Crystal structures of resorcin[4]arene and pyrogallol[4]arene complexes with proline: A model for proline recognition through C[sbnd]H···π interaction.

Resorcin[ 4 ]arene (resorcinol cyclic tetramer, abbreviated as RCT) or pyrogallol[ 4 ]arene (pyrogallol cyclic tetramer, PCT) form host–guest 1:1 complexes with DL-proline (DL-Pro) or l -proline (L-Pro), [RCT·DL-Pro]·2MeOH·3.5H 2 O ( 1 ) and 2[PCT·L-Pro]·2EtOH·10H 2 O ( 2 ), whose crystal structures have been determined. In each complex, the proline ligand is incorporated into the bowl-shaped cavity of RCT or PCT host molecules through C H … π interactions between alkyl protons of the pyrrolidine ring of proline and π-rings of RCT or PCT, forming an [RCT/PCT·Pro] structural fragment. In the crystal lattice, two [RCT/PCT·Pro] fragments self-associate to form a ligand-mediated dimeric structure, [RCT·D-Pro·L-Pro·RCT] in 1 or [PCT·L-Pro·L-Pro·PCT] in 2 . A 1 H NMR solution study gave the host‒ligand binding constants of 10.0 ± 1.1 M –1 for the RCT–DL-Pro system and 17.3 ± 1.3 M –1 for the PCT–L-Pro system. These complexes provide a synthetic model for the recognition of the proline residue in proline-containing substrates or inhibitors by enzymes through C H … π interaction. The CSD survey revealed that the absolute value of the torsion angle N–C α –C O1 (O1 is cis to N) about the carboxyl C α –C bond of proline is significantly smaller than that of the C β –C α –C O2 (O2 is cis to C β ) torsion angle. [ABSTRACT FROM AUTHOR]