Center for Molecular Spectroscopy and Dynamics

2016 118th KCS Meeting

Water near the membrane surface is a biological environment for many biochemical reactions and maintenance of the structural integrity of the membrane. Therefore, molecular level understanding of water structure and dynamics near the membrane is prerequisite to perceive the role of water in such relevant biological environment.

The OD stretch mode of HOD molecules as IR probe was used to study the water dynamics at the lipid multibilayer (mimic the biological membrane) using femtosecond IR pump-probe spectroscopy [1,2]. The azido stretch mode of HN₃ is a promising IR probe for studying local water structure in confined systems due to its narrow spectral bandwidth and large transition dipole moment [3]. Here, we used two different IR probes (OD stretch mode and azido stretch mode) to study water hydrogen-bonding structure and dynamics near the lipid multibilayer.

We observed two distinct vibrational lifetime components of OD stretch mode of HOD molecule near the lipid multibilayer. The fast component (0.6 ps) is associated with water interacting with the phosphate moiety, whereas the slow component (1.9 ps) is bulk-like choline-associated water. However, with azido probe, we observed only bulk-like choline-associated water. Because of the tight packing of the head part of the lipid in the gel phase, HN₃ molecules mostly stay at the very end part of the lipid (in the bulk region). Indeed, the vibrational lifetime (∼2.4 ps) is close to azido stretch of bulk HN₃ (∼2.5 ps). We anticipate these different type of water near the lipid multibilayer have an important role for many biochemical reactions at these biological environment.

References:

[1] Kundu, A.; Bartosz, B.; Lim, J.; Kwak, K.; Cho, M. J. Phys. Chem. Lett. 2016. 7, 741.

[2] Zhao, W.; Moilanen, D. E.; Fenn, E. E.; Fayer, M. D. J. Am. Chem. Soc. 2008, 130, 13927.

[3] Lee, J.; Maj, M.; Kwak, K.; Cho, M. J. Phys. Chem. Lett. 2014. 5, 3404.