Model-free analysis of a thermophilic Fe7S8 protein compared with a mesophilic Fe4S4 protein

N-15 T-1, T-2 and H-1-N-15 NOE were measured for the thermophilic Fe7S8 protein from Bacillus schlegelii and for the Fe4S4 HiPIP protein from Chromatium vinosum, which is a mesophilic protein. The investigation was performed at 276, 300, and 330 K at 11.7 T for the former, whereas only the 298 K data at 14.1 T for the latter were acquired. The data were analyzed with the model-free protocol after correcting the measured parameters for the effect of paramagnetism, because both proteins are paramagnetic, Both thermophilic and mesophilic proteins are quite rigid, with an average value of the generalized order parameter S-2 at room temperature of 0.92 and 0.94 for Fe7S8 and Fe4S4 proteins, respectively. The analyzed nitrogens for the Fe7S8 protein showed a significant decrease in S-2 with increasing temperature, and at the highest temperature >70% of the residues had an internal correlation time. This research shows that subnanosecond rigidity is not related to thermostability and provides an estimate of the effect of increasing temperature on this time scale. Proteins 2000;41:75-85. (C) 2000 Wiley-Liss, Inc.