4-(O-nitrofurfuryl N-methyl-N-(4-chlorobutyl) phosphono(difluoromethyl))phenylacetyl-aspartyl-4-(O-nitrofurfuryl N-methyl-N-(4-chlorobutyl) phosphono(difluoromethyl))-N-(9-fluorenylmethoxycarbonyl)-phenylalanine amide