Additionally, specific activities based on initial reaction velocities of HheG and HheG-682 in the azidolysis of the different epoxide substrates were determined using an adapted, pH-based spectrophotometric assay. Reactions were performed at 30°C using 10 mM epoxide, 20 mM azide and 100 µg mL -1 purified enzyme. As expected, both HheG and HheG-682 displayed low specific activity with epoxide 1 (Figure ), while for all other tested epoxides the specific activities of HheG-682 were significantly higher compared to HheG. The highest (13.6-fold) increase in specific activity was obtained with epoxide 11. Hence, HheG-682 is significantly more active, especially also with the sterically more demanding substrates 7, 9 and 11, than HheG. ), (+)-cis/trans-limonene oxide (9) and trans-1-phenylpropylene oxide (11) determined with a spectrophotometric assay. Reactions were carried out in duplicate in a total volume of 1 mL with 10 mM epoxide and 20 mM azide in 2 mM buffer at 30 °C using 100 µg mL -1 purified enzyme. Samples were taken after 30, 60, 180, 270 and 360 s and quenched in equal volumes of MeOH. Chemical background of negative control reactions without enzyme addition were subtracted. Resulting specific activities exhibit standard deviations between 0.0 and 0.1 U mg -1 .