The Kondo lattice Hamiltonian with ferromagnetic Hund's coupling as a model for manganites is investigated. The classical limit for the spin of the (localized) $t_{2g}$ electrons is analyzed on lattices of dimension 1,2,3 and $\infty$ using several numerical methods. The phase diagram at low temperature is presented. A regime is identified where phase separation occurs between hole undoped antiferromagnetic and hole-rich ferromagnetic regions. Experimental consequences of this novel regime are discussed. Regions of incommensurate spin correlations have also been found. Estimations of the critical temperature in 3D are compatible with experiments.Comment: Accepted in Phys. Rev. Letter

The heart of the H ؉ conductance mechanism in the homotetrameric M2 H ؉ channel from influenza A is a set of four histidine side chains. Here, we show that protonation of the third of these imidazoles coincides with acid activation of this transmembrane channel and that, at physiological pH, the channel is closed by two imidazole-imidazolium dimers, each sharing a low-barrier hydrogen bond. This unique construct succeeds in distributing a pair of charges over four rings and many atoms in a low dielectric environment to minimize charge repulsion. These dimers form with identical pK as of 8.2 ؎ 0.2, suggesting cooperative H ؉ binding and clearly illustrating high H ؉ affinity for this channel. The protonation behavior of the histidine side chains has been characterized by using solid-state NMR spectroscopy on the M2 transmembrane domain in fully hydrated lipid bilayers where the tetrameric backbone structure is known. Furthermore, electrophysiological measurements of multichannel and single-channel experiments confirm that these protein constructs are functional.M2 channel ͉ proton channel ͉ solid-state NMR ͉ low-barrier hydrogen bond ͉ histidine ionization constants A histidine tetrad in the pore of the tetrameric M2 protein has long been associated with key channel features of H ϩ selectivity, pH activation, gating, inhibition, and the specific conductance mechanism. M2 protein from influenza A virus conducts protons into the viral core after endocytosis, which leads to the uncoating and release of genetic material into the cytoplasm after fusion of the viral coat with the endosomal wall (1, 2). Much is known about this system from its tetrameric state (2-4), the backbone structure of the transmembrane (TM) domain (5), and numerous electrophysiological (6, 7), biophysical (8-10), and modeling (11) studies that have cast a fascinating tale for this important influenza drug target and the only proton channel of its kind to be characterized in such detail. However, the specific role of His-37 in the tetrameric protein has not been elucidated. Here, we have characterized the pK a s associated with this cluster of four histidine residues in the hydrophobic interstices of the membrane. These pK a values have led us to substantial mechanistic conclusions.There are many lines of evidence, reviewed by Kelly et al. (6), that support the conclusion that M2 is responsible for viral acidification. In vivo ion conductance recordings have shown pH sensitive conductance resulting in rapid acidification of the Xenopus oocytes (12, 13) and mammalian cells (13-15) containing M2 protein.Preparations of purified M2 protein have also been used to show proton conductance in synthetic lipid bilayers (16,17). Singlechannel conductance measurements with membranes containing M2 protein give clear evidence that it is H ϩ conductance, not counterion conductance, that is observed. Furthermore, the channel conductance is unchanged by addition of an excess of NaCl (18). Conductance measurements for the isolated TM domain of M2 protein have als...

Avibactam is a β-lactamase inhibitor that is in clinical development, combined with β-lactam partners, for the treatment of bacterial infections comprising Gram-negative organisms. Avibactam is a structural class of inhibitor that does not contain a β-lactam core but maintains the capacity to covalently acylate its β-lactamase targets. Using the TEM-1 enzyme, we characterized avibactam inhibition by measuring the on-rate for acylation and the offrate for deacylation. The deacylation off-rate was 0.045 min −1 , which allowed investigation of the deacylation route from TEM-1. Using NMR and MS, we showed that deacylation proceeds through regeneration of intact avibactam and not hydrolysis. Other than TEM-1, four additional clinically relevant β-lactamases were shown to release intact avibactam after being acylated. We showed that avibactam is a covalent, slowly reversible inhibitor, which is a unique mechanism of inhibition among β-lactamase inhibitors.antibacterial | drug discovery | enzymology T here is an urgent need for new antibacterial agents that are active against drug-resistant bacteria. In particular, some Gram-negative pathogens have accumulated enough resistance mechanisms to render them virtually untreatable by modern antibacterial chemotherapy (1, 2). A mainstay for treatment of Gram-negative infections is the β-lactam classes of drugs. The most common form of resistance to β-lactam antibiotics is the expression of various β-lactamase enzymes capable of hydrolyzing the β-lactam ring of β-lactam drugs, rendering them ineffective. As new β-lactams have been introduced into clinical use, a changing landscape of β-lactamases has been selected and disseminated. Presently, over 1,000 β-lactamases have been documented comprising several structural classes and a wide range of substrate promiscuities and catalytic efficiencies (3, 4).In efforts to restore the efficacy of β-lactam antibiotics, β-lactamases have also been targeted with a variety of inhibitors (5, 6). The three inhibitors approved for clinical use are clavulanic acid, tazobactam, and sulbactam, all of which contain a β-lactam core. A challenge for the development of broad-spectrum β-lactamase inhibitors is the mechanistic diversity in β-lactamase enzymes, with the largest distinction being between the enzyme classes that use a serine residue as the nucleophilic species and the metallo-β-lactamases, which directly activate water for hydrolysis (7). A shared mechanistic feature of the marketed β-lactam-based inhibitors is their reaction with the serine enzymes to form a covalent acylenzyme intermediate. On ring opening, the acyl-enzyme intermediate can undergo additional rearrangements or be released through hydrolysis to regenerate the active β-lactamase enzyme (8). Originally designed to combat class A serine β-lactamase enzymes such as TEM-1, the clinical use of β-lactam-based inhibitors has been diminished by the emergence of enzymes against which they are ineffective. Despite intense investigation by pharmaceutical companies, no new β-lactamas...

Background:Avibactam is a ␤-lactamase inhibitor with a broad spectrum of activity. Results: Kinetic parameters of inhibition as well as acyl enzyme stability are reported against six clinically relevant enzymes. Conclusion: Inhibition efficiency is highest against class A, then class C, and then class D. Significance: These base-line inhibition values across enzyme classes provide the foundation for future structural and mechanistic enzymology experiments.