Specific ion effects on water dynamics and local solvation structure around a peptide are important in understanding the Hofmeister series of ions and their effects on protein stability in aqueous solution. Water dynamics is essentially governed by local hydrogen-bonding interactions with surrounding water molecules producing hydration electric field on each water molecule. Here, we show that the hydration electric field on the OD bond of HOD molecule in water can be directly estimated by measuring its OD stretch infrared (IR) radiation frequency shift upon increasing ion concentration. For a variety of electrolyte solutions containing Hofmeister anions, we measured the OD stretch IR bands and estimated the hydration electric field on the OD bond to be about a hundred MV∕cm with standard deviation of tens of MV∕cm. As anion concentration increases from 1 to 6 M, the hydration electric field on the OD bond decreases by about 10%, indicating that the local H-bond network is partially broken by dissolved ions. However, the measured hydration electric fields on the OD bond and its fluctuation amplitudes for varying anions are rather independent on whether the anion is a kosmotrope or a chaotrope. To further examine the Hofmeister effects on H-bond solvation structure around a peptide bond, we examined the amide I' and II' mode frequencies of N-methylacetamide in various electrolyte D(2)O solutions. It is found that the two amide vibrational frequencies are not affected by ions, indicating that the H-bond solvation structure in the vicinity of a peptide remains the same irrespective of the concentration and character of ions. The present experimental results suggest that the Hofmeister anionic effects are not caused by direct electrostatic interactions of ions with peptide bond or water molecules in its first solvation shell. Furthermore, even though the H-bond network of water is affected by ions, thus induced change of local hydration electric field on the OD bond of HOD is not in good correlation with the well-known Hofmeister series. We anticipate that the present experimental results provide an important clue about the Hofmeister effect on protein structure and present a discussion on possible alternative mechanisms.
IR probes have been extensively used to monitor local electrostatic and solvation dynamics. Particularly, their vibrational frequencies are highly sensitive to local solvent electric field around an IR probe. Here, we show that the experimentally measured vibrational frequency shifts can be inversely used to determine local electric potential distribution and solute-solvent electrostatic interaction energy. In addition, the upper limits of their fluctuation amplitudes are estimated by using the vibrational bandwidths. Applying this method to fully deuterated N-methylacetamide (NMA) in D(2)O and examining the solvatochromic effects on the amide I' and II' mode frequencies, we found that the solvent electric potential difference between O(═C) and D(-N) atoms of the peptide bond is about 5.4 V, and thus, the approximate solvent electric field produced by surrounding water molecules on the NMA is 172 MV/cm on average if the molecular geometry is taken into account. The solute-solvent electrostatic interaction energy is estimated to be -137 kJ/mol, by considering electric dipole-electric field interaction. Furthermore, their root-mean-square fluctuation amplitudes are as large as 1.6 V, 52 MV/cm, and 41 kJ/mol, respectively. We found that the water electric potential on a peptide bond is spatially nonhomogeneous and that the fluctuation in the electrostatic peptide-water interaction energy is about 10 times larger than the thermal energy at room temperature. This indicates that the peptide-solvent interactions are indeed important for the activation of chemical reactions in aqueous solution.
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