A quantum-chemical picture of hemoglobin affinity.

Understanding the molecular mechanism of hemoglobin cooperativity remains an enduring challenge. Protein forces that control ligand affinity are not directly accessible by experiment. We demonstrate that computational quantum mechanics/molecular mechanics methods can provide reasonable values of ligand binding energies in Hb, and of their dependence on allostery. About 40% of the binding energy differences between the relaxed state and tense state quaternary structures result from strain induced in the heme and its ligands, especially in one of the pyrrole rings. The proximal histidine also contributes significantly, in particular, in the alpha-chains. The remaining energy difference resides in protein contacts, involving residues responsible for locking the quaternary changes. In the alpha-chains, the most important contacts involve the FG corner, at the "hinge" region of the alpha(1)beta(2) quaternary interface. The energy differences are spread more evenly among the beta-chain residues, suggesting greater flexibility for the beta- than for the alpha-chains along the quaternary transition. Despite this chain differentiation, the chains contribute equally to the relaxed substitute state energy difference. Thus, nature has evolved a symmetric response to the quaternary structure change, which is a requirement for maximum cooperativity, via different mechanisms for the two kinds of chains.

Micrometer-sized short-lived radioactive aerosol particles for convenient use in laboratory measurements.

For calibration and testing of radioactive aerosol measuring equipment such as continuous air monitors and cascade impactors, and other research applications, it is helpful to have a convenient and relatively safe means of producing radioactive aerosol particles of controlled size and activity. We describe a technique for producing such particles in the micrometer-diameter size range using electrostatic deposition of radon decay products onto otherwise nonradioactive powders of different sizes. An electric field focuses radon decay products (primarily 218Po) onto the surface of a powdered substrate that is then suspended by a technique such as pneumatic dry dispersion. Only a modest-activity commercial 222Rn source (e.g., containing as little as 10(5) Bq of 226Ra) is required, and issues of radioactive cleanup and contamination are minimized due to the short half-lives (26.8 min or less) of the decay products. We report representative results using powders of glass beads, iron oxide, and iron and gold metals in the size range of 0.3 to 30 microm. Yields for the deposited radioactivity per unit concentration of 222Rn gas were of the order of 5 x 10(-7) Bq (214Bi) per milligram substrate per Bq m(-3) of 222Rn for an electrostatic collection time of 30 min.