Tyrosyl-carboxylate ion hydrogen bond

Fig. 44. Illustration of temperature dependence of xtj for tyrosyl-carboxylate ion hydrogen bonds with parameters of Fig. 43 together with AUi 6 kcal. per mole andAHj = 0 (Scheraga, 1960d).

Fio. 45. Values of Ahn as a function of pH at 300 K., computed from Eq. (IV-33) with the parameters of Figs. 43 and 44, together with AHi 6 kcal. per mole and AJSTi 7 kcal. per mole for the tyrosyl-histidyl hydrogen bond (B). Curve A corresponds to the tyrosyl-carboxylate ion hydrogen bond (Soheraga, 1960d). 

Fig. 66. Temperature dependence of the degree of denaturation at pH 2 and 7 for a hypothetical protein stabilized by tyrosyl-carboxylate ion hydrogen bonds [Eq. (IV-64) and the parameters of Fig. 63] (Scheraga, 1960d). 

Fig. 58. Effect of a tyrosyl-carboxylate ion hydrogen bond on the sharpness of the transition of Fig. 55 (Scheraga el al., 1961).

It may be argued that the effective concentration of carboxylate ion at the protein surface due to a glutamic acid side chain is also very high if it is held close to a tyrosyl residue. However, as already stated, it becomes difficult to distinguish such a situation from a tyrosyl-carboxylate ion hydrogen bond. 

Fig. 48. The pH dependence of Tm for A tyrosyl-carboxylate ion, B tyrosyl-histidine, and C carboxyl-carboxyl hydrogen bonds (Scheraga, 1960d).

Fio. 50. Equilibrium melting temperature as a function of pH for A, tyrosyl-carboxylate ion, B, tyrosyl-histidine, and C, carboxyl-carboxyl hydrogen bonds, all at a constant force of 3 X 10 dynes per square centimeter for the hypothetical fiber discussed in the text (Scheraga, lOOOd). 

Fig. 63. Contribution of a heterologous single hydrogen bond to AFg as a function of pH at 300°K., computed from Eq. (IV-32) for A, tyrosyl-carboxylate ion, and B, tyrosyl-histidine hydrogen bonds, with the parameters of Fig. 43 (Scheraga, 1960d).

Fig. 19. Heterologous single hydrogen bond between the hydroxyl group of a tyrosyl residue and the carboxylate ion of a glutamyl residue between two ri d, helical peptide chains. The symbols a-f indicate specific bonds to which reference is made in the text (Laskowski and Scheraga, 1954).

As an example of a cooperative hydrogen bond we shall consider the interaction between two tyrosyl groups with one carboxylate ion, as illustrated in Fig. 18. The equihbrium constant for the formation of such a bond is A , given by Eq. (II-4). The expression for Aob for the ionization of a specified donor is... 

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