Fluorescent probe for detecting activity of calpain

A compound represented by general formula (I) below or a salt thereof:

• wherein
• R1 is a hydrogen atom or represents 1 to 4 monovalent substituents present on a benzene ring, which are the same or different;
• R2 is a monovalent substituent;
• R3 and R4 are, independently, a hydrogen atom or C1-6 alkyl group;
• R5 and R6 are, independently, a C1-6 alkyl group or aryl group;
• R7 and R8 are, independently, a hydrogen atom or C1-6 alkyl group;
• R9 and R10 are, independently, a hydrogen atom or C1-6 alkyl group;
• R9 or R10 optionally forms, together with R3 or R7, a 5-7-member heterocyclyl or heteroaryl containing a nitrogen atom to which R9 or R10 binds, and optionally contains 1-3 additional heteroatoms selected from the group consisting of O, N, and S as ring-constituting members;
• the heterocyclyl or heteroaryl optionally being substituted by a C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl, C6-10 aralkyl, or C6-10 aralkenyl group;
• R11 is a monovalent substituent which is to be cleaved by contact with calpain;
• and
• X is a silicon, germanium, or tin atom.