A study of blast wave loading on cantilevers

A study has been made of the response of elastic-plastic and brittle circular-cross-section cantilevers when subjected to blast wave loading. It is demonstrated how the deformation or failure of such cantilevers enables them to be used as blast wave gauges. In addition, the deformation of cantilever-type structures can be used to assess the characteristics of accidental explosions. Two numerical models have been developed to describe the deformation of a dynamically loaded cantilever. Both models assume that the plastic deformation is localized in a region near the fixed end, and that the loading force is a function of the dynamic-pressure time-history and a variable drag coefficient, which depends on the Reynolds number, Mach number and angle of attack of each discretized element of the cantilever. The first model assumes a rigid-plastic response of the cantilever. It was found that this model accurately described the response of cantilevers made of 50/50 lead/tin alloy. It overestimated the deformation of cantilevers made of more elastic materials when exposed to blast waves from high explosives and in a shock tube. The second model assumes an elastic-plastic response. The algorithm is based on the premise that the elastic curvature of the cantilever is limited by the plastic yield stress of the material and that as the curvature approaches this limit the cantilever rotates by the amount needed to keep the curvature constant and equal to this maximum. It has been shown that this algorithm minimizes the curvature of the cantilever at the base. This model provided good predictions of the deformation of cantilevers made of aluminum and steel. The numerical models were evaluated by studying the response of cantilevers exposed to shock waves in a shock tube, and to the blast waves from two explosions of ammonium-nitrate/fuel-oil charges of approximately 2.5 kt. The response to the shock tube flows was recorded by high speed photography which showed good agreement between the observed modes of deflection and those predicted by the elastic-plastic model. The models also provided good predictions of the deformation of a wide range of cantilevers, made of a variety of materials and of different diameters and lengths, when exposed to the free field blast waves. It is demonstrated how the numerical models can be used to determine the type of cantilever that might be used as a gauge for monitoring the blast wave from an explosion, or for evaluating the deformation of a cantilever exposed to the blast wave from an accidental explosion so as to characterize the explosion. Received 23 September 1996 / Accepted 11 November 1996     

Energy transfer studies from excited cyclobutane chemically activated by nuclear recoil reaction

Relative vibrational energy transfer efficiencies are determined for cyclobutane-t chemically activated to an average energy of 5 eV by recoil tritium replacement reaction. The pressure and composition dependence of the stabilization-decomposition ratio indicates relative efficiencies of 1.00, 1.05, and 0.32 for c-C₄H₈, CF₄, and Ne bath gases.     

A compact water-soluble porphyrin bearing an iodoacetamido bioconjugatable site

A broad range of applications requires access to porphyrins that are compact, water-soluble, and bioconjugatable. A symmetrically branched hydrocarbon chain ('swallowtail') bearing polar end groups imparts high (>10 mM) aqueous solubility upon incorporation at one of the meso positions of a trans-AB-porphyrin. Two such swallowtail-porphyrins (1a, 1b) equipped with a conjugatable group (carboxylic acid, bromophenyl) have been prepared previously. The synthesis of three new water-soluble trans-AB-porphyrins is reported, where each porphyrin bears a diphosphonate-terminated swallowtail group and an amino (2a), acetamido (2b), or iodoacetamido (2c) group. The amine affords considerable versatility for functionalization. The iodoacetamide provides a sulfhydryl-reactive site for bioconjugation. Porphyrins were fully characterized in aqueous solution by 1H NMR spectroscopy (in D2O), ESI-MS, static absorption spectroscopy, and static and time-resolved fluorescence spectroscopy. Porphyrins 2a-2c exhibit characteristic porphyrin absorption and emission bands in aqueous solution, with a strong, sharp absorption band in the blue region (approximately 401 nm) and emission in the red region (approximately 624, 686 nm). Porphyrin 2b in aqueous phosphate buffer or phosphate-buffered saline solution exhibits a fluorescence quantum yield of approximately 0.04 and an excited singlet-state lifetime of approximately 11 ns. Collectively, the facile synthesis, amenability to bioconjugation, large spacing between the main absorption and fluorescence features, and long singlet excited-state lifetime make this molecular design quite attractive for a range of biomedical applications.     

Temperature dependence of electron transfer to the M-side bacteriopheophytin in rhodobacter capsulatus reaction centers

Subpicosecond time-resolved absorption measurements at 77 K on two reaction center (RC) mutants of Rhodobacter capsulatus are reported. In the D(LL) mutant the D helix of the M subunit has been substituted with the D helix from the L subunit, and in the D(LL)-FY(L)F(M) mutant, three additional mutations are incorporated that facilitate electron transfer to the M side of the RC. In both cases the helix swap has been shown to yield isolated RCs that are devoid of the native bacteriopheophytin electron carrier HL (Chuang, J. I.; Boxer, S. G.; Holten, D.; Kirmaier, C. Biochemistry 2006, 45, 3845-3851). For D(LL), depending whether the detergent Deriphat 160-C or N-lauryl-N,N-dimethylamine-N-oxide (LDAO) is used to suspend the RCs, the excited state of the primary electron donor (P*) decays to the ground state with an average lifetime at 77 K of 330 or 170 ps, respectively; however, in both cases the time constant obtained from single-exponential fits varies markedly as a function of the probe wavelength. These findings on the D(LL) RC are most easily explained in terms of a heterogeneous population of RCs. Similarly, the complex results for D(LL)-FY(L)F(M) in Deriphat-glycerol glass at 77 K are most simply explained using a model that involves (minimally) two distinct populations of RCs with very different photochemistry. Within this framework, in 50% of the D(LL)-FY(L)F(M) RCs in Deriphat-glycerol glass at 77 K, P* deactivates to the ground state with a time constant of approximately 400 ps, similar to the deactivation of P* in the D(LL) mutant at 77 K. In the other 50% of D(LL)-FY(L)F(M) RCs, P* has a 35 ps lifetime and decays via electron transfer to the M branch, giving P+HM- in high yield (> or =80%). This result indicates that P* --> P(+)H(M)(-) is roughly a factor of 2 faster at 77 K than at 295 K. In alternative homogeneous models the rate of this M-side electron-transfer process is the same or up to 2-fold slower at low temperature. A 2-fold increase in rate with a reduction in temperature is the same behavior found for the overall L-side process P* --> P(+)H(L)(-) in wild-type RCs. Our results suggest that, as for electron transfer on the L side, the M-side electron-transfer reaction P* --> P(+)H(M)(-) is an activationless process.     

Regiospecifically alpha-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Insight into the electronic communication between the individual constituents of multicomponent molecular architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-linked zinc porphyrin dyad was prepared wherein one porphyrin bears two (13)C atoms and the other porphyrin is unlabeled. The (13)C atoms are located at the 1- and 9-positions (alpha-carbons symmetrically disposed to the position of linker attachment), which are sites of electron/spin density in the a(1u) HOMO of the porphyrin. The (13)C labels were introduced by reaction of KS(13)CN with allyl bromide to give the allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methylthio)pyrrole-2-(13)C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave dipyrromethane-1,9-(13)C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing (13)C labels at the 1,9-positions and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospecifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin constituents of the dyad is slow (<10(6) s(-1)) on the EPR time scale at room temperature. The slow rate stems from the a(1u) HOMO of the electron-deficient porphyrins, which has a node at the site of linker connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a(2u) and has a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5 x 10(7) s(-1)) on the EPR time scale at room temperature.     

Effects of substituents on synthetic analogs of chlorophylls. Part 1: Synthesis, vibrational properties and excited-state decay characteristics

Understanding the effects of substituents on the spectra of chlorins is essential for a wide variety of applications. Recent developments in synthetic methodology have made possible systematic studies of the properties of the chlorin macrocycle as a function of diverse types and patterns of substituents. In this paper, the spectral, vibrational and excited-state decay characteristics are examined for a set of synthetic chlorins. The chlorins bear substituents at the 5,10,15 (meso) positions or the 3,13 (beta) positions (plus 10-mesityl in a series of compounds) and include 24 zinc chlorins, 18 free base (Fb) analogs and one Fb or zinc oxophorbine. The oxophorbine contains the keto-bearing isocyclic ring present in the natural photosynthetic pigments (e.g. chlorophyll a). The substituents cause no significant perturbation to the structure of the chlorin macrocycle, as evidenced by the vibrational properties investigated using resonance Raman spectroscopy. In contrast, the fluorescence properties are significantly altered due to the electronic effects of substituents. For example, the fluorescence wavelength maximum, quantum yield and lifetime for a zinc chlorin bearing 3,13-diacetyl and 10-mesityl groups (662 nm, 0.28, 6.0 ns) differ substantially from those of the parent unsubstituted chlorin (602 nm, 0.062, 1.7 ns). Each of these properties of the lowest singlet excited state can be progressively stepped between these two extremes by incorporating different substituents. These perturbations are associated with significant changes in the rate constants of the decay pathways of the lowest excited singlet state. In this regard, the zinc chlorins with the red-most fluorescence also have the greatest radiative decay rate constant and are expected to have the fastest nonradiative internal conversion to the ground state. Nonetheless, these complexes have the longest singlet excited-state lifetime. The Fb chlorins bearing the same substituents exhibit similar fluorescence properties. Such combinations of factors render the chlorins suitable for a range of applications that require tunable coverage of the solar spectrum, long-lived excited states and red-region fluorescence.