Hofmeister anionic effects on hydration electric fields around water and peptide

Abstract

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.

Hydrogen Bonding and Solvation of a Proline-Based Peptide Model in Salt Solutions

“…It is noteworthy that the more pronounced upshift of the proline amide band was induced by the addition of KCl and KF salts to a water solution of GPG-NH 2 . This finding seems consistent with the effect of the halide anions on the peptides’ structure, as already reported in the literature [ 43 , 44 ].…”

Ion aggregation in high salt solutions. IV. Graph-theoretical analyses of ion aggregate structure and water hydrogen bonding network

“…, in the Hofmeister series can stabilize water H-bonding network. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] However, there also exist evidences showing that the water structure is little influenced by dissolved ions except for those water molecules in the immediate vicinity of ions. 27,40 Thus, the specific ion effects on water structure still remain to be a hotly debated issue.…”

Hofmeister‐Effekte unter der Lupe: Die direkte Anion‐Amid‐Bindung ist schwächer als die Kation‐Amid‐Bindung

“…B. das Amidgrundgerüst, die Aminosäure-Seitenketten oder die Te rmini eines Proteins) aufweisen. [9,[11][12][13][14] Direkte Wechselwirkungen werden manchmal infrage gestellt, [15] allerdings zeigen NMR-Untersuchungen, dass die chemische Verschiebung der Protonen der Amidgruppe durch Anionen beeinflusst wird [9,11,12] und dass der Abstand von Anionen zur Amidgruppe der Hofmeister-Reihe folgt. [3] Direkte Wechselwirkungen zwischen Ionen und dem Amidgrundgerüst, dem grundlegenden Strukturmerkmal aller Proteine,w urden mit verschiedensten spektroskopi-schen Methoden untersucht.…”

“…[3] Direkte Wechselwirkungen zwischen Ionen und dem Amidgrundgerüst, dem grundlegenden Strukturmerkmal aller Proteine,w urden mit verschiedensten spektroskopi-schen Methoden untersucht. [9,[11][12][13][14] Direkte Wechselwirkungen werden manchmal infrage gestellt, [15] allerdings zeigen NMR-Untersuchungen, dass die chemische Verschiebung der Protonen der Amidgruppe durch Anionen beeinflusst wird [9,11,12] und dass der Abstand von Anionen zur Amidgruppe der Hofmeister-Reihe folgt. [16] Gleichermaßen werden die C = O-Schwingung der Amide [14] und die chemische Verschiebung des Kohlenstoffatoms der C = O-Gruppe durch Kationen verändert, [17] was mit direkten Kation-Amid-Kontakten vereinbar ist.…”