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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution
Water Dynamics in the Hydration Shell of Amphiphilic Macromolecules Jiaqi Zhang, Liyuan Liu, Yu Chen, Bin Wang, Chunmei Ouyang, Zhen Tian, Jianqiang Gu, Xueqian Zhang, Mingxia He, Jiaguang Han, and Weili Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b02040 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019
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The Journal of Physical Chemistry
Water Dynamics in the Hydration Shell of Amphiphilic Macromolecules Jiaqi Zhang,† Liyuan Liu,∗,† Yu Chen,‡ Bin Wang,∗,¶ Chunmei Ouyang,† Zhen Tian,† Jianqiang Gu,† Xueqian Zhang,† Mingxia He,† Jiaguang Han,† and Weili Zhang∗,†,§ Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, and Key Laboratory of Optoelectronics Information and Technology, Ministry of Education, Tianjin University, Tianjin 300072, China, Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300354, China, Tianjin Engineering Technology Center of Chemical Wastewater Source Reduction and Recycling, School of Science, Tianjin Chengjian University, Tianjin 300384, P. R. China, and School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA E-mail:
[email protected];
[email protected];
[email protected] ∗ To
whom correspondence should be addressed for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, and Key Laboratory of Optoelectronics Information and Technology, Ministry of Education, Tianjin University, Tianjin 300072, China ‡ Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300354, China ¶ Tianjin Engineering Technology Center of Chemical Wastewater Source Reduction and Recycling, School of Science, Tianjin Chengjian University, Tianjin 300384, P. R. China § School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA † Center
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Abstract In present work, the dynamics of water in hydration shell of amphiphilic hyperbranched polyglycerol (HPG) molecules was investigated using THz and GHz dielectric relaxation spectroscopies. Different from the typical small amphiphilic molecules, HPG macromolecule provides a rather dispersionless background and allows the direct observation of four types of water molecules in the hydration shell, including bulk-like water, undercoordinated water, slow water (water molecules hydrating hydrophobic groups and water molecules accepting hydrogen bond from OH hydroxyl groups), and super slow water that are strongly hydrogen bonded to ether groups. For bulk-like water and undercoordinated water, the time constant remains invariant with HPG concentration. While, the time constant of slow water molecules increases from 15 ps to 24 ps with increase of HPG concentration. Differently, the time constant of super slow water molecules decrease dramatically with HPG concentration because of the strong hydrogen bonds to ether groups.
Introduction The couplings between water and amphiphilics, in particular interplay between water and hydrophobic/hydrophilic via hydrogen bonding interactions, is of great interest to basic chemistry and biology. Essential biological molecules such as proteins affect the structure and dynamics of surrounding water molecules, whereas in turn the conformational changes of proteins and thus its biological functions appear to be coupled to the dynamics and hydrogen bone network of surrounding water molecules. 1–4 Even with intensive studies, there is still no consensus reached on detailed mechanistic picture and properties of hydration shell. The discussion about the existence of ’iceberg’ mode of the hydration can be dated back to the work of Frank in 1940’s, which indicated hydrophobic hydrating water molecules formed ’iceberg’ structure based on the observation of decreased entropy and increased heat capacity caused by hydrophobic groups arising from enhanced water hydrogen bond. 5 Recent studies provided experimental evidence on the exitance of ice-like water in the hydration 2
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The Journal of Physical Chemistry
shell of hydrophobes. 6,7 On the other hand, it was also thought as that the immobilization of hydrating water were explained by the decrease of configurational space available to water molecules around hydrophobic solutes, in particular with MD simulation, 8 neutron diffraction studies, 9 femtosecond infrared pump-probe (fs-IR) 10,11 and Raman scattering measurements with multivariate curve resolution (Raman-MCR). 12 Most of studies on hydration dynamics of amphiphilics focuses on small molecules, such as trimetlylamine oxide (TMAO), tetramethylurea (TMU), 1,1-dimethyurea (1,1-DMU) and Dimethyl sulfoxide (DMSO), using various experimental and theoretical approaches including time-resolved infrared experiments, 10,11 vibrational sum frequency generation (VSFG) spectroscopy, 13–15 NMRrelaxation studies, molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations 7,8,12 and GHz-THz dielectric relaxation spectroscopy. 16–19 Specially GHz-THz dielectric relaxation spectroscopy is widely used to study the dynamics of water molecules, because orientational and translational motions of liquid water, as well as stretch and bending modes of hydrogen bond, are in GHz to THz frequency range. 20 By adding solute, the dielectric response of water in GHz-THz frequency range is expected to be different from pure water, which gives the information on the dynamics of hydrating water. However, small amphiphilics including TMAO, urea, TMU and DMU have dielectric response in similar frequency region with hydrating water, and thus the analysis relied on the effective dipole moment of these small amphiphilics obtained with Cavell equation and the result might be different if the spherical approximations of solvent is varied. 16–18,21,22 Here, we report the water dynamics in hydration shell of amphiphilic hyperbranched polyglycerols (HPG) whose dielectric response is down to MHz region, using combined GHz and THz dielectric relaxation spectroscopy. HPG provides a small and dispersionless background in the studied frequency region and thus allows a direct characterization of effect of hydrophobic, hydroxyl and ether groups on the slowdown of water reorientation. Four types of hydrating water molecules are observed: bulk-like water, undercoordinated water, slow water with reorientation time of tens of picosecond and super slow water with reorientation time over 100 ps.
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Experiment Dielectric Relaxation Spectroscopy Dielectric relaxation measurement probes the macroscopic polarization of a sample as a function of frequency, which for an uncharged polar liquid reflects the reorientation of dipoles. For water molecules, the dielectric relaxation response contains the information on how well the permanent dipoles of water molecules can follow up the oscillating electromagnetic field. By driving water molecules with an oscillating electromagnetic field, it is possible to determine the frequency dependence of the reorientation polarization. In oscillating electromagnetic field, the permanent dipole of water molecules tends to align with the field. If oscillation frequency is low, the dipoles can follow up. When frequency of oscillating field is high, the dipoles start lagging behind. For a typical polar system with only one reorientation time, the frequency dependent orientation polarization leads to dielectric relaxation function which is described by Debye model:
ε(ω) =
εS − ε∞ + ε∞ 1 + iωτ
(1)
where, εS is static permittivity of sample, ε∞ is the high frequency limit or unrelaxed permittivity, τ is relaxation time, and dielectric strength is given by S = εS − ε∞ . If the system has more than one reorientation processes, each process can be described by introducing an extra Debye term, which is similar to the Eqn. 1. For example, pure liquid water has two reorientational relaxation processes and its dielectric relaxation can be described as the sum of two terms: ε(ω) =
S f ast Sbulk + + ε∞ 1 + iωτbulk 1 + iωτ f ast
(2)
The first term with τbulk of ∼8.4 ps and Sbulk of ∼74, is assigned to the hydrogen bonded water molecules. The second term which has a much smaller time constant of ∼250 fs and a much weaker strength S f ast
