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Hydration of Phospholipids and the DNA Backbone Studied by Femtosecond 2D-Infrared Spectroscopy


Hydration of Phospholipids and the DNA Backbone Studied by Femtosecond 2D-Infrared Spectroscopy

T. Elsaesser1*

1Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Str. 2a, 12489 Berlin, Germany

Ionic phosphate groups are key functional groups of numerous macromolecules and play a central role in the hydration of biomolecular structures. While x-ray diffraction, nuclear magnetic resonance, and linear vibrational spectroscopy have identified time-averaged hydration patterns, in particular around phosphates [1], ultrafast structural dynamics and fluctuations originating from phosphate-water interactions have remained mainly unexplored. Here, new insight into phosphate hydration from femtosecond two-dimensional infrared (2D-IR) spectroscopy will be presented for a series of model systems of increasing structural complexity. Results for phosphate ions solvated in H2O [2] serve as a benchmark for the behavior of the much more complex phospholipid [3,4] and DNA structures. Phosphate stretch and DNA backbone vibrations are highly sensitive local probes of hydration dynamics. Femtosecond 2D-IR spectroscopy in the frequency range from 900 to 1300 cm-1 maps anharmonic couplings and population transfer between the different modes and establishes the basic femtosecond time scales of structure fluctuations in the hydrated systems. In DNA, the basic coupling pattern within the manifold of backbone vibrations is deciphered and their delocalized character established. The fastest fluctuations of bulk water around solvated phosphate ions occur on a 50-fs time scale, resulting in ultrafast spectral diffusion of the phosphate’s vibrational bands. In contrast, hydrated phospholipids and DNA display a slower 300-fs time scale of limited fluctuations, leaving the inherent structural disorder and the hydrogen bonds between the biomolecule and the first water layer intact for longer than 10 ps. In phospholipids and DNA, orientation of water molecules in the strong interfacial electric fields and steric hindrance reduces the flexibility of the water shell. Fluctuations on the 300-fs time scale are connected with low-frequency motions of the biomolecules and hindered water motions. Processes of energy exchange with the water shell will be discussed as well [3,5].


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[4] R. Costard, I. A. Heisler, T. Elsaesser, J. Phys. Chem. Lett. 5, 506 (2014).

[5] Ł. Szyc, M. Yang, T. Elsaesser, J. Phys. Chem. B 114, 7951 (2010).