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Ligand binding can change the pKa of protein residues and influence enzyme catalysis. Herein, we report three ultrahigh resolution X-ray crystal structures of CTX-M β-lactamase, directly visualizing protonation state changes along the enzymatic pathway: apo protein at 0.79 Å, precovalent complex with nonelectrophilic ligand at 0.89 Å, and acylation transition state (TS) analogue at 0.84 Å. Binding of the noncovalent ligand induces a proton transfer from the catalytic Ser70 to the negatively charged Glu166, and the formation of a low-barrier hydrogen bond (LBHB) between Ser70 and Lys73, with a length of 2.53 Å and the shared hydrogen equidistant from the heteroatoms. QM/MM reaction path calculations determined the proton transfer barrier to be 1.53 kcal/mol. The LBHB is absent in the other two structures although Glu166 remains neutral in the covalent complex. Our data represents the first X-ray crystallographic example of a hydrogen engaged in an enzymatic LBHB, and demonstrates that desolvation of the active site by ligand binding can provide a protein microenvironment conducive to LBHB formation. It also suggests that LBHBs may contribute to stabilization of the TS in general acid/base catalysis together with other preorganized features of enzyme active sites. These structures reconcile previous experimental results suggesting alternatively Glu166 or Lys73 as the general base for acylation, and underline the importance of considering residue protonation state change when modeling protein-ligand interactions. Additionally, the observation of another LBHB (2.47 Å) between two conserved residues, Asp233 and Asp246, suggests that LBHBs may potentially play a special structural role in proteins.

The geometries and relative stabilities of the singlet and triplet states of carbenes were investigated. It was found that the resonance stabilization of both singlet and triplet carbenes increases modestly with the size of the arene substituent.

Small hydrocarbon complexes (X @ cage) incorporating cage-centered endohedral atoms and ions (X = H(super +), H, He, Ne, Ar, Li(super 0,+), Be(super 0,+,2+), Na(super 0,+), Mg(super 0,+,2+) are studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. The study shows endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.

Research is presented concerning the use of density functional theory and ab initio methods to investigate the stability of the singlet and triplet states of 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarb ene.

The formation of different vacuum and solvent ab initio surfaces by the stereoelectronic and entropic stabilization of the exocyclic torsion of 5-(hydroxymethyl)tetrahydropyran is discussed. The solvent surface is shown to be more stable as compared to the vacuum and solvent surfaces.

Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H[sup +], H, He, Ne, Ar, Li[sup 0,+], Be[sup 0,+,2+], Na[sup 0,+], Mg[sup 0,+,2+]) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C[sub 4]H[sub 4], T[sub d]) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C[sub 8]H[sub 8], O[sub h]) and bicyclo[2.2.2]octane (C[sub 8]H[sub 14], D[sub 3h]) minima are limited to encapsulating species smaller than Ne and Na[sup +]. Despite its intermediate size, adamantane (C[sub 10]H[sub 16], T[sub d]) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li[sup 0,+], Be[sup 0,+,2+], Na[sup 0,+], and Mg[sup 2+]. In contrast, the truncated tetrahedrane (C[sub 12]H[sub 12], T[sub d]) encapsulates fewer species, while the D[sub 4d] symmetric C[sub 16]H[sub 16] hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg[sup +] species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E[sub endo]) to the sum of their isolated components (E[sub inc] = E[sub endo] - E[sub cage] - E[sub x]) and to their exohedral isomer energies (E[sub isom] = E[sub endo] - E[sub exo]). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., E[sub isom] is always exothermic), Be[sup 2+] @C[sub 10]H[sub 16] (T[sub d]; -235.5 kcal/mol), Li[sup +] @C[sub 12]H[sub 12] (T[sub d]; 50.2 kcal/mol), Be[sup 2+]@C[sub 12]H[sub 12] (T[sub d]; -181.2 kcal/mol), Mg[sup 2+]@C[sub 12]H[sub 12] (T[sub d]; -45.0 kcal/mol), Li[sup +]@C[sub 16]H[sub 16] (D[sub 4d]; 13.3 kcal/mol), Be[sup +]@C[sub 16]H[sub 16] (C[sub 4v]; 31.8 kcal/mol), Be[sup 2+]@C[sub 16]H[sub 16] (D[sub 4d]; -239.2 kcal/mol), and Mg[sup 2+]-C[sub 16]H[sub 16] (D[sub 4d]; -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C[sub 20]H[sub 20] (l[sub h]), which has an E[sub inc] = 37.9 kcal/mol and E[sub isorn] = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets. [ABSTRACT FROM AUTHOR]