Quantification of Rapid Cooling of Glycerol/Water Solutions Based on Photoluminescence from Thioflavin T

Rapid cooling to a solid state allows intermediates in chemical and biomolecular processes that occur in solution near room temperature to be trapped for subsequent measurements by magnetic resonance spectroscopies, electron microscopy, or other techniques. In time-resolved solid state nuclear magnetic resonance and rapid freeze-quench electron paramagnetic resonance studies, solutions are typically frozen by spraying into a cold bath or onto a cold metal surface. Although simulations suggest freezing on millisecond or submillisecond time scales, direct experimental measurements of cooling rates have been elusive. Here, we describe a method for quantification of rapid cooling rates based on measurements of temperature-dependent photoluminescence from thioflavin T (ThT). In our experiments, a jet of ThT solution in glycerol/water, with 10.8 m/s jet velocity and 30 μm diameter, freezes on a cold, rotating copper surface. Images of ThT photoluminescence on the copper surface indicate that the cooling rate of the solution increases linearly with the surface velocity over the 0.45-6.2 m/s range. At surface velocities greater than 3.8 m/s, the time to cool from 300 to 260 K or from 300 to 230 K is less than 100 μs or less than 700 μs. The experimental results do not agree quantitatively with calculations in which a layer of glycerol/water cools by thermal conduction when suddenly brought in contact with a cold copper surface. Discrepancies between experimental results and simplistic calculations illustrate the importance of direct measurements of cooling rates.