Atomic force microscope manipulations of single polysaccharide molecules have recently expanded conformational chemistry to include force-driven transitions between the chair and boat conformers of the pyranose ring structure. We now expand these observations to include chair inversion, a common phenomenon in the conformational chemistry of six-membered ring molecules. We demonstrate that by stretching single pectin molecules (1 3 4-linked ␣-D-galactouronic acid polymer), we could change the pyranose ring conformation from a chair to a boat and then to an inverted chair in a clearly resolved two-step conversion: 4 C 1 i boat i 1 C 4 . The two-step extension of the distance between the glycosidic oxygen atoms O 1 and O 4 determined by atomic force microscope manipulations is corroborated by ab initio calculations of the increase in length of the residue vector O 1 O 4 on chair inversion. We postulate that this conformational change results from the torque generated by the glycosidic bonds when a force is applied to the pectin molecule. Hence, the glycosidic bonds act as mechanical levers, driving the conformational transitions of the pyranose ring. When the glycosidic bonds are equatorial (e), the torque is zero, causing no conformational change. However, when the glycosidic bond is axial (a), torque is generated, causing a rotation around COC bonds and a conformational change. This hypothesis readily predicts the number of transitions observed in pyranose monomers with 1a-4a linkages (two), 1a-4e (one), and 1e-4e (none). Our results demonstrate single-molecule mechanochemistry with the capability of resolving complex conformational transitions.Atomic force microscope (AFM) manipulations of single polysaccharide molecules have recently expanded conformational chemistry (1) to include force-driven transitions between the chair and boat conformers of the pyranose ring structure (2). The application of a force to a single molecule will deform it elastically and also induce conformational transitions. Although it is easy to understand the origin of an elastic deformation, the mechanics of the conformational transition is less clear.Pyranose-based sugars have two distinct chair conformations, 4 C 1 and 1 C 4 (3), separated by an energy barrier of Ϸ11 kcal͞mol (4). In addition to the chair conformers, pyranoses have intermediate conformers corresponding to the boat conformation, whose energy is Ϸ5-8 kcal͞mol above the energy of the 4 C 1 chair (5). Thermally driven transitions do occur between these conformers. However, in the absence of an applied force, the most stable conformation of a pyranose is that of the 4 C 1 chair (4-9). Application of a force of Ϸ200 pN to polymers of ␣-D-glucopyranose such as amylose drives a conformational change in the pyranose ring that is evident as a sudden elongation of the molecule, marking a prominent enthalpic component of the elasticity of the molecule (2, 10). This enthalpic component results from an increase in the distance between glycosidic oxygen atoms caused by a for...

Dimeric acetylcholinesterase (AChE) inhibitors containing a single 9-amino-1,2,3,4-tetrahydroacridine (tacrine) unit were constructed in an effort to further delineate structural requirements for optimal binding to the AChE peripheral site. Basic amines of differing hydrophobicity were selected as peripheral site ligands, and in each case, improvements in inhibitory potency and selectivity were seen relative to tacrine itself. AChE IC(50) values of the optimum dimers decrease significantly as the peripheral site ligand was permuted in the series ammonia > dimethylamine > 4-aminopyridine > 4-aminoquinoline > tacrine. Calculated desolvation free energies of the optimum dimers match the trend in IC(50) values, suggesting the importance of ligand hydrophobicity for effective cation-pi interaction with the peripheral site.

Despite intensive studies of zinc's role in proteins and recent growing appreciation of zinc in modern biology, the knowledge of the protonation state of common zinc ligands in proteins remains controversial. Water, the side chains of Glu, Asp, and Cys residues, and even the peptide nitrogen atom are treated as a deprotonated, negatively charged ligand in the zinc complexes in proteins, whereas the side chain of His residue is treated as a neutral ligand in the zinc complexes regardless of the common knowledge that the imidazole nitrogen proton is more acidic than the peptide nitrogen proton. In an attempt to resolve this controversy, we performed large basis set DFT calculations of proton dissociation energies of common zinc ligands in the presence and absence of Zn 2+ . Herein, we report the results of our calculations revealing that the proton dissociation energies of H 2 O, MeOH, MeSH, imidazole, and N-methylacetamide are dramatically reduced when they coordinate to Zn 2+ and that the proton dissociation energy of the zinc-bound imidazole is 4 kcal/mol lower than that of the zinc-bound N-methylacetamide that is known to be deprotonated according to the X-ray and NMR studies. The result thus suggests further investigations of the possibility of imidazolate as a zinc ligand in proteins.