Quantification\nof Rapid Cooling of Glycerol/Water\nSolutions Based on Photoluminescence from Thioflavin T

Rapid cooling to a solid state allows intermediates in\nchemical\nand biomolecular processes that occur in solution near room temperature\nto be trapped for subsequent measurements by magnetic resonance spectroscopies,\nelectron microscopy, or other techniques. In time-resolved solid state\nnuclear magnetic resonance and rapid freeze-quench electron paramagnetic\nresonance studies, solutions are typically frozen by spraying into\na cold bath or onto a cold metal surface. Although simulations suggest\nfreezing on millisecond or submillisecond time scales, direct experimental\nmeasurements of cooling rates have been elusive. Here, we describe\na method for quantification of rapid cooling rates based on measurements\nof temperature-dependent photoluminescence from thioflavin T (ThT).\nIn our experiments, a jet of ThT solution in glycerol/water, with\n10.8 m/s jet velocity and 30 μm diameter, freezes on a cold,\nrotating copper surface. Images of ThT photoluminescence on the copper\nsurface indicate that the cooling rate of the solution increases linearly\nwith the surface velocity over the 0.45–6.2 m/s range. At surface\nvelocities greater than 3.8 m/s, the time to cool from 300 to 260\nK or from 300 to 230 K is less than 100 μs or less than 700\nμs. The experimental results do not agree quantitatively with\ncalculations in which a layer of glycerol/water cools by thermal conduction\nwhen suddenly brought in contact with a cold copper surface. Discrepancies\nbetween experimental results and simplistic calculations illustrate\nthe importance of direct measurements of cooling rates.