Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo2(4+) compounds and their bicyclic guanidinate precursors.

A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo2(4+) units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo2(TEhpp)4 complex which has a broad first ionization band centered around 4.27 +/- 0.03 eV and an ionization onset at the very low energy of 3.93 +/- 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (eta5-C9Me7)2Co and (eta5-C5Me5)2Cr.

Evolutionary bridges to new protein folds: design of C-terminal Cro protein chameleon sequences.

Regions of amino-acid sequence that are compatible with multiple folds may facilitate evolutionary transitions in protein structure. In a previous study, we described a heuristically designed chameleon sequence (SASF1, structurally ambivalent sequence fragment 1) that could adopt either of two naturally occurring conformations (α-helical or ß-sheet) when incorporated as part of the C-terminal dimerization subdomain of two structurally divergent transcription factors, P22 Cro and λ Cro. Here we describe longer chameleon designs (SASF2 and SASF3) that in the case of SASF3 correspond to the full C-terminal half of the ordered region of a P22 Cro/λ Cro sequence alignment (residues 34-57). P22-SASF2 and λ(WDD)-SASF2 show moderate thermal stability in denaturation curves monitored by circular dichroism (T(m) values of 46 and 55°C, respectively), while P22-SASF3 and λ(WDD)-SASF3 have somewhat reduced stability (T(m) values of 33 and 49°C, respectively). (13)C and (1)H NMR secondary chemical shift analysis confirms two C-terminal α-helices for P22-SASF2 (residues 36-45 and 54-57) and two C-terminal ß-strands for λ(WDD)-SASF2 (residues 40-45 and 50-52), corresponding to secondary structure locations in the two parent sequences. Backbone relaxation data show that both chameleon sequences have a relatively well-ordered structure. Comparisons of (15)N-(1)H correlation spectra for SASF2 and SASF3-containing proteins strongly suggest that SASF3 retains the chameleonism of SASF2. Both Cro C-terminal conformations can be encoded in a single sequence, showing the plausibility of linking different Cro folds by smooth evolutionary transitions. The N-terminal subdomain, though largely conserved in structure, also exerts an important contextual influence on the structure of the C-terminal region.

Relationship between sequence determinants of stability for two natural homologous proteins with different folds.

In the Cro protein family, an evolutionary change in secondary structure has converted an alpha-helical fold to a mixture of alpha-helix and beta-sheet. P22 Cro and lambda Cro represent the ancestral all-alpha and descendant alpha+beta folds, respectively. The major structural differences between these proteins are at the C-terminal end of the domain (residues 34-56), where two alpha-helices in P22 Cro align with two beta-strands in lambda Cro. We sought to assess the possibility that smooth evolutionary transitions could have converted the all-alpha structure to the alpha+beta structure through sequences that could adopt both folds. First, we used scanning mutagenesis to identify and compare patterns of key stabilizing residues in the C-terminal regions of both P22 Cro and lambda Cro. These patterns exhibited little similarity to each other, with structurally important residues in the two proteins most often occurring at different sequence positions. Second, "hybrid scanning" studies, involving replacement of each wild-type residue in P22 Cro with the aligned wild-type residue in lambda Cro and vice versa, revealed five or six residues in each protein that strongly destabilized the other. These results suggest that key stability determinants for each Cro fold are quite different and that the P22 Cro sequence strongly favors the all-alpha structure while the lambda Cro sequence strongly favors the alpha+beta structure. Nonetheless, we were able to design a "structurally ambivalent" sequence fragment (SASF1), which corresponded to residues 39-56 and simultaneously incorporated most key stabilizing residues for both P22 Cro and lambda Cro. NMR experiments showed SASF1 to stably fold as a beta-hairpin when incorporated into the lambda Cro sequence but as a pair of alpha-helices when incorporated into P22 Cro.

A trade between similar but nonequivalent intrasubunit and intersubunit contacts in Cro dimer evolution.

The homodimeric lambda Cro protein has a "ball-and-socket" interface that includes insertion of an aromatic side chain, Phe 58, from each subunit into a cavity in the hydrophobic core of the other subunit. This overlap between the subunit core and dimer interface hypothetically explains the strong dimerization and weak monomer stability of lambda Cro in comparison to homologues. According to a model developed here and in a previous study [LeFevre, K. R., and Cordes, M. H. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 2345-2350], the socket cavity evolved in part by replacement of a buried tryptophan in an ancestral stable monomer with a smaller side chain (Ala 33 in lambda Cro). The resulting core defect was in effect repaired by insertion of a different side chain (Phe 58) from a second subunit, generating the ball and socket. Consistent with such an evolutionary trade between intrasubunit and intersubunit interactions, we showed in the previous study that restoration of the ancestral Trp 33 in lambda Cro stabilized the monomer and reduced the extent of dimerization. Here, we report the solution structure of a stable lambda Cro monomer containing the Ala33Trp mutation, which confirms that the restored tryptophan fulfills its ancestral role as a core side chain, filling part of the socket cavity occupied by Phe 58 in the wild-type dimer. The structure also reveals, however, that the cavity is not completely filled by Trp 33, suggesting that its formation could have involved multiple mutations that reduced side chain volume. We offer suggestive evidence of a role of mutations at a second position.

Facilitating access to the most easily ionized molecule: an improved synthesis of the key intermediate, W2(hpp)4Cl2, and related compounds.

A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.

Closed-shell molecules that ionize more readily than cesium.

We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.