Mechanism-Based Inactivation of Serine Proteases by Dichlorocyclopropane Fused Lactone Derivatives.-The novel lactone (XII) shows inhibitory activity towards α-chymotrypsin. However the enzyme activity recovered after six hours. -(OHBA, T.; IKEDA, E.; TSUCHIYA, N.; NISHIMURA, K.; TAKEI, H.; Bioorg. Med.