Time-resolved chemically induced dynamic nuclear polarization studies of structure and reactivity of methionine radical cations in aqueous solution as a function of pH

Using time-resolved chemically induced dynamic nuclear polarization (CIDNP) techniques, we have studied the mechanism of the photoreactions of triplet excited 4-carboxybenzophenone (CBP) with l-methionine (Met) and 3-(methylthio)propylamine (MTPA) in aqueous solution and the details of the formation of CIDNP at pH from 6.7 to 13.6. At a pH below the pKa of the nitrogen atom of Met, the CIDNP is strongly affected by degenerate electron exchange between the S-S cationic radical dimer and the zwitterionic form of Met with the rate constant kex = 3.4 x 10(8) s(-1) providing an exhaustive explanation of the pH dependence of steady-state CIDNP that was previously interpreted as a manifestation of fast interconversion among three different methionine radical species (Goez, M.; Rozwadowski, J. J. Phys. Chem. A 1998, 102, 7945-7953). By analyzing the polarization of different protons formed in geminate recombination as a function of the pH, we obtained the branching ratio between two reaction pathways for oxidative quenching of (T)CBP via electron transfer from the sulfur and nitrogen atoms of Met and MTPA. Nuclear spin-lattice relaxation times were determined in the dimeric cation radical of Met (T1,S = 8.5 micros). In the cyclic radical cation of MTPA with a three-electron two-center S-N bond, the estimated paramagnetic relaxation is comparatively slow for all protons. Fast deprotonation of the primary aminium radical cation of MTPA and Met in strongly basic solution takes place on the submicrosecond time scale leading to efficient formation of CIDNP in the neutral aminyl radical.     

Relative reactivity of peracids versus dioxiranes (DMDO and TFDO) in the epoxidation of alkenes. A combined experimental and theoretical analysis

Comparative analysis of the calculated gas-phase activation barriers (DeltaE++) for the epoxidation of ethylene with dimethyldioxirane (DMDO) and peroxyformic acid (PFA) [15.2 and 16.4 kcal/mol at QCISD(T)// QCISD/6-31+G(d,p)] and E-2-butene [14.3 and 13.2 kcal/mol at QCISD(T)/6-31G(d)//B3LYP/6-311+G(3df,2p)] suggests similar oxygen atom donor capacities for both oxidants. Competition experiments in CH(2)Cl(2) solvent reveal that DMDO reacts with cyclohexene much faster than peracetic acid/acetic acid under scrupulously dried conditions. The rate of DMDO epoxidation is catalyzed by acetic acid with a reduction in the classical activation barrier of 8 kcal/mol. In many cases, the observed increase in the rate for DMDO epoxidation in solution may be attributed to well-established solvent and hydrogen-bonding effects. This predicted epoxidative reactivity for DMDO is not consistent with what has generally been presumed for a highly strained cyclic peroxide. The strain energy (SE) of DMDO has been reassessed and its moderated value (about 11 kcal/mol) is now more consistent with its inherent gas-phase reactivity toward alkenes in the epoxidation reaction. The unusual thermodynamic stability of DMDO is largely a consequence of the combined geminal dimethyl- and dioxa-substitution effects and unusually strong C-H and C-CH(3) bonds. Methyl(trifluoromethyl)dioxirane (TFDO) exhibits much lower calculated activation barriers than DMDO in the epoxidation reaction (the average DeltaDeltaE++ values are about 7.5 kcal/mol). The rate increase relative to DMDO of approximately 10(5), while consistent with the higher strain energy for TFDO (SE approximately 19 kcal/mol) is attributed largely to the inductive effect of the CF(3) group. We have also examined the effect of alkene strain on the rate of epoxidation with PFA. The epoxidation barriers are only slightly higher for the strained alkenes cyclopropene (DeltaE++ = 14.5 kcal/mol) and cyclobutene (DeltaE++ = 13.7 kcal/mol) than for cyclopentene (DeltaE++ = 12.1 kcal/mol), reflecting the fact there is little relief of strain in the transition state. Alkenes strained by twist or pi-bond torsion do exhibit much lower activation barriers.     

Rapid detection of an anthrax biomarker by surface-enhanced Raman spectroscopy

A rapid detection protocol suitable for use by first-responders to detect anthrax spores using a low-cost, battery-powered, portable Raman spectrometer has been developed. Bacillus subtilis spores, harmless simulants for Bacillus anthracis, were studied using surface-enhanced Raman spectroscopy (SERS) on silver film over nanosphere (AgFON) substrates. Calcium dipicolinate (CaDPA), a biomarker for bacillus spores, was efficiently extracted by sonication in nitric acid and rapidly detected by SERS. AgFON surfaces optimized for 750 nm laser excitation have been fabricated and characterized by UV-vis diffuse reflectance spectroscopy and SERS. The SERS signal from extracted CaDPA was measured over the spore concentration range of 10(-14)-10(-12) M to determine the saturation binding capacity of the AgFON surface and to calculate the adsorption constant (Kspore=1.7 x 10(13) M(-1)). At present, an 11 min procedure is capable of achieving a limit of detection (LOD) of approximately 2.6 x 10(3) spores, below the anthrax infectious dose of 10(4) spores. The data presented herein also demonstrate that the shelf life of prefabricated AgFON substrates can be as long as 40 days prior to use. Finally, these sensing capabilities have been successfully transitioned from a laboratory spectrometer to a field-portable instrument. Using this technology, 10(4) bacillus spores were detected with a 5 s data acquisition period on a 1 month old AgFON substrate. The speed and sensitivity of this SERS sensor indicate that this technology can be used as a viable option for the field analysis of potentially harmful environmental samples.     

Conformation of DNA modified by monofunctional Ru(II) arene complexes: recognition by DNA binding proteins and repair. Relationship to cytotoxicity

We analyzed DNA duplexes modified at central guanine residues by monofunctional Ru(II) arene complexes [(eta(6)-arene)Ru(II)(en)(Cl)](+) (arene = tetrahydroanthracene or p-cymene, Ru-THA or Ru-CYM, respectively). These two complexes were chosen as representatives of two different classes of Ru(II) arene compounds for which initial studies revealed different binding modes: one that may involve DNA intercalation (tricyclic-ring Ru-THA) and the other (mono-ring Ru-CYM) that may not. Ru-THA is approximately 20 times more toxic to cancer cells than Ru-CYM. The adducts of Ru-THA and Ru-CYM have contrasting effects on the conformation, thermodynamic stability, and polymerization of DNA in vitro. In addition, the adducts of Ru-CYM are removed from DNA more efficiently than those of Ru-THA. Interestingly, the mammalian nucleotide excision repair system has low efficiency for excision of ruthenium adducts compared to cisplatin intrastrand crosslinks.     

Gold(I) Complexes with the nido-Diphosphino Ligand [7,8-(Ph(2)P)(2)-7,8-C(2)B(9)H(10)](-). Preparation of the First Metallocarborane Complex of this Ligand. Crystal Structures of [Au{(PPh(2))(2)C(2)B(9)H(10)}(PPh(3))].CH(2)Cl(2) and [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2){(PPh(2))(2)(CH(2))(3)}].3Me(2)CO

The reaction of [AuCl(PR(3))] with [1,2-(Ph(2)P)(2)-1,2-C(2)B(10)H(10)] in refluxing ethanol proceeds with partial degradation (removal of a boron atom adjacent to carbon) of the closo species to give [Au{(PPh(2))(2)C(2)B(9)H(10)}(PR(3))] [PR(3) = PPh(3) (1), PPh(2)Me (2), PPh(2)(4-Me-C(6)H(4)) (3), P(4-Me-C(6)H(4))(3) (4), P(4-OMe-C(6)H(4))(3) (5)]. Similarly, the treatment of [Au(2)Cl(2)(&mgr;-P-P)] with [1,2-(Ph(2)P)(2)-1,2-C(2)B(10)H(10)] under the same conditions leads to the complexes [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2)(&mgr;-P-P)] [P-P = dppe = 1,2-bis(diphenylphosphino)ethane (6), dppp = 1,3-bis(diphenylphosphino)propane (7)], where the dppe or dppp ligands bridge two gold nido-diphosphine units. The reaction of 1 with NaH leads to removal of one proton, and further reaction with [Au(PPh(3))(tht)]ClO(4) gives the novel metallocarborane compound [Au(2){(PPh(2))(2)C(2)B(9)H(9)}(PPh(3))(2)] (8). The structure of complexes 1 and 7 have been established by X-ray diffraction. [Au{(PPh(2))(2)C(2)B(9)H(10)}(PPh(3))] (1) (dichloromethane solvate) crystallizes in the monoclinic space group P2(1)/c, with a = 17.326(3) ?, b = 20.688(3) ?, c = 13.442(2) ?, beta = 104.710(12) degrees, Z = 4, and T = -100 degrees C. [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2)(&mgr;-dppp)] (7) (acetone solvate) is triclinic, space group P&onemacr;, a = 13.432(3) ?, b = 18.888(3) ?, c = 20.021(3) ?, alpha = 78.56(2) degrees, beta = 72.02(2) degrees, gamma = 73.31(2) degrees, Z = 2, and T = -100 degrees C. In both complexes the gold atom exhibits trigonal planar geometry with the 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate(1-) acting as a chelating ligand.     

Synthesis of new 6-substituted purinyl-5′-nor-1′-homocarbanucleosides based on indanol

A series of new 6-substituted purinyl-5′-nor-1′-homocarbanucleosides based on indanol were synthesized from (±)-cis-3-hydroxymethyl-1-indanol, an appropriately functionalized derivative of which was reacted with 6-chloropurine in the presence of NaH and 18-crown-6 ether to prepare a key intermediate that gave access to the target molecules, purinylcarbanucleosides in which position 6 is occupied by a chloro, hydroxy, methoxy, amino or substituted phenyl group.     

Isotopic scrambling in Di-13C-labeled 2-butyl cation: evidence for a protonated cyclopropane intermediate

The (13)C NMR spectrum of 2-butyl-1,2-(13)C(2) cation (1) is unchanged on heating the sample to -78 degrees C, indicating no isomerization to another isotopomer. In contrast, the spectrum of 2-butyl-2,3-(13)C(2) cation (2) shows rapid formation of all of the other isotopomers except 1. These results are consistent with a protonated cyclopropane intermediate in the rearrangement process. In this mechanism, either C(1) and C(2) or C(3) and C(4) interchange. Only the bond between C(2) and C(3) breaks.     

Acylaminoacetyl derivatives of active methylene compounds. 2. The cyclization of the acetylaminoacetyl derivatives to α-substituted tetramic acids and the formation of N-acetyl-α-substituted tetramic acids

The reaction of aceturic acid p-nitrophenyl ester with active methylene compounds Y-CH₂-CO₂R has been found to give either the normally expected C-acylation compounds 2 (Y = -CN, -CO₂R, -COCH₃) or N-acetyl-α-Y-substituted tetramic acids 3 (Y = -CO₂R,-COCH₃), depending on the reaction conditions. Moreover, the intramolecular condensation of the C-acylation compounds 2 derived from malonic and acetoacetic esters (Y = -CO₂R, -COCH₃) to α-Y-substituted tetramic acids is shown to proceed through initial cyclization to the corresponding N-acetyl-α-Y-substituted tetramic acids 3 (Y = -CO₂R, -COCH₃).     

Electrostatics of PEGylated micelles and liposomes containing charged and neutral lipopolymers

The electrostatics of large unilamellar vesicles (LUVs) of various lipid compositions were determined and correlated with steric stabilization. The compositional variables studied include (a) degree of saturation, comparing the unsaturated egg phosphatidylcholine (EPC) and the fully hydrogenated soy phosphatidylcholine (HSPC) as liposome-forming lipids; (b) the effect of 40 mol % cholesterol; (c) the effect of mole % of three methyl poly(ethylene glycol) (mPEG)-lipids (the negatively charged mPEG-distearoyl phosphoethanolamine (DSPE) and two uncharged lipopolymers, mPEG-distearoyl glycerol (DSG) and mPEG-oxycarbonyl-3-amino-1,2-propanediol distearoyl ester (DS)); and (d) the negatively charged phosphatidyl glycerol (PG). The lipid phases were as follows: liquid disordered (LD) for the EPC-containing LUV, solid ordered (SO) for the HSPC-containing LUV, and liquid ordered (LO) for either of those LUV with the addition of 40 mol % cholesterol. The LUV zeta potential and electrical surface potential (psi(0)) were determined. It was found that progressive addition of mPEG(2k)-DSPE to liposomes increases negative surface potential and reduces surface pH to a similar extent as the addition of PG. However, due to the "hidden charge effect", zeta potential was more negative for liposomes containing PG than for those containing mPEG(2k)-DSPE. Replacing mPEG-DSPE with mPEG(2k)-DS or mPEG-DSG had no effect on surface pH and surface potential, and zeta potential was approximately zero. Addition of low concentrations of cationic peptides (protamine sulfate and melittin) to PG- or mPEG-DSPE-containing liposomes neutralized the liposome negative surface potential to a similar extent. However, only in liposomes containing PG, did liposome aggregation occur. Replacing the negatively charged lipopolymer mPEG-DSPE with the neutral lipopolymers mPEG-DS or mPEG-DSG eliminates or reduces such interactions. The relevance of this effect to the liposome performance in vivo is discussed.     

A very large diversity space of synthetically accessible compounds for use with drug design programs

We have constructed a very large virtual diversity space containing more than 10¹³ chemical compounds. The diversity space is built from about 400 combinatorial libraries, which have been expanded by choosing sizeable collections of suitable R-groups that can be attached to each link point of their scaffolds. These R-group collections have been created by selecting reagents that have drug-like properties from catalogs of available chemicals. As members of known combinatorial libraries, the compounds in the diversity space are in general synthetically accessible and useful as potential drug leads. Hence, the diversity space can be used as a vast source of compounds by a de novo drug design program. For example, we have used such a program to generate inhibitors of HIV integrase enzyme that exhibited activity in the micromolar range.