Efficient total synthesis of pulchellalactam,a CD45 protein tyrosine phosphatase inhibitor

A new approach to a CD45 protein tyrosine phosphatase inhibitor, pulchellalactam, is described. The key step of the sequence involves addition and elimination of an enolic lactam in a single step and 70% yield, employing an organocuprate reagent. The resulting alpha,beta-unsaturated lactam could be condensed with isobutyraldehyde to produce Z-pulchellalactam or converted into siloxypyrrole, which was subjected to the BF(3) x Et(2)O-promoted coupling reaction with isobutyraldehyde to afford E-pulchellalactam after E1-cB elimination and TFA deprotection. This first total synthesis afforded Z-pulchellalactam in six steps and 32% overall yield from Boc-glycine. The same sequence of reactions could also be applied to the liquid- or solid-phase synthesis of trifunctionalized pulchellalactam derivatives.     

Substitution and oxidation reactions of bis(dithiolene)tungsten complexes of potential relevance to enzyme sites

Structurally characterized tungstoenzymes contain mononuclear active sites in which tungsten is coordinated by two pterin-dithiolene ligands and one or two additional ligands that have not been identified. In this and prior investigations (Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275; J. Am. Chem. Soc. 2001, 123, 1931), stable coordination units of bis(dithiolene)tungsten(IV,V,VI) complexes potentially related to enzyme sites have been sought by exploratory synthesis. In this work, additional members of the sets [WL(S2C2Me2)2](2-,-) and [WLL'(S2C2Me2)2](2-,-) have been prepared and structurally characterized. Tungsten(IV) complexes obtained by substitution are carbonyl displacement products of [W(CO)2(S2C2Me2)2] and include those with the groups W(IV)S (4), W(IV)(O2CPh) (5), and W(IV)(2-AdQ)(CO) (Q = S (6), Se (7); Ad = adamantyl). Those obtained by oxidation reactions contain the groups W(V)O (9), W(V)(QPh)2 (Q = S (10), Se (11)), W(VI)S(OPh) (12), and W(VI)O2 (14). The latter two complexes were obtained from W(IV) precursors using sulfur and oxygen atom transfer reactions, respectively. Complexes 4 and 9 are square pyramidal; 6, 7, 10, and 11 are distorted trigonal prismatic with cis ligands LL'; and 12 and 14 are distorted octahedral. Complexes 4, 10, and 11 support three-membered electron transfer series. Attempts to oxidize 4 to the W(V)S complex results in the formation of binuclear [W2(mu2-S)2(S2C2Me2)4](2-) having distorted octahedral coordination. The 21 known functional groups WL and WLL' in mononuclear bis(dithiolene) complexes prepared in this and prior investigations are tabulated. Of those with physiological-type ligands, it remains to be seen which (if any) of these ligation modes are displayed by enzyme sites.     

Sequential injection analysis system for on-line monitoring of l-cysteine concentration in biological processes

A sequential injection analysis (SIA) system was developed to monitor the concentration of l-cysteine in biological processes on-line. It is based on the redox reaction of l-cysteine with iron(III) in the presence of 1,10-phenanthroline (phen) and the detection of the red-iron(II)-phen complex with a spectrophotometry. The system was fully automated using software written in the LabVIEW™ development environment. A number of system variables such as the flow rate of the carrier buffer solution, the volume ratio of the sample to the reagents, and the reaction coil length, etc., were evaluated to increase the sensitivity and performance of the SIA system. Under partially optimized operating conditions the performance of the SIA system was linear up to a concentration of l-cysteine of 1 mM (R² = 0.998) with a detection limit of 0.005 mM and a sample frequency of 15 hr⁻¹. The SIA system was employed to monitor the concentration of l-cysteine on-line in a continuously stirred reactor and a fermentation process of Saccharomyces cerevisiae. The on-line monitored data were in good agreement with the off-line data measured by a HPLC with a fluorescence detector (n = 15, R² = 09899).     

Comparative investigation on non-IPR C68 and IPR C78 fullerenes encaging Sc3N molecules

A computational study on the experimentally detected Sc(3)N@C(68) cluster is reported, involving quantum chemical analysis at the B3LYP/6-31G level. Extensive computations were carried out on the pure C(68) cage which does not conform with the isolated pentagon rule (IPR). The two maximally stable C(68) isomers were selected as initial Sc(3)N@C(68) cage structures. Full geometry optimization leads to a confirmation of an earlier assessment of the Sc(3)N@C(68) equilibrium geometry (Nature 2000, 408, 427), namely an eclipsed arrangement of Sc(3)N in the C(68) 6140 frame, where each Sc atom interacts with one pentagon pair. From a variety of theoretical procedures, a D(3h) structure is proposed for the free Sc(3)N molecule. Encapsulated into the C(68) enclosure, this unit is strongly stabilized with respect to rotation within the cage. The complexation energy of Sc(3)N@C(68) cage is found to be in the order of that determined for Sc(3)N@C(80) and exceeding the complexation energy of Sc(3)N@C(78). The cage-core interaction is investigated in terms of electron transfer from the encapsulated trimetallic cluster to the fullerene as well as hybridization between these two subsystems. The stabilization mechanism of Sc(3)N@C(68) is seen to be analogous to that operative in Sc(3)N@C(78). For both cages, C(68) and C(78), inclusion of Sc(3)N induces aromaticity of the cluster as a whole.     

Dynamic identification of phosphopeptides using immobilized metal ion affinity chromatography enrichment, subsequent partial beta-elimination/chemical tagging and matrix-assisted laser desorption/ionization mass spectrometric analysis

The enrichment of phosphopeptides using immobilized metal ion affinity chromatography (IMAC) and subsequent mass spectrometric analysis is a powerful protocol for detecting phosphopeptides and analyzing their phosphorylation state. However, nonspecific binding peptides, such as acidic, nonphosphorylated peptides, often coelute and make analyses of mass spectra difficult. This study used a partial chemical tagging reaction of a phosphopeptide mixture, enriched by IMAC and contaminated with nonspecific binding peptides, following a modified beta-elimination/Michael addition method, and dynamic mass analysis of the resulting peptide pool. Mercaptoethanol was used as a chemical tag and nitrilotriacetic acid (NTA) immobilized on Sepharose beads was used for IMAC enrichment. The time-dependent dynamic mass analysis of the partially tagged reaction mixture detected intact phosphopeptides and their mercaptoethanol-tagged derivatives simultaneously by their mass difference (-20 Da for each phosphorylation site). The number of new peaks appearing with the mass shift gave the number of multiply phosphorylated sites in a phosphopeptide. Therefore, this partial chemical tagging/dynamic mass analysis method can be a powerful tool for rapid and efficient phosphopeptide identification and analysis of the phosphorylation state concurrently using only MS analysis data.     

Efficient excitation energy transfer in long meso-meso linked Zn(II) porphyrin arrays bearing a 5,15-bisphenylethynylated Zn(II) porphyrin acceptor

Electronically coupled porphyrin arrays are suitable for artificial light harvesting antenna in light of a large absorption cross-section and fast excitation energy transfer (EET). Along this line, an artificial energy transfer model system has been synthesized, comprising of an energy donating meso-meso linked Zn(II) porphyrin array and an energy accepting 5,15-bisphenylethynylated Zn(II) porphyrin linked via a 1,4-phenylene spacer. This includes an increasing number of porphyrins in the meso-meso linked Zn(II) porphyrin array, 1, 2, 3, 6, 12, and 24 (Z1A, Z2A, Z3A, Z6A, Z12A, and Z24A). The intramolecular singlet-singlet EET processes have been examined by means of the steady-state and time-resolved spectroscopic techniques. The steady-state fluorescence comes only from the acceptor moiety in Z1A-Z12A, indicating nearly the quantitative EET. In Z24A that has a molecular length of ca. 217 A, the fluorescence comes largely from the acceptor moiety but partly from the long donor array, indicating that the intramolecular EET is not quantitative. The transient absorption spectroscopy has provided the EET rates in real time scale: (2.5 ps)(-1) for Z1A, (3.3 ps)(-1) for Z2A, (5.5 ps)(-1) for Z3A, (21 ps)(-1) for Z6A, (63 ps)(-1) for Z12A, and (108 ps)(-1) for Z24A. These results have been well explained by a revised F?rster equation (Sumi formula), which takes into account an exciton extending coherently over several porphyrin pigments in the donor array, whose length is not much shorter than the average donor-acceptor distance. Advantages of such strongly coupled porphyrin arrays in light harvesting and transmission are emphasized in terms of fast EET and a large absorption cross-section for incident light.     

X-ray spectroscopy of enzyme active site analogues and related molecules: bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands

The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L2,3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [MoIVL(S2C2Me2)2]- (L = O2-, R3SiO-, RO-, RS-, RSe-) and [WIV(OR)(S2C2Me2)2]-, distorted trigonal prismatic [MoIV(CO)(SeR)(S2C2Me2)2]- and [WIV(CO)L(S2C2Me2)2]- (L = RS-, RSe-), and distorted octahedral [WVIO(OR)(S2C2Me2)2]-. The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to < or = 0.02 A as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [MoIV(QR)(S2C2Me2)2]- simulate protein ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)2]- and six-coordinate [M(CO)L(S2C2Me2)2]- are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Soc. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.     

Immobilizing a Fluorescent Dye Offers Potential to Investigate the Glass/Resin Interface

Silane coupling agents are commonly applied to glass fibers to promote fiber/resin adhesion and enhance durability in composite parts. In this study, a coupling agent multilayer on glass was doped with trace levels of the dimethylaminonitrostilbene (DMANS) fluorophore. The fluorophore was immobilized on the glass surface by tethering the molecule to a triethoxy silane coupling agent, creating the DMANS/silane coupling agent molecule (DMSCA). DMSCA was then diluted with commonly used coupling agents and grafted to a glass microscope coverslip to create a model composite interface. A 53-nm blue shift in fluorescence from the immobilized DMSCA can be followed during cure of an epoxy resin overlayer, giving this technique potential to monitor the properties of the fiber/resin interface during composite processing. Contact angle measurements on these coupling agent layers were similar in the presence or absence of the DMSCA molecule, suggesting that trace levels of the fluorescent probe did not affect the structure of the layer. The immobilized DMSCA molecule behaved similarly to the DMANS precursor in solution. Both showed longer wavelength fluorescence in more polar environments. Copyright 2000 Academic Press.     

Metal-oxygen-metal arrays in lamellar hybrid materials: cobalt and manganese 4-cyclohexene-1,2-dicarboxylates

We have obtained three layered hybrid materials from the hydrothermal reaction of 4-cyclohexene-1,2-dicarboxylic acid with Co and Mn salts: Co(C(8)H(8)O(4))[1], Mn(H(2)O)(C(8)H(8)O(4))[2], and Mn(4)(H(2)O)(C(8)H(8)O(4))(4).0.3(H(2)O)[3]. The structures for all materials were solved by single-crystal XRD ([1]P1, a=4.805(2) A, b=6.650(3) A, c=12.960(6) A, alpha=98.285(7) degrees, beta=98.986(7) degrees, gamma=95.689(7) degrees, V= 401.6(3) A(3), R(1)= 0.0438; [2] P2(1)/c, a=11.151(2) A, b=11.330(2) A, c=7.6560(15) A, beta=108.813(3) degrees , V=915.6(3) A(3), R(1)=0.0412; [3] P1, a= 11.412(3) A, b=12.136(4) A, c=13.809(4) A, alpha=104.703(6) degrees, beta=103.207(6) degrees, gamma=92.468(5) degrees, V=1790.6(9) A(3), R(1)=0.1056). While all three structures are two-dimensional overall, the metal-oxygen-metal dimensionality within the layers varies from isolated metal atoms in the case of [1] to 1D ribbons of vertex sharing MnO(6) octahedra [2] and 2D arrays of edge- and vertex-sharing polyhedra in [3].     

Integral equation theory for two-dimensional polymer melts

The polymer reference interaction site model theory is investigated for two-dimensional polymer melts composed of freely-jointed hard disk chains and tangent-disk rods. Exact results for the intramolecular pair correlation functions are input into the theory, and predictions of the theory for the intermolecular pair correlation functions are tested via comparison with simulation. The theory is not as accurate for this system as it is for three-dimensional polymer melts, and the quantitative predictions are not good except at the highest area fractions. Possible reasons for the deficiency in the theory are discussed.