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The MM2 potential functions for amides and peptides have been further extended by examining the experimental crystal structures for cyclo-(-Ala-Ala-Gly-Gly-Ala-Gly-), I, and cyclo-(-Ala-Ala-Gly-Ala-GlyGly-1, 11. The force field obtained was then applied to a study of the structure of the hydrophobic protein Crambin, for which a high resolution crystal structure is available. The energy minimization was carried out using a version of MM2 adapted to the CYBER 205.

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The thermodynamic properties of (CO2) N molecular aggregates of size 2 ⩽ N ⩽ 13 have been investigated. These crystallites exhibit well defined orientational order–disorder rotational transitions accompanied by a structural transition into a plastic crystallite phase. In addition, they exhibit melting and disassociation transitions. It is shown that the interpretation of experimental data, based upon dimer properties, depends crucially on these results. Equilibrium structures and orientations are also given.

Abstract:The refractive index (RI) is an important parameter in describing the radiative impacts of aerosols. It is important to constrain the RI of aerosol components, since there is still significant uncertainty regarding the RI of biomass burning aerosols. Experimentally measured extinction cross-sections, scattering cross-sections, and single scattering albedos for white pine biomass burning (BB) aerosols under two different burning and sampling conditions were modeled using T-matrix theory. The refractive indices were extracted from these calculations. Experimental measurements were conducted using a cavity ring-down spectrometer to measure the extinction, and a nephelometer to measure the scattering of size-selected aerosols. BB aerosols were obtained by burning white pine using (1) an open fire in a burn drum, where the aerosols were collected in distilled water using an impinger, and then re-aerosolized after several days, and (2) a tube furnace to directly introduce the BB aerosols into an indoor smog chamber, where BB aerosols were then sampled directly. In both cases, filter samples were also collected, and electron microscopy images were used to obtain the morphology and size information used in the T-matrix calculations. The effective radius of the particles collected on filter media from the open fire was approximately 245 nm, whereas it was approximately 76 nm for particles from the tube furnace burns. For samples collected in distilled water, the real part of the RI increased with increasing particle size, and the imaginary part decreased. The imaginary part of the RI was also significantly larger than the reported values for fresh BB aerosol samples. For the particles generated in the tube furnace, the real part of the RI decreased with particle size, and the imaginary part was much smaller and nearly constant. The RI is sensitive to particle size and sampling method, but there was no wavelength dependence over the range considered (500-680 nm). Our values for the RI of fresh (white pine) biomass burning aerosols ranged from 1.33 + i0.008 to 1.74 + i0.008 for 200-nm, 300-nm, and 400-nm diameter particles. These are within the range of RI values in the most recent study conducted during the Fire Laboratory at Missoula Experiments (FLAME I and II), which were 1.55 to 1.80 for the real part, and 0.01-0.50 for the imaginary part, for fresh BB aerosols with diameters of 200-570 nm. There is no clear trend on the dependence of the RI values on particle size. The RI values derived from measurements of aerosols produced from the combustion of hydrocarbons and diesel cannot be used for BB aerosols.

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