pH Dependence of Interfacial Water in the Presence of Amino Acid Side Chains Revealed by Heterodyne-Detected Sum-Frequency Generation Spectroscopy

Abstract

Proteins are built of amino acid residues that differ from each other only by their side chain. The pH-dependent response of these side chains on protein surface is vital for various biological processes. Here we have investigated the aqueous interface in the presence of different amino acid side chains at varying pH (bulk) using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. It is observed that amine/imidazolic (e.g., lysine/histidine) and alcoholic (e.g., serine and threonine) side chains preferentially orient the interfacial water as "H-down" (i.e., the water hydrogens are pointed toward the aqueous bulk) in acidic solution (pH ∼2). At physiological pH (7.4), the interfacial water takes "H-up" orientation (i.e., the water hydrogens are pointed away from the aqueous bulk) for the alcoholic/imidazolic side chains but remains "H-down" for the amine containing side chain. On further increasing the pH (up to 12.0), the interfacial water becomes increasingly H-up oriented, revealing the charging of the interface for all side chains investigated. Because the side chains are uncharged at high pH (12.0), the charging of the aqueous interface implies the adsorption of OH − anion at the interface. Moreover, in the case of imidazolic side chain (i.e., histidine), the flip-flop of interfacial water within a narrow range of pH 6.0−7.5 shows the reversal of interfacial electric field within a little variation of [H + ] (or [OH − ]) near the physiological pH. This observation provides direct experimental evidence in support of the hypothesis that the pH-dependent oxygen affinity of hemoglobin (Bohr effect) is due to changing electrostatics of histidine at the surface of hemoglobin.

Interaction of α‐Synuclein with Phospholipids and the Associated Restructuring of Interfacial Lipid Water: An Interface‐Selective Vibrational Spectroscopic Study

“…It is important to note that previous phase‐sensitive/heterodyne‐detected VSFG measurements [20, 27, 37, 38] showed a positive Imχ (2) band around 3200 cm −1 and was assigned to the “ice‐like” or “strongly H‐bonded water pair”. The positive band however is not so prominent in the carefully measured recent Imχ (2) spectra, [35, 36, 39–41] which is in conformity with the present data (black‐dashed curve in Figure 2 a ) measured using high resolution displacement sensor (see the Supporting Information) as well as our previously reported spectra that were phase‐corrected by internal referencing [42–44] . With the addition of 20 μM αS in water, the surface pressure ( π =surface tension of neat water (72.8 mN m −1 )—surface tension of αS solution) becomes ≈20 mN m −1 , indicating adsorption of αS on water surface.…”

Bulk Contributions Modulate the Sum-Frequency Generation Spectra of Water on Model Sea-Spray Aerosols

“…17,26,50,78 Disentangling c ð2Þ ijk and c ð3Þ ijkz contributions to the vSFG response at charged interfaces is particularly challenging and has recently been the focus of several experimental and theoretical studies on model systems. 10,17,26,[28][29][30]32,40,42,43,47,50,69,75,76,78,103,105,106 Perhaps most importantly, these studies have now revealed that the second-and third-order contributions to the SFG response from charged interfaces are not merely additive but instead modulated non-linearly by the ionic strength-dependence of the c ð3Þ ijkz phase angle 76 and the phase interference 32,105 terms shown in Equation 1. Recent work on the salt concentration-dependent water OH stretching spectra obtained from bilayers, as opposed to monolayers, of zwitterionic and anionic lipids has identified the an additional complication, namely, the choice of a truly zero-potential reference state.…”

Interaction of α‐Synuclein with Phospholipids and the Associated Restructuring of Interfacial Lipid Water: An Interface‐Selective Vibrational Spectroscopic Study

“…Thep ositive band however is not so prominent in the carefully measured recent Imc (2) spectra, [35,36,[39][40][41] which is in conformity with the present data (black-dashed curve in Figure 2a )m easured using high resolution displacement sensor (see the Supporting Information) as well as our previously reported spectra that were phase-corrected by internal referencing. [42][43][44] With the addition of 20 mM aSi nw ater, the surface pressure (p = surface tension of neat water (72.8 mN m À1 )-surface tension of aS solution) becomes % 20 mN m À1 ,i ndicating adsorption of aS on water surface.Inthe HD-VSFG measurement too,the CH stretch region (< 3000 cm À1 )w hich was featureless for the neat air-water interface,s hows sharp negative bands around 2880 and 2940 cm À1 (yellow curve) corresponding to the C À H stretch of aS, confirming the presence of aSatt he air-water interface.The OH stretch band ( % 3000-3580 cm À1 )becomes positive in sign due to interfacial aS(yellow curve,expanded view of the OH stretch is shown in the inset, Figure 2a). The Positive sign implies that the interfacial water which was Hdown oriented (negative Imc (2) )a tt he neat air-water interface,a dopts H-up orientation in presence of aS.…”