggests that Hofmeister ions alter protein solvation. These results also show that a combination of CD and infrared spectroscopy is useful in studying how Hofmeister ions and co-solvents influence protein solvation and thermal stability.