Catalytic esterase peptide dendrimers with a core active site were discovered by functional screening of a 65 536-member combinatorial library of third-generation peptide dendrimers using fluorogenic 1-acyloxypyrene-3,6,8-trisulfonates as substrates. In the best catalyst, RMG3, ((AcTyrThr)(8)(DapTrpGlY)(4)-(DapTrgGly)(2)(DapHisSerNH(2)), ester hydrolysis is catalyzed by a single catalytic histidine residue at the dendrimer core. A pair of arginine residues in the first-gene ration branch assists substrate binding. The catalytic proficiency of dendrimer RMG(3) (k(cat)/K-M = 860 M-1 min(-1) at pH 6.9) per catalytic site is comparable to that of the multivalent esterase dendrimer A3 ((AcHisSer)(8)(DapHisSer)(4)(DapHisSer)(2)DapHisSerNH(2)) which has fifteen histidines and five catalytic sites (Delort, E. et al. J. Am. Chem. Soc. 2004, 126, 15642-15643). Remarkably, catalysis in the single site dendrimer RMG3 is enhanced by the outer dendritic branches consisting of aromatic amino acids. These interactions take place in a relatively compact conformation similar to a molten globule protein as demonstrated by diffusion NMR. In another dendrimer, HG3 ((AcillePro)(8)(DaplleThr)(4)(DapHisAla)(2)DapHisLeuNH(2)) by contrast, catalysis by a core of three histidine residues is unaffected by the outer dendritic layers. Dendrimer HG3 or its core HG1 exhibit comparable activity to the f irst-gene ration dendrimer A1 ((AcHisSer)(2)DapHisSerNH(2)). The compactness of dendrimer HG3 in solution is close to that a denatured pepticle. These experiments document the first esterase peptide dendrimer enzyme models with a single catalytic site and suggest a possible relationship between packing and catalysis in these systems.