Optical methods were used to study the internal motions of myoglobin and cytochrome $c$. The experiments show that these proteins exhibit conformational fluctuations at temperatures as low as 2 K. The distribution of fluctuation rates can be measured in real time and turns out to be very sharp. The temperature dependence of the structural relaxation of myoglobin follows a simple Arrhenius law. The results are in agreement with existing models for protein dynamics.

• Received 16 September 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.2138