The exploitation of biological macromolecules, such as nucleic acids, for the fabrication of advanced materials is a promising area of research. Although a greater variety of structural and functional uses can be envisioned for protein-based materials, systematic approaches for their construction have yet to emerge. Consistent with theoretical models of polymer macrocyclization, we have demonstrated that, in the presence of dimeric methotrexate (bisMTX), wild-type Escherichia coli dihydrofolate reductase (DHFR) molecules tethered together by a flexible peptide linker (ecDHFR(2)) are capable of spontaneously forming highly stable cyclic structures with diameters ranging from 8 to 20 nm. The nanoring size is dependent on the length and composition of the peptide linker, on the affinity and conformational state of the dimerizer, and on induced protein-protein interactions. Delineation of these and other rules for the control of protein oligomer assembly by chemical induction provides an avenue to the future design of protein-based materials and nanostructures.

Chemically Controlled Self-Assembly of Protein Nanorings

The aggregates in heated bovine β-lactoglobulin solutions were separated by high-performance size-exclusion chromatography, and molecular characteristics were measured on-line using multiangle laser-light scattering detection (SEC-MALLS). This technique was proven to be a very useful and valuable approach for characterization of heat-induced β-lactoglobulin aggregates. Using TSK G2000 SW XL and TSK G4000 SW XL silica gel columns connected in series, aggregates up to a molecular mass of 4 x 10 6 Da could be separated, and complete molecular mass distributions were derived. Up to a molecular mass of 1.5 x 10 6 Da the molecular masses calculated by the MALLS agreed very well with the results obtained by conventional calibration. The measured molecular mass distributions of the heat-induced β-lactoglobulin aggregates varied strongly with experimental heating conditions (β-lactoglobulin concentration, pH, heating temperature, ionic strength). The results obtained with several β-lactoglobulin concentrations at neutral pH were consistent with a kinetic aggregation model based on thiol/disulfide exchange reactions. The average molecular mass and the radius of gyration of the heat-induced β-lactoglobulin aggregates increased with increasing initial β-lactoglobulin concentration.

Molecular Mass Distributions of Heat-Induced β-Lactoglobulin Aggregates

The stereochemistry of sigmatropic rearrangements. Tests of the predictive power of orbital symmetry rules

Aus dem sterischen Verlauf und aus der Geschwindigkeit der thermischen Isomerisierung von Cyclobutenen zu Butadienen folgt ein grundsatzlicher Unterschied im Mechanismus fur monocyclische Cyclobutene und fur Cyclobutene, die in 3.4-Stellung mit 4-, 5- oder 6-gliedrigen Ringen kondensiert sind. Nur im ersten Fall leistet die Doppelbindung Hilfestellung fur die Ringoffnung, wahrend im zweiten Fall die Ringspannung als einzige treibende Kraft wirkt.

Valenzisomerisierungen von Cyclobutenen

QSPR correlation equations were developed for the prediction of the solubilities of organic gases and vapors in water. A two-parameter correlation with the squared correlation coefficient R2 = 0.977 gives excellent predictions for 95 alkanes, cycloalkanes, alkenes, alkylarenes, and alkynes. A satisfactory description (R2 = 0.941) of the gas solubilities of 406 organic compounds with a large structural variablity was obtained using a five-parameter QSPR equation. Notably, all the parameters involved in these equations can be derived solely from the chemical structure of the compounds which makes them very useful for the prediction of the solubilities of unknown or unavailable compounds.

W. D. Walters

Papers

The kinetics of the thermal decomposition of isopropylcyclobutane1

The Kinetics of the Thermal Decomposition of Methyl Cyclobutanecarboxylate