INTRAMOLECULAR VIBRATIONS FROM MOLECULAR-DYNAMICS SIMULATIONS OF LIQUID WATER

Des simulations, par dynamique moléculaire, de l'eau en utilisant un modèle moléculaire rigide conduisent à une description statistique des forces agissant sur les degrés de liberté internes. Ceci détermine la perturbation des oscillateurs internes, à partir de laquelle les spectres peuvent être calculés au niveau fondamental. On montre que les spectres sont déterminés par une distribution de forces, qui induisent un déplacement et un élargissement inhomogène dus à l'anharmonicité des oscillateurs. Cet élargissement et ce déplacement sont en relation avec le nombre et la force des liaisons hydrogène dans lesquelles la molécule participe comme donneur de liaisons hydrogène. Les autres causes de perturbations spectrales (élargissement homogène, couplage intermoléculaire, modulation par le mouvement rotationnel) sont négligeables. La dépendance en température des spectres simulés conduit à un point isosbestique, observé expérimentalement. Ceci confirme la validité d'une description à deux états.Molecular dynamics simulations of liquid water using a rigid molecular model provide statistical characteristics of the forces acting on the internal degrees of freedom. This determines the perturbation of the internal oscillators, from which fundamental spectra can be derived. It turns out that the spectra are determined by the distribution of forces, causing a shift and an inhomogeneous broadening due to anharmonicity of the oscillators. This broadening and shift are related to the number and strength of the hydrogen bonds in which the molecule participates as a hydrogen bonding donor. Further causes of spectral perturbation (homogeneous broadening, intermolecular coupling, modulation by rotational motion) are negligeable. Temperature dependence of simulated spectra produce an isosbestic point, as observed experimentally, indicating the validity of a two state description

Preparation Sequence of a 3-Unit Fixed Partial Denture

Abstract This resource is a video that illustrates a straightforward tooth-preparation sequence for a three-unit fixed partial denture. Reduction accuracy and preparation draw are emphasized through the use of reduction guides and paralleling grooves. The demonstration is recorded on a typodont with Ivorine teeth, using carbide and diamond burs. The video depicts the step-by-step preparation process taught to dental students, prosthodontics residents, and preceptors by the Prosthodontics Department at the University of Texas Dental Branch at Houston. This video is an excellent communication tool that places tooth-preparation techniques in the hands of dental students and residents any time they choose. It also maximizes the students' comprehension and learning experience since they have infinite opportunities to review the material prior to and after class. The video could also be used as a continuing clinical aid, which students can rely upon prior to treating their patients in clinic. It is, by far, a superior communication tool when compared to a static PowerPoint presentation, since the students can observe a live demonstration of abutment preparation for a three-unit fixed partial denture

THE CRYSTAL-STRUCTURE OF THE OPEN AND THE CLOSED CONFORMATION OF THE FLEXIBLE LOOP OF TRYPANOSOMAL TRIOSEPHOSPHATE ISOMERASE

Triosephosphate isomerase has an important loop near the active site which can exist in a "closed" and in an "open" conformation. Here we describe the structural properties of this "flexible" loop observed in two different structures of trypanosomal triosephosphate isomerase. Trypanosomal triosephosphate isomerase, crystallized in the presence of 2.4 M ammonium sulfate, packs as an asymmetric dimer of 54,000 Da in the crystallographic asymmetric unit. Due to different crystal contacts, peptide 167-180 (the flexible loop of subunit-1) is an open conformation, whereas in subunit-2, this peptide (residues 467-480) is in a closed conformation. In the closed conformation, a hydrogen bond exists between the tip of the loop and a well-defined sulfate ion which is bound to the active site of subunit-2. Such an active site sulfate is not present in subunit-1 due to crystal contacts. When the native (2.4 M ammonium sulfate) crystals are transferred to a sulfate-free mother liquor, the flexible loop of subunit-2 adopts the open conformation. From a closed starting model, this open conformation was discovered through molecular dynamics refinement without manual intervention, despite involving C alpha shifts of up to 7 A. The tip of the loop, residues 472, 473, 474, and 475, moves as a rigid body. Our analysis shows that in this crystal form the flexible loop of subunit-2 faces a solvent channel. Therefore the open and the closed conformations of this flexible loop are virtually unaffected by crystal contacts. The actual observed conformation depends only on the absence or presence of a suitable ligand in the active site