Introduction
The current experiment involves the preparation of the racemic mixture of the benzoyl derivatives of the alpha-amino acid Alanine. This is then followed by their separation into their optically pure components through the enzyme-catalyzed reaction of papain. The products can then be characterised based on their optical purity through the technique of Polarimetry.
Aim
The report is aimed at addressing additional reactions involved in the resolution of (R, S)-Benzoylamino acids.
Scope
The report covers a number of issues regarding the various reactions and reaction mechanisms involved in the resolution of the racemic components of alanine as will be listed in the following discussion.
Resolution of (R, S)-Benzoylamino acids
The purpose of washing the solid product from the enzymatic resolution reaction with 1.0M NaOH solution
The enzyme catalyzed reaction of papain takes place in an acidic environment and the purpose of washing the solid with a neutral solution is to neutralize it since the process of racemization that follows is sensitive to changes in the ionic concentration.
The purpose of controlling the pH of the reaction mixture when (R, S)-benzoylamino acid reacts with aniline in the presence of papain
According to studies that have been carried out, the rate of racemization of free amino acids in aqueous solutions has been shown to be dependent on a number of factors. These factors include; the buffer concentration, ionic strength and the type of buffer among others. For instance, the rate of racemization increases with an increase in the concentration of phosphate ions. Furthermore, it is indicated that ionic strength has no effect on the rate of racemization at pH 7.2 but when the pH increases to about pH 10, the rate increases. Therefore, these changes in the rate of racemization necessitate the control of pH.
The equation for the activation of papain by (S)-(+)-Cysteine hydrochloride
The reaction of the enzyme and the activating compound follows the following reaction mechanism:
- R-S.S-R’ + HCl or HCN→R-SH + R’-SCl or R’-SCN (Source)
From the equation above, R-S.S-R is the papain-like cysteine and HCl is the (S)-(+)-Cysteine hydrochloride.
Mechanism for the formation of the acyl papain from the Michaelis-Menten complex
Assuming that the reaction for the formation of acyl papain follows the normal acyl addition-elimination reaction pathway, it is possible for an experimenter to determine the formation of the acylated enzyme from the Michaelis-Menten complex. The individual rate constants for the reaction have been shown to be pH dependent.
Experiments that have been carried out indicate that the papain-catalyzed hydrolysis of the benzoylamino acids can be performed over a range of pH values at 35˚C. The mechanism involved in this case is a three-step reaction through a number of intermediates as shown by the following reaction pathway:
- E + RCOX ↔ E.RCOX → RCOR + XH →E + RCO2H (Source).
From the above equation, E.RCOX represents the enzyme-substrate complex, ECOR is the acyl papain, RCOX is the substrate, E is the free papain, XH is an intermediate after hydrolysis of the Michaelis-Menten complex and RCO2H is the liberated acyl group.
Conclusions
The report has elaborated on a number of important issues that should be taken into consideration when performing reactions involving resolution of the Benzoylamino acids. The report covered various issues including the factors that affect the reactions involved in the experiment.
Recommendations
As indicated in the discussion above, a number of factors determine the feasibility of the experiment in question; therefore it is important for the experimenter to bear this in mind when performing these kinds of reactions. One such parameter to be considered is the pH value.
Reference List
Fruton, JS & M Bergmann, The activation of papain, The Rockefeller Institute for Medical research, New York, 1940.
Lucas, E & A Williams, The pH dependencies of individual rate constants in papain catalyzed reactions, American Chemical Society, Washington, DC, 1969.
Mohrig, JR & SMJ Shapiro, Resolution of (R, S)-Benzoylamino acids, Chem. Educ., 53, 1976, pp. 586-589.
Smith, GG, KM Williams & DM Wonnacott, Factors affecting the rate of Racemization of amino acids and their significance to Geochronology, Utah University Press, Logan, Utah, 1977.