Effect of water activity on lipase enantioselectivity
Chemical Product Design and Engineering (CPD&E)
Chemical Product Design & Engineering - III (CPD&E - 3)
Keywords: Water activity, salt hydrate pair, saturated salt solution, kinetic resolution
The advantages of using organic solvents or low water activity media to operate enzymatic reactions have been pointed out [1].It is well known that water content in the reaction media is an important parameter when using enzymes in organic media It can influence the rate and equilibrium yield as well as the stability of enzymes. It is therefore important to measure and control water content in these systems. The effect of water in organic solvents is best described by the water activity (aw).
In Lipase catalyzed esterification reactions in organic solvents, water plays a crucial role. A minimal amount of water is necessary for the enzyme to ensure its optimal conformation and to become optically active but an excess of water decreases the enzyme catalytic activity.
In order to control aw several methods can be used. To maintain constant aw during a reaction, aw of headspace above the reaction medium is adjusted by circulation of headspace gas through a drying column. Another aw control method is to perform the reaction in a vessel with a saturated salt solution in contact with the reaction mixture via the gas phase, so that the saturated salt solution continuously absorbs and releases water vapor to keep the aw constant.
Another way for the continuous control of aw is the use of a salt hydrate pair. A salt hydrate pair controls the water level in the reaction mixture by taking up or releasing water as required to keep a constant aw condition during the reaction [2]
In this study, lipase (Novozym 435) catalyzed enantioselective esterification of 1-phenyl-1-propanol with capric acid is carried out in trimethylpentane solvent under controlled water activity conditions.
The aw is controlled by adding salt hydrate pairs which are Na2HPO4 (2/7)( aw:0.69), Na2HPO4(7/12)(aw:0.90), Na2SO4(10/0)( aw:0.76) C2H3NaO2(3/0)(0.33), MnCl2(6/0 )(aw:0.33), MgCl2(6/0)(0.06). The equal amounts of each salt form were added directly into reaction medium to keep the water activity constant during the reaction.
And using saturated salt solutions which are LiCl, (aw:0.11), LiBr (aw:0.15), CH3COOK (aw:0.23), (MgNO3).6H2O (aw:0.54), NaCl (aw:0.75), KCl (aw:0.85), K2Cr2O7 (aw:0.98) are used to investigate the role of initial water activity. In the experiments with saturated salt solutions, lipase and solvent were incubated in desiccators containing saturated salt solutions for 48 hours. When salt hydrated pairs were used, conversion and reaction rate increased with increasing aw from 0.06 to 0.33 and above this aw value they decreased. Similarly, ee increased from 2% to 84% with increasing aw 0.06 to 0.33. When saturated salt solutions were used, higher conversion, reaction rate and ee (68%) was obtained at the low aw (0.11). At high aw values, they decreased.
References:
1. Colombie S., Tweddell R.J., Condoret J.S., Marty A., Water activity control: A way to improve the efficiency of continuous lipase esterification, Biotechnology and Bioengineering, 1998, 60:362-368.
2. Han J. J. And Rhee J. S. Effect of salt hydrate pairs for water activity control on lipase-catalyzed synthesis of lysophospholipids in a solvent free system, Enzyme and Microbial Technology 1998, 22:158-164
Presented Thursday 20, 10:11 to 10:30, in session Chemical Product Design & Engineering - III (CPD&E - 3).
