Cyclo-β-peptides: Structure and tubular stacking of cyclic tetramers of 3-aminobutanoic acid as determined from powder diffraction data

The solid-state structures of three stereoisomer, 1–3 , of the cyclic tetramer of 3-aminobutanoic acid are presented. These cyclo-β-peptides were found to be highly insoluble materials, and it proved to be impossible to grow crystals of sufficient quality for X-ray single-crystal analysis. The samples of 1–3 were, however, suitable candidates for structure determination from powder diffraction data (Fig. 1), and the application of this method is described. All three isomers have been found to adopt tubular structures (Figs. 2–4) in a fashion similar to those already observed for certain cyclo-α-peptides. The stacks of 16-membered rings are held together by four nonlinear C?O…?H? N H-bonds between pairs of molecules (Fig.5).     

Intramolecular energy transfer involving heisenberg spin-coupled dinuclear iron-oxo complexes

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing pendant naphthalene groups are described. The compounds [Fe(2)O(O(2)CCH(2)-C(10)H(7))(tren)(2)](BPh(4))(NO(3))(2) (8), [Fe(2)O(O(2)CCH(2)-C(10)H(7))(TPA)(2)](ClO(4))(3) (9), Fe(2)O(O(2)CCH(2)-C(10)H(7))(2)(Tp)(2) (10), and Fe(2)O((O(2)CCH(2)CH(2))(2)-C(10)H(6))(Tp)(2) (11) (where tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. -130 cm(-)(1) for H = -2JS(1).S(2)). Room-temperature steady-state emission spectra for compounds 8-11 in deoxygenated CH(3)CN solution reveal spectral profiles similar to methyl-2-naphthyl acetate and [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) (13, where TACN-Me(3) is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to simple exponential decay models with tau(obs)(S)()1 = 36 +/- 2, 32 +/- 4, 30 +/- 5, and 39 +/- 3 ns for compounds 8-11, respectively. The decays are assigned to the S(1) --> S(0) fluorescence of naphthalene; all of the lifetimes are less than that of the zinc model complex (tau(obs)(S)()1 = 45 +/- 2 ns), indicating quenching of the S(1) state by the iron-oxo core. Nanosecond time-resolved absorption data on [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) reveal a feature at lambda(max) = 420 nm that can be assigned as the T(1) --> T(n) absorption of the naphthalene triplet; the rise time of 50 +/- 10 ns corresponds to an intersystem crossing rate of 2 x 10(7) s(-1). A similar feature (though much weaker in intensity) is also observed for compound 8. The order-of-magnitude reduction in the T(1) lifetime of the pendant naphthalene for all of the iron-oxo complexes (tau(obs)(T)1 = 5 +/- 2 micros vs 90 +/- 10 micros for [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4))) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene radical cation nor the reduced Fe(II)Fe(III) species were observed by transient absorption spectroscopy, implying that energy transfer is the most likely origin for the quenching of both the S(1) and T(1) states. Spectral overlap considerations strongly support a F?rster (i.e., dipolar) mechanism for energy transfer from the S(1) state, whereas the lack of phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the diiron(III) core is in part responsible for the unusual ability of the iron-oxo core to engage in energy transfer from both the singlet and triplet manifolds of naphthalene is discussed.     

Picosecond X-ray absorption spectroscopy of a photoinduced iron(II) spin crossover reaction in solution

In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron-nitrogen (Fe-N) bond is lengthened by 0.21+/-0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change in the metal-ligand interactions as reflected by the altered density of states of the unoccupied metal orbitals. Our results constitute the first direct measurements of the dynamic atomic and electronic structural rearrangements occurring during a photoinduced FeII spin crossover reaction in solution via picosecond X-ray absorption spectroscopy.     

Intramolecular excimer formation in a naphthalene-appended dinuclear iron-oxo complex

The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(O)(O2CCH2C10H7)2(TACN-Me3)2]2+ (3) crystallizes in space group P1 with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c = 15.23(2) A, alpha = 101.12(7) degrees, beta = 90.8(1) degrees, gamma = 114.14(7) degrees, V = 2605(6) A3, and Z = 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin FeIII centers are antiferromagnetically coupled with J = -105 cm-1 (H = -2 JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CH3CN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2]+ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, phi = 0.13) or the ZnII analogue (2, phi = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with tau 1 = 4.6 +/- 1 ns and tau 2 = 47 +/- 1 ns. The latter compares favorably with that found for 2 (tau = 47 +/- 1 ns) and is assigned as the S0-S1 fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.     

Asset replacement for an urban railway using a modified two-cycle replacement model

This paper considers the application of capital replacement models at Mass Transit Railway Corporation Limited (MTRCL), Hong Kong. A particular characteristic of the replacement problems considered is that costs relating to existing equipment are generally constant or increasing only slowly. Consequently, replacement is often driven by technical obsolescence, but other criteria are used for informing decisions. The applicability of traditional OR models of replacement is then problematic. We recommend the use of a modified two-cycle replacement model and compare this model to existing capital replacement models. Issues relating to the estimation of delay costs and failure consequences and their influence on the replacement decision are also considered—this is done using a fixed horizon model, which is a special case of the modified two-cycle model. Track points and escalators are used as particular examples. In addition to modelling recommendations, we discuss the management of asset replacement with emphasis on the procedures necessary to ensure that asset replacement requirements are considered appropriately and effectively. The paper treats, in particular, the procedural issues of asset replacement, and the discussion of asset replacement system methodology reflects the current practise at MTRCL, Hong Kong, and developments within that organization through collaboration with academia. The modified two-cycle replacement model is recommended by us for general replacement applications. The asset replacement procedure is presented as an exemplar for business and industry.     

Metal-peptide frameworks (MPFs): "bioinspired" metal organic frameworks

Chiral metal-organic frameworks (MOFs) have attracted a growing interest for their potential use in energy technologies, asymmetric catalysis, chiral separation, and on a more basic level, the creation of new topologies in inorganic materials. The current paper is the first report on a peptide-based MOF, a metal peptide framework (MPF), constructed from an oligovaline peptide family developed earlier by our group (Mantion, A.; et al. Macromol. Biosci. 2007, 7, 208). We have used a simple oligopeptide, Z-(L-Val)2-L-Glu(OH)-OH, to grow porous copper and calcium MPFs. The MPFs form thanks to the self-assembling properties of the peptide and specific metal-peptide and metal-ammonia interactions. They are stable up to ca. 250 degrees C and have some internal porosity, which makes them a promising prototype for the further development of MPFs.