Design of a peptide hairpin containing a central three-residue loop

The construction of a designed beta-hairpin structure, containing a central three-residue loop has been successfully achieved in the synthetic nonapeptide Boc-Leu-Phe-Val-(D)Pro-(L)Pro-(D)Ala-Leu-Phe-Val-OMe (2). The design is based on expanding the two-residue loop established in the peptide beta-hairpin Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (1). Characterization of the registered beta-hairpins in peptides 1 and 2 is based on the observation of key nuclear Overhauser effects (NOEs) in CDCl(3) and CD(3)OH. Solvent titration and temperature dependence of NH chemical shifts establish the identity of NH groups involved in interstrand hydrogen bonding. In peptide 2, the antiparallel registry is maintained, with the formation of a (D)Pro-(L)Pro-(D)Ala loop, stabilized by a 5-->1 hydrogen bond between Val3 CO and Leu7 NH groups (C(13), alpha-turn) and a 3-->1 hydrogen bond between (D)Pro4 CO and (d)Ala6 NH groups (C(7), gamma-turn). NMR derived structures suggest that in peptide 2, (d)Ala(6) adopts an alpha(L) conformation. In peptide 1, the (D)Pro-(L)Pro segment adopts a type II' beta-turn. Replacement of (D)Ala (6) in peptide 2 by (L)Ala in peptide 3 yields a beta-hairpin conformation, with a central (D)Pro-(L)Pro two-residue loop. Strand slippage at the C-terminus results in altered registry of the antiparallel strands.     

Discontinuum between a thiolate and a thiol ligand

The effect of H-bond donation to the thiolate ligand of (eta(5)-C(5)H(5))Fe(CO)(2)SR (1) to give H-bond adducts (1 small middle dotHX) and eventually protonation to give [(eta(5)-C(5)H(5))Fe(CO)(2)(HSR)](+) (1H(+)()) has been investigated experimentally and computationally. The electronic structures of 1(R = Me), several derivatives of 1(R = Me) small middle dotHX, and 1(R = Me)H(+)() have been investigated using DFT (density functional theory) computational methods. As previously suggested, these calculations indicate the HOMO of 1 is Fedpi-Sppi antibonding and largely sulfur in character. The calculations indicate the electronic structure of 1 is not altered markedly by H-bond donation to the S center, but protonation results in a reorganization of the electronic structure of 1H(+)() and a HOMO that is largely metal in character. The reduction of Fe-S distances upon protonation of 1(R = Ph) to give 1(R = Ph)H(+)() small middle dotBF(4)()(-)() (2.282(2) and 2.258(2) A, respectively), as determined by single-crystal X-ray crystallography, also indicates diminished Fedpi-Sppi antibonding. Using the carbonyl stretching frequencies as a gauge of the donor ability of the thiolate ligand, we conclude that H-bonding has a continuous effect on the donor properties of the thiolate ligand of 1 (i.e., is a function of the pK(a) of the H-bond donor). A discontinuous effect results when the pK(b) of 1 is reached and the complex is protonated. For our study of 1, the maximal effect of H-bonding is about 30% of protonation. Because the position of acid-base equilibrium depends on the relative basicities of the thiolate ligand and the conjugate base of the H-bond donor (and the relative heats of solvation of the acids and their conjugate bases), a true continuum of effects can be anticipated only for systems that are pK-matched in their given environments. Thus, when the conjugate base of the H-bond donor is a stronger base than the thiolate ligand (as in the present case), H-bond donation has a relatively small effect, but protonation triggers a large, discontinuous effect on the electronic structure of 1.     

Relaxation dynamics of a linear molecule in a random static medium: a scaling analysis

We present extensive molecular dynamics simulations of the motion of a single linear rigid molecule in a two-dimensional random array of fixed overlapping disklike obstacles. The diffusion constants for the center of mass translation, D(CM), and for rotation, D(R), are calculated for a wide range of the molecular length, L, and the density of obstacles, rho. The obtained results follow a master curve Drho(micro) approximately (L(2)rho)(-nu) with an exponent micro=-3/4 and 1/4 for D(R) and D(CM), respectively, that can be deduced from simple scaling and kinematic arguments. The nontrivial positive exponent nu shows an abrupt crossover at L(2)rho=zeta(1). For D(CM) we find a second crossover at L(2)rho=zeta(2). The values of zeta(1) and zeta(2) correspond to the average minor and major axis of the elliptic holes that characterize the random configuration of the obstacles. A violation of the Stokes-Einstein-Debye relation is observed for L(2)rho>zeta(1), in analogy with the phenomenon of enhanced translational diffusion observed in supercooled liquids close to the glass transition temperature.     

Utilizing a copper MOF as a reagent in a solvent mediated reaction to form a topologically distinct MOF

Employing a semi-rigid di-1,2,4-triazole ligand leads to the formation of new MOFs [Cu(4)(L)(4)(SO(4))(4)]·4[Cu(H(2)O)(6)(SO(4))] (3) and [Cu(6)(L)(3)(SO(4))(5)(OH)(2)(H(2)O)(6)]·13H(2)O (4). The frameworks can be synthesized independently, but a reaction occurs in water wherein kinetic product 3 is used as a reagent to synthesize the topologically distinct thermodynamic product 4.     

Reactivity of a silylsilylene bearing a functionalized diaminochlorosilyl substituent

The reactivity of the silylsilylene [{PhC(NtBu)(2)}SiSi(Cl){(NtBu)(2)C(H)Ph}] (2) towards diphenylacetylene, azobenzene, 2,6-diisopropylphenyl azide, sulfur, and selenium is described. The reaction of 2 with one equivalent of azobenzene in toluene afforded compound 3, which is the first example of a 1,2-diaza-3,4-disilacyclobutane containing a pentacoordinate silicon center. The formation of 3 can be explained by a [1+2] cycloaddition of the divalent Si center in 2 with PhN=NPh to form a diazasilacyclopropane intermediate, which then undergoes a 1,2-chlorine shift to release the ring strain to form 3. Similarly, the reaction of 2 with one equivalent of diphenylacetylene in toluene afforded the 1,2-disilacyclobutene 4, which contains a pentacoordinate silicon center. The reaction of 2 with 1.6 equivalents of 2,6-diisopropylphenylazide in toluene afforded the silaimine [LSi(=NAr)N(Ar)L'] (5, L=PhC(NtBu)(2) , L'=Si(Cl){(NtBu)(2)C(H)Ph}, Ar=2,6-iPr(2)C(6)H(3)). The formation of 5 can be explained by an oxidative addition of the divalent Si center in 2 with ArN(3) to afford a silaimine intermediate, which then reacts with another molecule of ArN(3) to give compound 5. The reaction of 2 with elemental sulfur in toluene afforded the chlorosilanethione [LSi(S)Cl] (6) and dithiodisiletane [{Ph(H)C(NtBu)(2) }Si(μ-S)](2) (7). Treatment of 2 with elemental selenium in THF afforded the di(silaneselone) [LSi(Se)Si(Se)L] (8). Evidently, the divalent Si center in 2 undergoes oxidative addition with chalcogens to afford a silylsilanechalcogenone intermediate, which then displaces ":Si{(NtBu)(2)C(H)Ph}" and "ClSi{(NtBu)(2) C(H)Ph}" to form 6 and 8, respectively. Moreover, compound 8 was synthesized by the reaction of [{PhC(NtBu)(2)}Si:](2) (10) with elemental selenium in THF. The results show that the reactions of 2 are initiated by oxidative addition of the divalent silicon center, and then the intermediate formed undergoes a rearrangement involving the diaminochlorosilyl substituent to form compounds 3-8. These products have been characterized by NMR spectroscopy and X-ray crystallography.     

Assembly of a supramolecular capsule on a molecular printboard

A molecular capsule based on ionic interactions between two oppositely charged calix[4]arenes, 1 and 2, was assembled both in solution and on a surface. In solution, the formation of the equimolar assembly 1.2 was studied by (1)H NMR, ESI-MS, and isothermal titration calorimetry, giving an association constant (K(a)) of 7.5 x 10(5) M(-1). A beta-cyclodextrin self-assembled monolayer (beta-CD SAM) on gold was used as a molecular printboard to anchor the tetraguanidinium calix[4]arene (2). The binding of tetrasulfonate calix[4]arene 1 was monitored by surface plasmon resonance spectroscopy. Rinsing of the surface with a high ionic strength aqueous solution allows the removal of the tetrasulfonate calix[4]arene (1), while by rinsing with 2-propanol it is possible to achieve the complete desorption of the tetraguanidinium calix[4]arene (2) from the beta-CD SAM. The K(a) for the capsule formation on a surface is 3.5 x 10(6) M(-1), thus comparing well with the K(a) determined in solution.     

Electrophoresis of a biocolloid covered with a cation-absorptive membrane

The electrophoretic behavior of a biocolloid covered with a charged membrane is theoretically analyzed in the present study. Here, the influences of nonuniformly distributed fixed groups, absorption of cations by fixed original functional groups, variation in dielectric constant in the electrophoretic system, and ionic sizes are considered. The results of numerical simulation suggest that a larger absolute value of the electrophoric mobility of biocolloids could be generated by larger membrane electricity. The absolute value of the electrophoric mobility for the nonlinear distribution of the fixed groups is larger than that for the linear distribution of the fixed groups. The absolute value of the electrophoric mobility increases with (1) the concentration of total fixed groups, (2) the cation-absorption equilibrium constant, (3) the nonuniform feature index for functional-groups distribution, (4) the dielectric constants of the inner uncharged membrane zone for only mobile cationic charge and for both mobile cationic and anionic charge, and (5) the effective size of the cations. An increase in the absolute value of the electrophoric mobility can also be resulted from a decrease in the following parameters: (1) the friction coefficient of the biocolloidal membrane phase, (2) the membrane thickness, (3) the dielectric constant of space for all charge and of outer uncharged membrane zone, (4) the effective sizes of anions and fixed groups, and (5) the number of cations and the fixed original functional groups involved in the formation of a cation-functional group complex.     

Thermal transformation of a layered multifunctional network into a metal-organic framework based on a polymeric organic linker

The preparation of layered [La(H(3)nmp)] as microcrystalline powders from optimized microwave-assisted synthesis or dynamic hydrothermal synthesis (i.e., with constant rotation of the autoclaves) from the reaction of nitrilotris(methylenephosphonic acid) (H(6)nmp) with LaCl(3)·7H(2)O is reported. Thermogravimetry in conjunction with thermodiffractometry showed that the material undergoes a microcrystal-to-microcrystal phase transformation above 300 °C, being transformed into either a three-dimensional or a two-dimensional network (two models are proposed based on dislocation of molecular units) formulated as [La(L)] (where L(3-) = [-(PO(3)CH(2))(2)(NH)(CH(2)PO(2))O(1/2)-](n)(3n-)). The two crystal structures were solved from ab initio methods based on powder X-ray diffraction data in conjunction with structural information derived from (13)C and (31)P solid-state NMR, electron microscopy (SEM and EDS mapping), FT-IR spectroscopy, thermodiffractometry, and photoluminescence studies. It is shown that upon heating the coordinated H(3)nmp(3-) anionic organic ligand undergoes a polymerization (condensation) reaction to form in situ a novel and unprecedented one-dimensional polymeric organic ligand. The lanthanum oxide layers act, thus, simultaneously as insulating and templating two-dimensional scaffolds. A rationalization of the various steps involved in these transformations is provided for the two models. Photoluminescent materials, isotypical with both the as-prepared ([(La(0.95)Eu(0.05))(H(3)nmp)] and [(La(0.95)Tb(0.05))(H(3)nmp)]) and the calcined ([(La(0.95)Eu(0.05))(L)]) compounds and containing stoichiometric amounts of optically active lanthanide centers, have been prepared and their photoluminescent properties studied in detail. The lifetimes of Eu(3+) vary between 2.04 ± 0.01 and 2.31 ± 0.01 ms (considering both ambient and low-temperature studies). [La(H(3)nmp)] is shown to be an effective heterogeneous catalyst in the ring opening of styrene oxide with methanol or ethanol, producing 2-methoxy-2-phenylethanol or 2-ethoxy-2-phenylethanol, respectively, in quantitative yields in the temperature range 40-70 °C. The material exhibits excellent regioselectivity to the β-alkoxy alcohol products even in the presence of water. Catalyst recycling and leaching tests performed for [La(H(3)nmp)] confirm the heterogeneous nature of the catalytic reaction. Catalytic activity may be attributed to structural defect sites. This assumption is somewhat supported by the much higher catalytic activity of [La(L)] in comparison to [La(H(3)nmp)].     

Vibrations of a chelated proton in a protonated tertiary diamine

Vibrational spectra of the conjugate acid of Me(2)NCH(2)CH(2)CH(2)CH(2)NMe(2) (N,N,N',N'-tetramethylputrescine) have been examined in the gaseous and crystalline phases using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy, Inelastic Neutron Scattering (INS), and high pressure Raman spectroscopy. A band observed near 530 cm(-1) is assigned to the asymmetric stretch of the bridging proton between the two nitrogens, based on deuterium substitution and pressure dependence. The NN distance measured by X-ray crystallography gives a good match to DFT calculations, and the experimental band position agrees with the value predicted from theory using a 2-dimensional potential energy surface. The reduced dimensionality potential energy surface, which treats the ion as though it possesses a linear NHN geometry, shows low barriers to proton transit from one nitrogen to the other, with zero point levels close to the barrier tops. In contrast, two other related systems containing strong hydrogen bonds do not exhibit the same spectroscopic signature of a low barrier hydrogen bond (LBHB). On the one hand, the IRMPD spectra of the conjugate acid ions of the amino acid N,N,N',N'-tetramethylornithine (in which the two nitrogens have different basicities) show fewer bands and no comparable isotopic shifts in the low frequency domain. On the other hand, the IRMPD spectrum of the shorter homologue Me(2)NCH(2)CH(2)CH(2)NMe(2) (N,N,N',N'-tetramethyl-1,3-propanediamine), for which the NHN bond angle deviates substantially from linearity, displays more than one band in the 1100-1400 cm(-1) domain, which vanish as a consequence of deuteration.     

Inverse-phosphocholine lipids: a remix of a common phospholipid

Zwitterionic inverse-phosphocholine (iPC) lipids contain headgroups with an inverted charge orientation relative to phosphocholine (PC) lipids. The iPC lipid headgroup has a quaternary amine adjacent to the bilayer interface and a phosphate that extends into the aqueous phase. Neutral iPC lipids with ethylated phosphate groups (CPe) and anionic iPC lipids nonethylated phosphate groups (CP) were synthesized. The surface potential of CPe liposomes remains negative across a broad pH range and in the presence of up to 10 mM Ca(2+). CP liposomes aggregate in the presence of Ca(2+), but at a slower rate than other anionic lipids. Hydrolysis of CP lipids by alkaline phosphatases generates a cationic lipid. CPe liposomes release encapsulated anionic carboxyfluorescein (CF) 20 times faster than PC liposomes and release uncharged glucose twice as fast as PC liposomes. As such, iPC lipids afford a unique opportunity to investigate the biophysical and bioactivity-related ramifications of a charge inversion at the bilayer surface.     

Harnessing a ratiometric fluorescence output from a sensor array

Ratiometric fluorescence-based sensors are widely sought after because they can effectively convert even relatively small changes in optical output into a strong and easy-to-read signal. However, ratiometric sensor molecules are usually difficult to make. We present a proof-of-principle experiment that shows that efficient ratiometric sensing may be achieved by an array of two chromophores, one providing an on-to-off response and the second yielding an off-to-on response in a complementary fashion. In the case that both chromophores emit light of different color, the result is a switching of colors that may be utilized in the same way as from a true ratiometric probe. The chromophore array comprises two sensor elements: i) a polyurethane membrane with embedded N-anthracen-9-yl-methyl-N-7-nitrobenzoxa-[1,2,5]diazo-4-yl-N',N'-dimethylethylenediamine hydrochloride and ii) a membrane with N,N-dimethyl-N'-(9-methylanthracenyl)ethylenediamine. A combination of photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET) allows for green-to-blue emission switching in the presence of Zn(II) ions. The sensing experiments carried out with different Zn(II) salts at controlled pH revealed that the degree of color switching in the individual sensor elements depends on both the presence of Zn(II) ions and the counter anion. These results suggest that sensing of both cations and anions may perhaps be extended to different cation-anion pairs.     

Expression of a supramolecular complex at a multivalent interface

The multivalent binding of a supramolecular complex at a multivalent host surface by combining the orthogonal beta-cyclodextrin (CD) host-guest and metal ion-ethylenediamine coordination motifs is described. As a heterotropic, divalent linker, an adamantyl-functionalized ethylenediamine derivative was used. This was complexed with Cu(II) or Ni(II). The binding of the complexes to a CD self-assembled monolayer (SAM) was studied as a function of pH by means of surface plasmon resonance (SPR) spectroscopy. A heterotropic, multivalent binding model at interfaces was used to quantify the multivalent enhancement at the surface. The Cu(II) complex showed divalent binding to the CD surface with an enhancement factor higher than 100 relative to the formation of the corresponding divalent complex in solution. Similar behavior was observed for the Ni(II) system. Although the Ni(II) system could potentially be trivalent, only divalent binding was observed at the CD SAMs, which was confirmed by desorption experiments.     

Selective detection and assignment of the solution NMR signals of bacteriochlorophyll a in a reconstituted subunit of a light-harvesting complex

High-resolution solution NMR spectra have been obtained for bacteriochlorophyll (BChl) a molecules in a biologically functional subunit of a bacterial core light-harvesting complex based on a modified reconstitution method. The reconstituted subunit of pigment-integral membrane polypeptides is stable and homogeneous at high concentrations at room temperature and exhibits a Q(y) absorption peak at 818 nm. (1)H and (13)C chemical shifts have been specifically assigned for BChl a using the fully and selectively (13)C-labeled pigments incorporated with natural abundance polypeptides in deuterated detergent solution. Remarkable signal broadening has been observed upon reconstitution, where the bacteriochlorin macrocycle is shown in a highly restricted molecular motion while the phytol side chain remains relatively mobile. Two sets of resonances are revealed for 3(2), 8(1), 10, 12(1), and 13(4) protons, and 8(2) methyl protons exhibit four resonances with large upfield complexation shifts. The result indicates a nonequivalent state for the two BChl a molecules in the subunit and can be best interpreted in terms of a parallel face-to-face configuration with partial overlap over the pyrrolic rings II, III, and V. In comparison with BChl a in acetone, 8(2), 13(2), and 13(4) protons are largely perturbed, and the propionic and phytol side chain may adopt a different conformation in the reconstituted subunit. The (13)C chemical shift of 3(1) carbonyl carbon shows a large change downfield, indicating strong hydrogen bonding for all the acetyl carbonyls. Carbonyl carbons at 13(1) give rise to two (13)C resonances with equal intensities, suggesting that the keto carbonyl in one BChl a molecule within a subunit forms a stronger hydrogen bond than that in another BChl a molecule.     

Forces between a Rigid Probe Particle and a Liquid Interface

The effect of disjoining pressure between a rigid spherical probe particle (attached to an AFM cantilever) and a liquid interface (e.g., oil/water or air/water) is treated in an analytic manner to describe the total force F exerted on the probe as a function of the distance X of the probe from the rigid substrate (AFM stage) on which the liquid interface resides. Two cases (i) a flat interface under gravity and (ii) a drop whose size is sufficiently small that gravity can be neglected have been examined. A simple numerical algorithm is given for computing F(X) (the AFM observable) from a given form for the disjoining pressure. Numerical results are displayed for electrostatic probe/interface interactions which reveal the linear compliance regime experimentally observed in AFM experiments on these systems. The slope of the linear compliance regime is shown to be a function of the properties of the interface (capillary length, particle radius, drop size, contact angle of drop on rigid substrate etc.). Copyright 2001 Academic Press.     

Trapping a pseudo-Hofmann rearrangement on a ruthenium cluster

In the clusters [Ru3(micro(3)-NPPh(3))(micro(3)-OSiMe(3))(micro-X)(micro(C,O)-OC[double bond]NPPh(3))(micro-CO)(CO)6] (X = NCO, 2; X = Cl, 3), which were prepared by a pyrolytic reaction of Ph(3)PNSiMe(3) with Ru(3)(CO)12, the ligands result from a cluster-mediated pseudo-Hofmann rearrangement involving the micro-NCO, micro(C,O)-Ph(3)P[double bond]N-CO and micro(3)-NPPh(3) fragments.     

Sequential protonation and methylation of a hydride-osmium complex containing a cyclopentadienyl ligand with a pendant amine group

Complex OsH{eta5-C5H4(CH2)2NMe2}(P(i)Pr3)2 (1) reacts with 1 equiv of trifluoromethanesulfonic acid (HOTf) and trifluoromethanesulfonic acid-d1 (DOTf) to produce the dihydride and hydride-deuteride complexes, [OsHE{eta5-C5H4(CH2)2NMe2}(P(i)Pr3)2]OTf (E = H (2), D (2-d1), respectively. Treatment of 2 and 2-d1 with a second equivalent of HOTf gives [OsHE{eta5-C5H4(CH2)2NHMe2}(P(i)Pr3)2][OTf]2 (E = H (3), D (3-d1) as a result of the protonation of the nitrogen atom. While the hydride and deuteride ligands of 2, 2-d1, 3, and 3-d1 do not undergo any H/D exchange process with the solvent, in acetone-d6, the NH proton of 3 and 3-d1 changes places with a deuterium atom of the solvent to yield [OsHE{eta5-C5H4(CH2)2NDMe2}(P(i)Pr3)2][OTf]2 (E = H (3-Nd1), D (3-d2)). Complex 3-Nd1 can also be obtained from the treatment of complex 2 with DOTf in dichloromethane. No exchange process between the hydride and the ND positions in 3-Nd1 or between the deuteride and NH positions in 3-d1 has been observed. Treatment of 3-Nd1 and 3-d1 with sodium methoxide results in a selective reaction of the base with the ammonium group to regenerate 2 and 2-d1, respectively. Complex 1 also reacts with methyl and methyl-d3 trifluoromethanesulfonate (CH3OTf and CD3OTf, respectively) to give [OsH{eta5-C5H4(CH2)2NMe2CE3}(P(i)Pr3)2]OTf (E = H (4), D (4-d3)) as a result of the addition of the CE3 (E = H, D) group to the nitrogen atom. Complex 4 has been characterized by an X-ray diffraction analysis. It reacts with a second molecule of CH3OTf or CD3OTf to produce [OsH{eta5-C5H4(CH2)2NMe3}{CH2CH(CH3)P(i)P2}(P(i)Pr3)[OTf]2 (5). Similarly, complex 4-d3 reacts with a second molecule of CH3OTf or CD3OTf to yield [OsH{eta5-C5H4(CH2)2NMe2CD3}{CH2CH(CH3)P(i)P2}(P(i)Pr3)[OTf]2 (5-d3). In acetonitrile, complex 5 evolves to an equilibrium mixture of the acetonitrile adducts [Os{eta5-C5H4(CH2)2NMe3}(NCCH3)(P(i)Pr3)2][OTf]2 (7) and [Os{eta5-C5H4(CH2)2NMe3}(NCCH3)2(P(i)Pr3)][OTf]2 (8). In methanol or methanol-d4, complex 4 is not stable and loses trimethylamine to give the vinylcyclopentadienyl derivatives [OsHE(eta5-C5H4CH=CH2)(P(i)Pr3)2]OTf (E = H (9), D (9-d1)) as a result of the protonation or deuteration of the metallic center and a subsequent Hofmann elimination. Protonation of 4 with HOTf gives the dihydride-trimethylammonium derivative [OsH2{eta5-C5H4(CH2)2NMe3}(P(i)Pr3)2][OTf]2 (10). Treatment of 9 with sodium methoxide produces OsH(eta5-C5H4CH=CH2)(P(i)Pr3)2 (11).     

PHOTOSENSITIZING PROPERTIES OF BACTERIOCHLOROPHYLLIN a and BACTERIOCHLORIN a, TWO DERIVATIVES OF BACTERIOCHLOROPHYLL a

Abstract The photosensitizing properties of two water soluble derivatives of bacteriochlorophyll a , bacteriochlorophyllin a and bacteriochlorin a (lacking the central Mg-ion) were investigated and compared to those of hematoporphyrin derivatives. At physiological pH the oxygen consumption rate of histidine, tryptophan, dithiothreitol and guanosine upon illumination was 3 to 4 times higher when bacteriochlorin a was used as photosensitizer than when hematoporphyrin derivatives were used. Especially bacteriochlorin a proved to be an effective sensitizer for the killing of L929 cells. Because bacteriochlorin a has an absorption maximum at 765 nm in phosphate buffered saline (allowing a light penetration in tissue about ten times larger than at 630 nm) and a high molar absorption coefficient (32 000 M cm⁻¹) it has promising possibilities for the application in photodynamic therapy.     

Dielectric response of a polar fluid trapped in a spherical nanocavity

We present extensive molecular dynamics simulation results for the structure and the static and dynamical responses of a droplet of 1000 soft spheres carrying extended dipoles and confined to spherical cavities of radii R=2.5, 3, and 4 nm embedded in a dielectric continuum of permittivity epsilon(')>or=1. The polarization of the external medium by the charge distribution inside the cavity is accounted for by appropriate image charges. We focus on the influence of the external permittivity epsilon(') on the static and dynamic properties of the confined fluid. The density profile and local orientational order parameter of the dipoles turn out to be remarkably insensitive to epsilon('). Permittivity profiles epsilon(r) inside the spherical cavity are calculated from a generalized Kirkwood formula. These profiles oscillate in phase with the density profiles and go to a "bulk" value epsilon(b) away from the confining surface; epsilon(b) is only weakly dependent on epsilon('), except for epsilon(')=1 (vacuum), and is strongly reduced compared to the permittivity of a uniform (bulk) fluid under comparable thermodynamic conditions. The dynamic relaxation of the total dipole moment of the sample is found to be strongly dependent on epsilon(') and to exhibit oscillatory behavior when epsilon(')=1; the relaxation is an order of magnitude faster than in the bulk. The complex frequency-dependent permittivity epsilon(omega) is sensitive to epsilon(') at low frequencies, and the zero-frequency limit epsilon(omega=0) is systematically lower than the bulk value epsilon(b) of the static permittivity.     

The size of a polymer in a symmetric solvent

Using a simple thermodynamic model, we derive an expression for the excluded volume parameter v of a polymer chain in a symmetric solvent (solvated by its own monomers). For a chain with a given segment length and number of monomers, this parameter determines whether the chain is collapsed or expanded. For the latter it determines the degree of expansion. Using a simple off-lattice version of Flory's model [P. J. Flory, Principles of Polymer Chemistry (Cornell University Press, Ithaca, 1953)] and relaxing the assumption of incompressibility, we obtain the result v=(1-kappa)rho(0), where kappa is the dimensionless compressibility and rho(0) the number density of solvent. In the incompressible limit (in the sense that kappa-->0) the chain is expanded and the inverse of the solvent number density determines the degree of expansion of the chain. Using the van der Waals equation of state to estimate kappa (allowing for nonzero compressibility in a system that can undergo a gas-liquid phase transition), the model predicts that upon raising the temperature at constant pressure there is both a lower (coil to globule) and upper (globule to coil) Flory temperature. This is in quantitative agreement with experiment and computer simulations.     

Enzyme-purification and catalytic transformations in a microstructured PASSflow reactor using a new tyrosine-based Ni-NTA linker system attached to a polyvinylpyrrolidinone-based matrix

The synthesis of a Ni-nitrilotriacetic acid (Ni-NTA) attached via a new tyrosine-based linker matrix on monolithic crosslinked poly(vinyl benzyl chloride)/poly(vinylpyrrolidinone) is described. This matrix is incorporated inside a microstructured PASSflow reactor which was used for automatic purification and immobilisation of His(6)-tagged proteins. These could be used as stable and highly active biocatalysts for the synthesis of (R)-benzoin (6), (R)-2-hydroxy-1-phenylpropan-1-one (7) and 6-O-acetyl-D-glucal (17) in a flow-through mode.