The bacterial toxin colicin E9 is secreted by producing Escherichia coli cells with its 9.5 kDa inhibitor protein Im9 bound tightly to its 14.5 kDa C-terminal DNase domain. Double- and triple-resonance NMR spectra of the isolated DNase domain uniformly labeled with 13C/15N bound to unlabeled Im9 contain more signals than expected for a single DNase conformer, consistent with the bound DNase being present in more than one form. The presence of chemical exchange cross peaks in 750 MHz 15N–1H–15N HSQC–NOESY–HSQC spectra for backbone NH groups of Asp20, Lys21, Trp22, Leu23, Lys69, and Asn70 showed that the bound DNase was in dynamic exchange. The rate of exchange from the major to the minor form was determined to be 1.1 ± 0.2 s−1 at 298 K. Previous NMR studies have shown that the free DNase interchanges between two conformers with a forward rate constant of 1.61 ± 0.11 s−1 at 288 K, and that the bound Im9 is fixed in one conformation. The NMR studies of the bound DNase show that Im9 binds similarly to both conformers of the DNase and that the buried Trp22 is involved in the dynamic process. For the free DNase, all NH groups within a 9 Å radius of any point of the Trp22 ring exhibit heterogeneity suggesting that a rearrangement of the position of this side chain is connected with the conformational interchange. The possible functional significance of this feature of the DNase is discussed.

Here we describe the NMR conformational study of a 20-residue linear peptide designed to fold into a monomeric three-stranded antiparallel β-sheet in aqueous solution. Experimental and statistical data on amino acid β-turn and β-sheet propensities, cross-strand side-chain interactions, solubility criteria, and our previous experience with β-hairpins were considered for a rational selection of the peptide sequence. Sedimentation equilibrium measurements and NMR dilution experiments provide evidence that the peptide is monomeric. Analysis of 1H and 13C-NMR parameters of the peptide, in particular NOEs and chemical shifts, and comparison with data obtained for two 12-residue peptides encompassing the N- and C-segments of the designed sequence indicates that the 20-residue peptide folds into the expected conformation. Assuming a two-state model, the exchange kinetics between the β-sheet and the unfolded peptide molecules is in a suitable range to estimate the folding rate on the basis of the NMR linewidths of several resonances. The time constant for the coil-β-sheet transition is of the order of several microseconds in the designed peptide. Future designs based on this peptide system are expected to contribute greatly to our knowledge of the many factors involved in β-sheet formation and stability.

Thermal, mechanical and chemical exchange occurs between felsic and mafic magmas in dynamic magma systems. The occurrence and efficiency of such exchanges are constrained mainly by the intensive parameters, the compositions, and the mass fractions of the coexisting magmas. As these interacting parameters do not change simultaneously during the evolution of the granite systems, the exchanges appear sequentially, and affect magmatic systems at different structural levels, i.e. in magma chambers at depth, in the conduits, or after emplacement. Hybridisation processes are especially effective in the plutonic environment because contrasting magmas can interact over a long time-span before cooling. The different exchanges are complementary and tend to reduce the contrasts between the coexisting magmas. They can be extensive or limited in space and time; they are either combined into mixing processes which produce homogeneous rocks, or only into mingling processes which produce rocks with heterogeneities of various size-scales. Mafic microgranular enclaves represent the most common heterogeneities present in the granite plutons. The composite enclaves and the many types of mafic microgranular enclaves commonly associated in a single pluton, or in polygenic enclave swarms, are produced by the sequential occurrence of various exchanges between coexisting magmas with constantly changing intensive parameters and mass fractions. The complex succession and repetition of exchanges, and the resulting partial chemical and complete isotopic equilibration, mask the original identities of the initial components.