No abstract

Infrared absorption bands that have previously been assigned to vibrations of the dangling bonds (db) of water molecules at the surfaces of crystalline ice clusters and the micropores of amorphous ice have been investigated to determine their usefulness in probing molecular-level events at icy surfaces. This study has established that the db bands are sensitive to adsorption of gases at the cluster or micropore surfaces, and that reversible adsorption/desorption at the db sites is readily monitored spectroscopically. Consequently, energetics for the interactions with adsorbents such as H2 and N2 are potentially measurable. It has also been demonstrated, for both clusters and the micropores, that surface HOD molecules give unique db band positions, and that the intensities of the db bands are indicative of a strong preference of the surface HOD molecules to engage in deuterium bonding to the subsurface molecules. The unique positions of the HOD db bands also signals a potential for using isotopic-exchange data to monitor point-defect activity at icy surfaces.

The details of isotopic exchange reactions in amorphous ice (amorphous solid water, ASW) have been followed spectroscopically in an effort to establish the degree to which the exchange mechanism is analogous to that of cubic ice for which exchange is based on sequential action of mobile ion (H+) and orientational (L) point defects. Because pure amorphous ice is proton inactive below its crystallization temperature, the necessary H+ have been injected into annealed ASW, containing intact isolated D2O molecules, by the photoexcitation of dopant 2-naphthol at 80 K. The limited isotopic exchange that occurred during UV irradiation and the rate data obtained for exchange at temperatures ranging from 108 to 125 K are shown to be consistent with a point-defect model having the following specific characteristics: (1) the protons injected at 80 K, while migrating short distances before being shallowly trapped, convert some D20 to dynamically coupled (HOD)2 units; (2) the "L" defects and trapped protons are immobilized below -100 K, but both are thermally activated at the temperatures of the rate measurements; (3) above -115 K quasi-equilibria develop between isotopomeric units as expected for an icelike point-defect mechanism having the "L" defects much more active than the H' ions. By contrast, the exchange data are inconsistent with the occurrence of any significant molecular diffusional motion, an observation that suggests that the molecular motion that develops at the glass transition temperature of ASW is orientational diffusion based on L-defect activity. From this view, fluidity is not expected for ASW when warmed above Tg (-130 K, measured values depend on warmup rates) any more than for cubic ice above its Tg (-140 K).

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We analyzed the 440-kDa transmembrane pore formed by the protective antigen (PA) moiety of anthrax toxin in the presence of GroEL by negative-stain electron microscopy. GroEL binds both the heptameric PA prepore and the PA pore. The latter interaction retards aggregation of the pore, prolonging its insertion-competent state. Two populations of unaggregated pores were visible: GroEL-bound pores and unbound pores. This allowed two virtually identical structures to be reconstructed, at 25-Å and 28-Å resolution, respectively. The structures were mushroom-shaped objects with a 125-Å-diameter cap and a 100-Å-long stem, consistent with earlier biochemical data. Thus, GroEL provides a platform for obtaining initial glimpses of a membrane protein structure in the absence of lipids or detergents and can function as a scaffold for higher-resolution structural analysis of the PA pore.