Myoglobin solvent effects

The enthalpy of ionization of the titratable histidine groups in myoglobin is 7.5 kcal mol-1 (Antonini and Brunori, 1971). These groups are involved at the isoelectric point level. The solvent effect on the pKa of histidine has been measured. From a value of 6.1 in pure water, it decreases to 5.85 in the mixed solvent, at +20°C. The conditions (AH buffer > AH of the pi) for isoelectric focusing are therefore satisfied. 

Effects of solvent mixtures can be seen in biochemical systems. Ligand binding to myoglobin in aqueous solution involves two kinetic components, one extramolecular and one intramolecular, which have been interpreted in terms of two sequential kinetic barriers. In mixed solvents and subzero temperatures, the outer barrier increases and the inner barrier splits into several components, giving rise to fast intramolecular recombination. Measurements of the corresponding solvent structural relaxation rates by frequency resolved calorimetry allows the discrimination between solvent composition and viscosity-related effects. The inner barrier and its coupling to structural relaxation appear to be independent of viscosity but change with solvent composition (Kleinert et al., 1998). 

A related reduced series (Figure 5.5) was also reported J74 Selectivity of these crowned arborols (100-102) towards alkali metal cation binding was examined and allosteric as well as conformational binding effects were studied. Also they showed that the sterically less crowded arborols (e.g., 102) are better for the dissolution of myoglobin in organic solvents, such as DMF. This example affords an interesting entrance to internal metal ion complexation at a specific loci see Section 8.3. 

To provide an understanding of the importance of solvent mobility and the intrinsic protein energy surface, an MDS of proteins and surrounding solvent molecules at different temperatures has been performed. The simulation of myoglobin dynamics showed that solvent mobility is the dominant factor in determining protein atomic fluctuations above 180 K (Vitkup et ah, 2000). The drastic effects of water molecule dynamics on the intramolecular motion of RNase and xylase was demonstrated in recent computer simulation studies (Reat et al., 2000 Tarek et al, 2000). Extensive simulations were carried out to identify the time-scale of water attachment to lysozyme (Steprone et... 

The effect of serum on rejection of solutes, as compared with rejection measured out of saline, can be addressed in light of data for cellulosic membranes. Figures 10 and 11 demonstrate an effect consistently observed for the solutes myoglobin and cytochrome C. The observed and corrected rejection values fell off at approximately J >0.6 x 10 cm/sec from a plateau when saline was the solvent, whereas in serum the R versus J graphs were as predicted from the Spiegler-Kedem equation. The relative effects of serum or BSA on R, , and on resultant values for a... 

When matter is transported in solution, the effective mass and the effective volume are not identical to the mass and volume of the dry macromolecule. A transported protein molecule such as myoglobin (see Figure 4-14) drags associated solvent along with it, which contributes to the frictional resistance. 

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