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Notes: We measured the νFe-His Raman band of horse heart deoxymyoglobin and human deoxyhemoglobin as a function of temperature between 10 and 300 K. A self-consistent spectral analysis of the deoxymyoglobin Raman band reveals that it is underlied by three different sublines with frequencies at Ο1 = 209 cm-1, Ο2 = 217 cm-1, and Ο3 = 225 cm-1 and an identical half-width of 13 cm-1. All these parameters were found to be independent of temperature. These sublines were attributed to different conformational substates of the Fe2+-His F8 linkage, which comprise different out-off-plane displacements of the heme iron and tilt angles of the Fe2+-Ng(His F8) bond. The intensity ratio I3/I2 exhibits a van't Hoff behavior between 150 and 300 K, bends over in a region between 150 and 80 K, and remains constant at lower temperature. In contrast, I2/I1 shows a maximum at 170 K and approaches a constant value at 80 K. These data can be fitted by a modified van't Hoff expression, which accounts for the freezing into nonequilibrium distributions of substrates in a temperature interval ΔT around a distinct temperature Tf and also for the linear temperature dependence of the protein's specific heat. The fits to the above intensity ratios yield a freezing temperature of Tf = 117 K and a transition region of ΔT = 55 K. The νFe-His Raman band of human deoxyhemoglobin was decomposed into seven sublines with frequencies at 195, 202, 211, 218, 226, 234, and 240 cm-1, with half-widths of 12 cm-1. While the low-frequency sublines are strong at 300 K, the high-frequency sublines dominate the band at cryogenic temperatures. In comparison, we also investigated the temperature dependence of the near-infrared band III at 760 nm. Band III of deoxymyoglobin can be decomposed into two subbands which are 165 cm-1 apart. The ratio of their absorption cross sections shows a temperature dependence which parallels that of the ratio I3/(I2 + I1) of the corresponding Raman sublines. Band III of deoxyhemoglobin was decomposed into three subbands, the absorption cross sections of which also depend on temperature, similar to what has been observed for the νFe-His subbands. These observations provide strong evidence that the frequency positions of the subbands of band III and the νFe-His sublines are governed by the same coordinates. For both proteins investigated, the frequency positions and the half-widths of the band III subbands depend significantly on temperature. This is rationalized in terms of an earlier proposed model (Cupane et al., Eur. Biophys. J. 21; 385 1993) which assumes that the corresponding electronic transition is coupled to a bath of low-frequency modes. Our data indicate that these modes are harmonic below 134 K but become anharmonic above this temperature. This onset of anharmonic motions is interpreted as resulting from conformational transitions within the protein which affect the prostethic group via heme-protein coupling. © 1996 John Wiley & Sons, Inc.