Determination of the second order doppler shift of iron in myoglobin by Mössbauer spectroscopy.

We have performed Mössbauer absorption experiments on a sample of deoxygenated myoglobin crystals from 5 K to 280 K. With two series of measurements, one with the source and sample at the same temperature and the other with the source always at 298 K, we are able to extract information from the second-order Doppler effect in the sample. Simple models consistent with a description of myoglobin with low lying electronic states which are thermally populated above 40 K indicate that the Debye temperature of myoglobin is 220 K, in agreement with measurements using the Lamb-Mössbauer factor. The second-order Doppler effect is proportional to the square of the velocity of the motion. We are unable to see any indication of protein specific motion from the second-order Doppler effect, thereby indicating that protein specific motions are relatively slow.