A secreted form of a class I major histocompatibility complex (MHC) molecule was denatured and renatured in vitro in the absence of peptide. The resulting empty class I heterodimer was immunologically reactive and structurally similar to a heterodimer renatured in the presence of an appropriate restricted peptide. Thermal stability profiles indicated that the two forms of heterodimer differed in their resistance to denaturation by heat but that a significant portion of the empty class I heterodimers had a native conformation at physiological temperatures. Free energies calculated from these data gave a direct measure of the stabilization of the class I MHC molecule that resulted from peptide binding.
The velocity of rouleaux formation (RF), as previously shown, increases with increasing dextran concentration up to a critical concentration (Ca), beyond which the addition of dextran reduces the RF velocity (RFV). de Gennes' model for polymer solutions suggests that dextrans exist in two conformations: a coil structure at low concentrations, which changes to a network beyond a critical concentration (C*). In the present study we examined the relation between Ca and C* for dextrans of different molecular weight, and found that they coincide. This suggests that the change in dextran behavior, from increasing to decreasing RFV, occurs when their conformation changes from coil to network. In addition, it has been reported that in dilute dextran solutions the intercellular distance (D) between RBC in rouleaux increases with the molecular weight of the dextran. We found that D correlates with Rf, the end-to-end distance of the polymer molecule, and for all dextrans D < or = 1.5 Rf. In accord with de Gennes' Model for polymers between surfaces, this corresponds to intercellular interaction with two overlapping surface-associated polymer layers, which may extend "tails" to interact with the opposing cells.
Conformational energy maps have been computed for the antiepileptic agents phenytoin and cannabidiol by the quantum-mechanical method of perturbative configuration interaction with localized orbitals (PCILO). The computation indicates that the spatial relationship between the two rings in the two drugs is similar and close to the respective structures in the crystal. This is supported by 1H and 13C NMR measurements. Hence, both compounds fulfill the stereochemical requirements suggested for anticonvulsant drug action.
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