Ultra-Fast Fluorescence Anisotropy Decay Of N-Acetyl-L-Tryptophanamide Reports On The Apparent Microscopic Viscosity Of Aqueous Solutions Of Guanidine Hydrochloride

The very fast fluorescence anisotropy decay of N-acetyl-L-tryptophanamide (NATA) in aqueous solutions has been measured with sub-picosecond laser excitation and detection with time-correlated single photon counting. By using global analysis of both parallel and perpendicular polarized fluorescence intensity decays involving deconvolution, the rotational correlation times of NATA in the tens of picosecond range are accurately recovered. Since rotational correlation times are directly proportional to viscosity, we have used these correlation times to derive the (relative) microscopic viscosity of increasing concentrations of guanidine hydrochloride (GuHCl) in buffered water. GuHCl is a well-known chaotropic agent of protein denaturation. We give a step-by-step description how to obtain the final results. Subsequently, we compare the obtained microscopic viscosities with macroscopic viscosity data reported half a century ago using capillary viscosimeters. From the results it is clear that GuHCl, present in molar concentration, associates with NATA making the apparent molecular volume larger.