SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS. XVIII. INDOMETHACIN

Abstract— The photochemistry, photophysics, and photosensitization (Type I and II) of indomethacin (IN) (N-[p-chlorobenzoyl]-5-methoxy-2-methylindole-3-acetic acid) has been studied in a variety of solvents using NMR, high performance liquid chromatography-mass spectroscopy, transient spectroscopy, electron paramagnetic resonance in conjunction with the spin trapping technique, and the direct detection of singlet molecular oxygen (^l O₂) luminescence. Photodecomposition of IN (λ_ex > 330 nm) in degassed or air-saturated benzene proceeds rapidly to yield a major (2; N-[p-chlorobenzoyl]-5-methoxy-2-methyl-3-methylene-indoline) and a minor (3; N-[p-chlorobenzoyl]-5-methoxy-2, 3-dimethyl-indole) decarboxylated product and a minor indoline (5; 1-en-5-methoxy-2-methyl-3-methylene-in-doline), which is formed by loss of the p-chlorobenzoyl moiety. In air-saturated solvents two minor oxidized products 4 (N-[p-chlorobenzoyl]-5-methoxy-2-methylindol-3-aldehyde) and 6 (5-methoxy-2-methyl-indole-3-aldehyde) are also formed. When photolysis was carried out in ¹⁸O₂-saturated benzene, the oxidized products 4 and 6 contained ¹⁸O, indicating that oxidation was mediated by dissolved oxygen in the solvent. In more polar solvents such as acetonitrile or ethanol, photodecomposition is extremely slow and inefficient. Phosphorescence of IN at 77 K shows strong solvent dependence and its emission is greatly reduced as polarity of solvent is increased. Flash excitation of IN in degassed ethanol or acetonitrile produces no transients. A weak transient is observed at 375 nm in degassed benzene, which is not quenched by oxygen. Irradiation of IN (λ_ex > 325 nm) in N₂-gassed C₆H₆ in the presence of 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) results in the trapping of two carbon-centered radicals by DMPO. One adduct was identified as DMPO/^.COC₆H₄-p-CI, while the other was probably derived from a radical formed during IN decarboxylation. In air-saturated benzene, (hydro) peroxyl and alkoxyl radical adducts of DMPO are observed. A very weak luminescence signal from ¹O₂ at 1268 nm is observed initially upon irradiation (λ_ex= 325 nm) of IN in air-saturated benzene or chloroform. The intensity of this ¹O₂ signal increases as irradiation is continued suggesting that the enhancement in ¹O₂ yield is due to photoproduct(s). Accordingly, when 2 and 3 were tested directly, 2 was found to be a much better sensitizer of ¹O₂ than IN. In air-saturated ethanol or acetonitrile no IN ¹O₂ luminescence is detected even on continuous irradiation. The inability of IN to cause phototoxicity may be related to its photo stability in polar solvents, coupled with the low yield of active oxygen species (¹O₂, O₂^?_-) upon UV irradiation.     

How does huperzine A enter and leave the binding gorge of acetylcholinesterase? Steered molecular dynamics simulations

The entering and leaving processes of Huperzine A (HupA) binding with the long active-site gorge of Torpedo californica acetylcholinesterase (TcAChE) have been investigated by using steered molecular dynamics simulations. The analysis of the force required along the pathway shows that it is easier for HupA to bind to the active site of AChE than to disassociate from it, which for the first time interprets at the atomic level the previous experimental result that unbinding process of HupA is much slower than its binding process to AChE. The direct hydrogen bonds, water bridges, and hydrophobic interactions were analyzed during two steered molecular dynamics (SMD) simulations. Break of the direct hydrogen bond needs a great pulling force. The steric hindrance of bottleneck might be the most important factor to produce the maximal rupture force for HupA to leave the binding site but it has a little effect on the binding process of HupA with AChE. Residue Asp72 forms a lot of water bridges with HupA leaving and entering the AChE binding gorge, acting as a clamp to take out HupA from or put HupA into the active site. The flip of the peptide bond between Gly117 and Gly118 has been detected during both the conventional MD and SMD simulations. The simulation results indicate that this flip phenomenon could be an intrinsic property of AChE and the Gly117-Gly118 peptide bond in both HupA bound and unbound AChE structures tends to adopt the native enzyme structure. At last, in a vacuum the rupture force is increased up to 1500 pN while in water solution the greatest rupture force is about 800 pN, which means water molecules in the binding gorge act as lubricant to facilitate HupA entering or leaving the binding gorge.     

A tetra-coordinate nickel(II) complex as neutral carrier for nitrate-selective PVC membrane electrode

The potentiometric response properties and applications of a tetra-coordinate nickel(II) complex with relatively high selectivity toward nitrate ion are described. The nickel(II) complex of 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-4,6,11,13-tetraene was used as a neutral carrier into plasticized poly(vinyl chloride) (PVC) membrane. The influence of several variables was investigated in order to optimize the potentiometric response and selectivity of the electrode. The resulting membrane electrode incorporating 31.0% PVC, 61.0% dioctyl phthalate (DOP) as plasticizer, 3% methyltrioctylammonium chloride (MTOAC) as a cationic additive and 5% carrier (all w/w) demonstrates a Nernstian response slope of −59.6 mV per decade over the concentration range of 5×10⁻⁶-1×10⁻¹ M NO₃⁻. The electrode exhibits a fast response time (≤10 s), a detection limit of 2.5×10⁻⁶ M, and can be used over a wide pH range of 4-12. The electrode shows improved selectivity in comparison to most of the previously reported nitrate-selective electrodes. It was successfully applied to the determination of nitrate ion in natural water samples.     

Paradigms and paradoxes: patterns and estimation of the entropy of formation of aqueous hydrogen containing mono and polynuclear oxyanions

“Simple” energetic patterns, where simple means the use of parameters derived only from the stoichiometry of these species, are relatively rarely discussed in the literature. In addition, entropy studies have been dominated by derivation of the absolute quantity S° rather than the entropy of formation (TΔ_f S ^o). Relationships between the entropy of formation and different parameters such as negative value of the charge of the species, the number of oxygen atoms, the natural logarithm of the molecular weight, the total number of atoms and the number of central atoms that are gases were recently discussed by us for aqueous polynuclear oxyanions. As shown here hydrogen containing anions do not follow this pattern. In this study, new approaches for the estimation of the entropy of formation of aqueous hydrogen containing mono and polynuclear oxyanions are suggested, evaluated and recommended.     

Pressure pinched injection of nanolitre volumes in planar micro-analytical devices

A new method for injecting and driving fluids by means of a multi-port injection valve and syringe pumps in a micro-channel network is described. A structure composed of two micro-channels arranged as a cross is connected with capillary tubes to an external multi-port injection valve. The fluid flows are driven by pressure and the multi-port valve controls the direction of the flow within the different sections of the structure. The first position of the multi-port valve allows the preparation of the loading of the sample, which is pinched in the cross section of the two micro-channels. The second position allows the precise injection of nL volumes. No dead volume exists between injection and separation modes. The system can be used to prepare a sample plug by pressure in order to perform chromatography with a broad range of buffered or non-buffered solutions. Thanks to the insensitivity to the ionic strength of the sample, this injection method is useful for the injection of complex biological samples in microchip analysis. In order to demonstrate the feasibility of the method, different solutions of ionic or fluorescent molecules were injected and detected in a photoablated planar polymer device.     

Existence,uniqueness, and nondegeneracy of positive solutions of semilinear elliptic equations

We study positive solutions of the Dirichlet problem: u(x)+f(u(x))=0,xD ⁿ ,u(x)=0,xD ⁿ , whereD ⁿ is ann-ball. We find necessary and sufficient conditions for solutions to be nondegenerate. We also give some new existence and uniqueness theorems.Research supported in part by NSF Contract Number MCS 80-02337     

Chemistry of Succinyl Succinic Acid Derivatives. Part V. On the transesterification of succinyl succinates

The unique stabilization of the ester link in dialkyl succinyl succinates is explained and a facile route to known as well as to hitherto inaccessible succinyl succinates is suggested. New representatives of this series are described.     

Clusters,metastability, and nucleation: Kinetics of first-order phase transitions

We describe and interpret computer simulations of the time evolution of a binary alloy on a cubic lattice, with nearest neighbor interactions favoring like pairs of atoms. Initially the atoms are arranged at random; the time evolution proceeds by random interchanges of nearest neighbor pairs, using probabilities compatible with the equilibrium Gibbs distribution at temperatureT. For temperatures 0.59T_c, 0.81 T_c, and 0.89T _c, with density of A atoms equal to that in the B-rich phase at coexistence, the density C₁ of clusters ofl A atoms approximately satisfies the following empirical formulas: C₁ w(1 –)³ andC ₁, (1 –)⁴Q₁w¹ (2 l 10). Herew is a parameter and we defineQ _l = _K e ^–E(K) , where the sum goes over all translationally nonequivalentl-particle clusters andE(K) is the energy of formation of the clusterK. Forl > 10,Q ₁ is not known exactly; so we use an extrapolation formulaQ _l Aw _s ^–l l ^– exp(–bl ), wherew _s is the value ofw at coexistence. The same formula (withw > w _s) also fits the observed values of C, (for small values ofl) at densities greater than the coexistence density (forT=0.59T_c): When the supersaturation is small, the simulations show apparently metastable states, a theoretical estimate of whose lifetime is compatible with the observations. For higher supersaturation the system is observed to undergo a slow process of segregation into two coexisting phases (andw therefore changes slowly with time). These results may be interpreted as a more quantitative formulation (and confirmation) of ideas used in standard nucleation theory. No evidence for a spinodal transition is found.Supported by AFOSR Grant No. 73-2430D and by ERDA Contract No. EY-76-C-02-3077*000.     

Transport properties of the Lorentz gas: Fourier's law

We investigate the stationary nonequilibrium (heat transporting) states of the Lorentz gas. This is a gas of classical point particles moving in a region gL containing also fixed (hard sphere) scatterers of radiusR. The stationary state considered is obtained by imposing stochastic boundary conditions at the top and bottom of , i.e., a particle hitting one of these walls comes off with a velocity distribution corresponding to temperaturesT ₁ andT ₂ respectively,T ₁ <T ₂. Letting be the average density of the randomly distributed scatterers we show that in the Boltzmann-Grad limit,,R 0 with the mean free path fixed, the stationary distribution of the Lorentz gas converges in theL ¹-norm to the stationary distribution of the corresponding linear Boltzmann equation with the same boundary conditions. In particular, the steady state heat flow in the Lorentz gas converges to that of the linear Boltzmann equation, which is known to behave as (T ₂-T ₁)/L for largeL, whereL is the distance from the bottom to the top wall: i.e., Fourier's law of heat conduction is valid in the limit. The heat flow converges even in probability. Generalizations of our result for scatterers with a smooth potential as well as the related diffusion problem are discussed.Research supported in part by NSF Grant no. Phy 77-22302.On leave of absence from the Fachbereich Physik der Universität, München. Work supported by a DFG fellowship.     

Growth of clusters in a first-order phase transition

The results of computer simulations of phase separation kinetics in a binary alloy quenched from a high temperature are analyzed in detail, using the ideas of Lifshitz and Slyozov. The alloy was modeled by a three-dimensional Ising model with Kawasaki dynamics. The temperature after quenching was 0.59T _c, whereT _c is the critical temperature, and the concentration of minority atoms was=0.075, which is about five times their largest possible single-phase equilibrium concentration at that temperature. The time interval covered by our analysis goes from about 1000 to 6000 attempted interchanges per site. The size distribution of small clusters of minority atoms is fitted approximately byc ₁(1-)³ w(t),c ₁ (1–)⁴ Q _l w(t)^l(2l10); wherec _l is the concentration of clusters of sizel;Q ₂,...,Q ₁₀ are known constants, the cluster partition functions;t is the time; andw(t)=0.015(1+7.17t ^–1/3). The distribution of large clusters (l20) is fitted approximately by the type of distribution proposed by Lifshitz and Slyozov,c _l ,(t)=–(d/dl) [lnt+p (l/t)], where is a function given by those authors and is defined by(x)=C _o e–x-C ₁ e ^–4x/3-C ₂ e ^–5x/3;C ₀,C ₁,C ₂ are constants determined by considering how the total number of particles in large clusters changes with time.Supported by the U.S. Air Force Office of Scientific Research under Grant No. 78-3522 and by the U.S. Department of Energy under Contract No. EY-76-C-02-3077*000.