N@a and N@d: Oligomer and Partner Specification by Asparagine in Coiled-Coil Interfaces.

The α-helical coiled coil is one of the best-studied protein-protein interaction motifs. As a result, sequence-to-structure relationships are available for the prediction of natural coiled-coil sequences and the de novo design of new ones. However, coiled coils adopt a wide range of oligomeric states and topologies, and our understanding of the specification of these and the discrimination between them remains incomplete. Gaps in our knowledge assume more importance as coiled coils are used increasingly to construct biomimetic systems of higher complexity; for this, coiled-coil components need to be robust, orthogonal, and transferable between contexts. Here, we explore how the polar side chain asparagine (Asn, N) is tolerated within otherwise hydrophobic helix-helix interfaces of coiled coils. The long-held view is that Asn placed at certain sites of the coiled-coil sequence repeat selects one oligomer state over others, which is rationalized by the ability of the side chain to make hydrogen bonds, or interactions with chelated ions within the coiled-coil interior of the favored state. We test this with experiments on de novo peptide sequences traditionally considered as directing parallel dimers and trimers, and more widely through bioinformatics analysis of natural coiled-coil sequences and structures. We find that when located centrally, rather than near the termini of such coiled-coil sequences, Asn does exert the anticipated oligomer-specifying influence. However, outside of these bounds, Asn is observed less frequently in the natural sequences, and the synthetic peptides are hyperthermostable and lose oligomer-state specificity. These findings highlight that not all regions of coiled-coil repeat sequences are equivalent, and that care is needed when designing coiled-coil interfaces.

Aminophobanes: hydrolytic stability, tautomerism and application in Cr-catalysed ethene oligomerisation.

9-Amino-9-phosphabicyclo[3.3.1]nonanes, (PhobPNHR'; R = Me or (i)Pr) are readily prepared by aminolysis of PhobPCl and are significantly less susceptible to hydrolysis than the acyclic analogues Cy2PNHR'. Treatment of Cy2PNHMe with Cy2PCl readily gave Cy2PNMePCy2. By contrast, treatment of PhobPCl with PhobPNHMe in the presence of Et3N does not afford PhobPNMePPhob but instead the salt [PhobP(= NMeH)PPhob]Cl is formed which, upon addition of [PtCl2(NC(t)Bu)2] gives the zwitterionic complex [PtCl3(PhobP(= NMeH)PPhob)]. The neutral PhobP(= NMe)PPhob is accessible from PhobNMeLi and is converted to the chelate [PdCl2(PhobPNMePPhob)] by addition of [PdCl2(cod)]. The anomalous preference of the PhobP group for the formation of PPN products is discussed. The unsymmetrical diphos ligands PhobPNMePAr2 (Ar = Ph, o-Tol) are prepared, converted to [Cr(CO)4(PhobPNMePAr2)] and shown to form Cr-catalysts for ethene oligomerisation, producing a pattern of higher alkenes that corresponds to a Schulz-Flory distribution overlaid on selective tri/tetramerisation.

Computational kinetics of cobalt-catalyzed alkene hydroformylation.

Density functional theory, coupled-cluster theory, and transition state theory are used to build a computational model of the kinetics of phosphine-free cobalt-catalyzed hydroformylation and hydrogenation of alkenes. The model provides very good agreement with experiment, and enables the factors that determine the selectivity and rate of catalysis to be determined. The turnover rate is mainly determined by the alkene coordination step.