Complex Formation in Aqueous Trimethylamine-N-oxide (TMAO) Solutions

We study aq. solns. of the amphiphilic osmolyte trimethylamine-N-oxide (TMAO) using broadband dielec. spectroscopy and femtosecond mid-IR spectroscopy. Both expts. provide strong evidence for distinctively slower rotation dynamics for water mols. interacting with the hydrophobic part of the TMAO mols. Further, water is found to interact more strongly at the hydrophilic site of the TMAO mols.: we find evidence for the formation of stable, TMAO·2H2O and/or TMAO·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 dielec. spectra. The strong interaction between water and TMAO also results in a red-shifted band for the O-D stretching vibration of HDO mols. in an isotopically dild. aq. TMAO soln. This O-D stretching vibration has a vibrational lifetime of 670 fs, which is significantly shorter than the lifetime of the O-D stretch vibration of bulk-like HDO mols., presumably due to efficient coupling to vibrational modes of TMAO. The rotational dynamics of these O-D groups are slowed down dramatically, and are limited by the rotation of the whole complex, while the O-D vector oriented away from TMAO probably shows an accelerated reorientation