Spectral diffusion at the water/lipid interface revealed by two-dimensional fourth-order optical spectroscopy: a classical simulation study |
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Authors: | Nagata Yuki Mukamel Shaul |
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Institution: | Department of Chemistry, University of California, Irvine, California 92697, United States. nagatay@uci.edu |
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Abstract: | Using a classical simulation protocol for nonlinear optical signals, we predict the two-dimensional (2D) spectra of water near a monolayer of 1,2-dimytristoyl-sn-glycero-3-phosphatidylcholine] (DMPC) generated by three IR probe pulses followed by one visible probe pulse. Sum-frequency-generation 1D spectra show two peaks of the OH stretch representing two environments: near-bulk water nonadjacent to DMPC and top-layer water adjacent to DMPC. These peaks create a 2D pattern in the fourth-order signal. The asymmetric cross-peak pattern with respect to the diagonal line is a signature of coherence transfer from the higher- to the lower-frequency modes. The nodal lines in the imaginary part of the 2D spectrum show that the near-bulk water has fast spectral diffusion resembling that of bulk water despite the orientation by the strong electrostatic field of DMPC. The top-layer water has slower spectral diffusion. |
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