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Histidine ball and stick model spinning. Histidine (symbol His or H) [2] is an essential amino acid that is used in the biosynthesis of proteins.It contains an α-amino group (which is in the protonated –NH 3 + form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO − form under biological conditions), and an imidazole side chain (which is partially ...
In this case, one can use various free energy calculation methods to obtain the protonation free energy [15] such as biased Metropolis MC, [24] free-energy perturbation, [25] [26] thermodynamic integration, [27] [28] [29] the non-equilibrium work method [30] or the Bennett acceptance ratio method. [31] Note that the pK HH
Both lower the kinetic barrier and speed up the attainment of chemical equilibrium. In acid catalysis and base catalysis, a chemical reaction is catalyzed by an acid or a base. By Brønsted–Lowry acid–base theory, the acid is the proton (hydrogen ion, H +) donor and the base is the proton acceptor.
Using Gibbs free energy: ΔG = −RT ln(K eq), where R is the universal gas constant, T is the temperature in kelvins, and K eq is the equilibrium constant of a reaction in equilibrium. The deprotonation of His31 is an acid equilibrium reaction with a special K eq known as the acid dissociation constant, K a: His31-H + ⇌ His31 + H +.
The protonation of isobutene in the formation of a carbocation: (CH 3) 2 C=CH 2 + HBF 4 ⇌ (CH 3) 3 C + + BF − 4; The protonation of ammonia in the formation of ammonium chloride from ammonia and hydrogen chloride: NH 3 + HCl → NH 4 Cl; Protonation is a fundamental chemical reaction and is a step in many stoichiometric and catalytic processes.
The endothelial protease vasohibin [f] uses a cysteine as the nucleophile, but a serine to coordinate the histidine base. [43] [44] Despite the serine being a poor acid, it is still effective in orienting the histidine in the catalytic triad. [43] Some homologues alternatively have a threonine instead of serine at the acid location. [43]
In that case, the equilibrium constants for ligand protonation are also defined as association constants. The numbering of association constants is the reverse of the numbering of dissociation constants; in this example log β 1 = p K a 2 {\displaystyle \log \beta _{1}=\mathrm {p} K_{{\ce {a2}}}}
In 1884, Jacobus van 't Hoff proposed the Van 't Hoff equation describing the temperature dependence of the equilibrium constant for a reversible reaction: = where ΔU is the change in internal energy, K is the equilibrium constant of the reaction, R is the universal gas constant, and T is thermodynamic temperature.