[Todos] Seminario de Química Orgánica, lunes 25. 11.30 hs

[Carla Marino]

SEMINARIO DE QUÍMICA ORGÁNICA 

http://www.qo.fcen.uba.ar/Cursos/semin.htm

Aula Dr. Venancio Deulofeu

Departamento de Química Orgánica

Ciudad Universitaria, Pab. II, Piso 3

 

Lunes 25 de Septiembre, 11.30 hs

 

Alfred D. French

USDA - SRRC (United States Department of Agriculture - Southern Regional Research Center), Nueva Orleáns

 

 Modeling of cellobiose- the shortest cellulose chain

 

Cellulose is important in many aspects of biology as well as being an important industrial molecule. It is the main component of plant cell walls, and is used in paper, textiles and building materials. In particular, it is the primary molecule in cotton fiber, the reason for our continuing interest at the U. S. Department of Agriculture's Southern Regional Research Center. The relation between cotton fiber and moisture is currently an interesting topic because the moisture affects the mechanical and other properties related to fiber quality. For example, cotton is an unusual carbohydrate because it is stronger when it is wet! Like other polysaccharides, cellulose is often crystalline, but it appears that non-crystalline components are more interesting. The noncrystalline components are very difficult to characterize experimentally, so molecular modeling is an appropriate tool for study.

Among our approaches, we have made Ramachandran energy surfaces with quantum mechanics to study the shortest cellulose chain, cellobiose. This was a massive study, involving some 15,000 energy minimizations that each took several hours of cpu time. More than 100 different arrangements of the primary and secondary groups were considered, and the results were examined in some detail to understand the contributions of these groups to the energy surface.

The final map was quite predictive of crystal structures, somewhat to our surprise, because similar surfaces made with molecular mechanics require elevated dielectric constants to provide predictive maps. Still, the work confirmed earlier reports that the global energy minimum for cellobiose is for a structure that is not near the structures observed by diffraction, as proposed by Strati et al. Although 24 different "starting geometries" contributed to the central 81 point portion of the map, the low energies were often obtained by distortions of several low energy structures. Using only the starting geometry that gave the lowest overall energy would give discrepancies of up to 6 kcal/mol with the final adiabatic map. Probing the high-energy regions showed that the maximum energy was about 30 kcal/mol.

For comparison we have made new energy surfaces with MM3, MM4 and AMBER/GLYCAM-

04 for both cellobiose and its analog based on tetrahydropyran. Hybrid QM/MM methods can be used, given the MM disaccharide and analog maps and QM analog maps. These will also be discussed.

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