Article pubs.acs.org/JPCB
Modeling Vibrational Spectra of Ester Carbonyl Stretch in Water and DMSO Based on Molecular Dynamics Simulation Bin Fang,†,∥ Tianjun Wang,‡,∥ Xian Chen,§ Tan Jin,† Ruiting Zhang,† and Wei Zhuang*,†
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 14, 2015 | doi: 10.1021/acs.jpcb.5b06541
†
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Liaoning, China ‡ Department of Chemistry, ShanghaiTech University, 19 Yueyang Road, Shanghai 200031, China § Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), Department of Physics, Jilin University, 2699 Qianjin Street, ChangChun 130012, China S Supporting Information *
ABSTRACT: On the basis of molecular dynamics simulation, we model the ester carbonyl stretch FTIR signals of methyl acetate in D2O and DMSO. An ab initio map is constructed at the B3LYP/6-311++G** level to relate the carbonyl stretch frequency to the external electric field. Using this map, fluctuating Hamiltonian of the carbonyl stretch is constructed from the MD simulation trajectory. The IR spectra calculated based on this Hamiltonian are found to be in good agreement with the experiment. For methyl acetate in D2O, hydrogen bonding on alkoxy oxygen causes a blue shift of frequency, while that on carbonyl oxygen causes a red shift. Two peaks observed in FTIR signals originate from the balance of these two effects. Furthermore, in both D2O and DMSO solutions, correlations are found between the instantaneous electric field on CO and the frequencies. Broader line width of the signal in D2O suggests a more inhomogeneous electric field distribution due to the complicated hydrogen-bonding environment. average electric fields on the ester group calculated based on molecular dynamics simulation, Gai establishes a linear frequency-electric field relation, which is used to probe the local electrostatic and hydration environment of different short peptides. Furthermore, the local dielectric constant in the interior of amyloid fibrils is also detected. Sensitivity of carbonyl stretch to the local electric field also makes it a good probe for fluctuation of the electrostatic environment as well as the electron-transfer processes.2 To explore these possibilities, a molecular level correlation between the vibration and the instantaneous electric field (A.K.A “ab initio map”) needs to be established. Similar work has been carried out extensively to simulate the line shape of the protein amide I band based on the molecular dynamics simulations.32−38 Herein we construct an ab initio map for the ester carbonyl stretch on methyl acetate (MA). We demonstrate that the Fourier transform infrared (FTIR) spectra of MA in water and in DMSO can be nicely reproduced using this map. Further analysis suggests that the much broader line width observed for MA−D2O signal is an indicator of the more inhomogeneous electrostatic environment in water.
I. INTRODUCTION Probing electrostatic interactions in condensed phases, such as solution, protein, biological membrane, and liquid crystal, is a fundamental problem of broad interest. Infrared (IR) spectroscopy, which is sensitive to the dependence of molecular vibration on the surrounding electrostatic environment, provides a valuable tool in these studies.1−25 IR measurements have been used extensively to, for instance, monitor the electrostatic environment in solutions, proteins,1−6 and the biological membranes,7 detect the protein conformational dynamics,8−19 and investigate the ion effect on water dynamics.20−24 A commonly used IR probe in the protein study is the carbonyl group. The amide I vibration arising mainly from the carbonyl stretch in the protein backbone amide units,26 for instance, has strong IR intensity and significantly correlates with the electrostatic environment.27−31 It has been used extensively to study protein conformational dynamics. It is difficult, however, to monitor the local electric field in protein using the amide I vibration because the amide units spread all over the protein backbone. Gai and coworkers1 recently demonstrated that the ester carbonyl vibration can serve as an alternative for this purpose because it absorbs in a spectral region between 1700−1800 cm−1, which is ∼100 cm−1 higher than the typical amide I mode (1600−1700 cm−1), and therefore does not overlap with any other protein IR band at the neutral pH value. By comparing the measured ester carbonyl stretch frequencies in different solvents with the © XXXX American Chemical Society
Received: July 8, 2015 Revised: August 31, 2015
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DOI: 10.1021/acs.jpcb.5b06541 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 14, 2015 | doi: 10.1021/acs.jpcb.5b06541
The Journal of Physical Chemistry B
forms on MA and electric field is nondirectional. In D2O, on the contrary, electric field becomes directional due to the hydrogen bonding. Most of the electric-field components have their maxima significantly shifted from zero point. Furthermore, the electric-field components in D2O have much broader distributions than those in DMSO, which suggests a more inhomogeneous local environment in water. Water molecules can form a hydrogen bond with both the carbonyl oxygen and alkoxy oxygen of MA. We define the H bond between MA and water as follows: The distance between donor (water) oxygen and acceptor (MA) oxygen (O1 or O2) is
