Complex Formation in Aqueous Trimethylamine-N-oxide (TMAO) Solutions
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- 作者：Johannes Hunger ; Klaas-Jan Tielrooij ; Richard Buchner ; Mischa Bonn ; Huib J. Bakker
- 刊名：The Journal of Physical Chemistry B
- 出版年：2012
- 出版时间：April 26, 2012
- 年：2012
- 卷：116
- 期：16
- 页码：4783-4795
- 全文大小：562K
- 年卷期：v.116,no.16(April 26, 2012)
- ISSN：1520-5207
文摘
We study aqueous solutions of the amphiphilic osmolyte trimethylamine-N-oxide (TMAO) using broadband dielectric spectroscopy and femtosecond mid-infrared spectroscopy. Both experiments provide strong evidence for distinctively slower rotation dynamics for water molecules interacting with the hydrophobic part of the TMAO molecules. Further, water is found to interact more strongly at the hydrophilic site of the TMAO molecules: we find evidence for the formation of stable, TMAO路2H2O and/or TMAO路3H2O complexes. While this coordination structure seems obvious, the lifetime of these complexes is found to be extraordinarily long (>50 ps). The existence of these long-lived complexes leads to pronounced parallel dipole correlations between water and TMAO, reflected in enhanced amplitudes in the dielectric spectra. The strong interaction between water and TMAO also results in a red-shifted band for the O鈥揇 stretching vibration of HDO molecules in an isotopically diluted aqueous TMAO solution. This O鈥揇 stretching vibration has a vibrational lifetime of 670 fs, which is significantly shorter than the lifetime of the O鈥揇 stretch vibration of bulk-like HDO molecules, presumably due to efficient coupling to vibrational modes of TMAO. The rotational dynamics of these O鈥揇 groups are slowed down dramatically, and are limited by the rotation of the whole complex, while the O鈥揇 vector oriented away from TMAO probably shows an accelerated reorientation.