STRUCTURAL HETEROGENEITY IN PROTEINS AT CRYOGENIC TEMPERATURES - COOLING RATE DEPENDENCE

The influence of the cooling rate on the structural heterogeneity of sperm whale myoglobin in solution at cryogenic temperatures was studied. Samples were either cooled slowly (0.03 K/s) or rapidly by immersing in liquid propane at 77 K, which yielded a cooling rate of more than 100 K/s. FTIR spectra of the stretch bands of the heme-bound CO showed that the population of the A substates depends on the cooling rate. The structural heterogeneity within each A substate was assessed by temperature-derivative spectroscopy (TDS), which yields the distribution of enthalpy barriers for CO rebinding after photodissociation. No significant changes of the barrier distribution were found. It is concluded that, within the range explored, the cooling rate plays a negligible role in the structural heterogeneity of protein solutions at cryogenic temperatures.