Multiple hydrogen bonding for reversible polymer surface adhesion

Specific and reversible adhesion of a terminal thymine-functionalized polystyrene (PS-thymine) was demonstrated for a silicon surface with complementary adenine recognition sites. A novel adenine-containing triethoxysilane (ADPTES), which was suitable for covalent attachment to silanol containing surfaces, was synthesized in one step from adenine and 3-isocyanatopropyl triethoxysilane (IPTES). 1H and 13C NMR spectroscopy and fast atom bombardment mass spectroscopy confirmed the chemical structure, and 29Si NMR spectroscopy indicated the absence of any premature hydrolysis of the alkoxysilane derivative. X-ray photoelectron spectroscopy (XPS) and water contact angle measurements indicated the attachment of PS-thymine to silicon surfaces that were modified with a mixture of ADPTES and 3-mercaptopropyl triethoxysilane (MPTES). PS-thymine attachment to surfaces that were modified with only MPTES was not observed. The exclusive attachment of PS-thymine to an ADPTES/MPTES-modified surface confirmed hydrogen bonding-mediated adenine-thymine association to silicon surfaces containing a sufficiently low concentration of adenine recognition sites. Although PS-thymine attachment to the ADPTES/MPTES-modified surfaces was insensitive to THF rinsing, the PS-thymine was completely removed from the surface upon DMSO rinsing because of the disruption of adenine-thymine hydrogen bonding with a more polar aprotic solvent. PS-thymine was successfully reattached to the ADPTES/MPTES-modified surface following the DMSO rinse, demonstrating the solvato-reversible nature of the adenine-thymine association.     

Adding explicit solvent molecules to continuum solvent calculations for the calculation of aqueous acid dissociation constants

Aqueous acid dissociation free energies for a diverse set of 57 monoprotic acids have been calculated using a combination of experimental and calculated gas and liquid-phase free energies. For ionic species, aqueous solvation free energies were calculated using the recently developed SM6 continuum solvation model. This model combines a dielectric continuum with atomic surface tensions to account for bulk solvent effects. For some of the acids studied, a combined approach that involves attaching a single explicit water molecule to the conjugate base (anion), and then surrounding the resulting anion-water cluster by a dielectric continuum, significantly improves the agreement between the calculated pK(a) value and experiment. This suggests that for some anions, particularly those concentrating charge on a single exposed heteroatom, augmenting implicit solvent calculations with a single explicit water molecule is required, and adequate, to account for strong short-range hydrogen bonding interactions between the anion and the solvent. We also demonstrate the effect of adding several explicit waters by calculating the pK(a) of bicarbonate (HCO(3)(-)) using as the conjugate base carbonate (CO(3)(2-)) bound by up to three explicit water molecules.     

Observation of solid-state 103Rh NMR by cross-polarization

Using 103Rh[1H] cross-polarization (CP) methods, we have obtained solid-state 103Rh NMR spectra for diamagnetic Rh(III) compounds. The isotropic chemical shift and chemical shift anisotropy (CSA) are reported for a crystalline form of the dihydroxy-bridged Rh(III) dimer and for a salt of the oxo-centered acetate-bridged Rh(III) trimer, from analysis of conventional CP/MAS spectra. Comparison of the CP kinetics of the dimer with new crystal structure data suggests a strategy for predicting 103Rh CP/MAS properties in solids.     

Corrigendum to solvent-dependent oxidative coupling of 1-aryl-1,3-dicarbonyls and styrene

This report describes the scope and mechanism of the solvent-dependent, chemoselective oxidative coupling of 1-aryl-1,3-dicarbonyls with styrene using Ce(IV) reagents. Dihydrofuran derivatives are obtained when reactions are performed in methanol, whereas nitrate esters can be selectively synthesized in acetonitrile and methylene chloride. Mechanistic studies are consistent with the rate of solvent-assisted deprotonation of a radical cation intermediate playing an integral role in the selective formation of products.     

A molecular dynamics investigation of hydrolytic polymerization in a metal-hydroxide gel

The early stages of the spontaneous hydrolytic polymerization of an active hydroxide in the anti-bayerite structure are followed by using molecular dynamics simulations. The polymer populations are calculated as a function of reaction progress and compared with a model governed by purely random bonding. In agreement with experimental observations in aqueous solutions, the transformation of singly bridged to doubly bridged metal ions is shown to have a significant autocatalytic component. The overall polymer populations, however, are very close to the predictions of the random bonding model, indicating that local autocatalytic behavior is decoupled from multimer populations at the larger scales. The calculations show that solid-state transformation processes do not preferentially give rise to higher order multimers, and suggest that interfacial processes, involving bulk solution, are required to facilitate rapid transformation to higher order oligomers. Two reaction pathways are identified for the dewatering of mu-H(3)O(2) bridges into mu-OH bridges. Both are activated primarily by undercoordination of one of the metals centers involved in the bridge.     

A molecular dynamics investigation of the titration of a trivalent aqueous ion

We carried out a series of molecular dynamics simulations of the hydrolysis of a model trivalent metal ion in aqueous solution. We use a dissociative model for water and examine the spontaneous speciation of M³⁺ into M(OH) _n ^(3-n)+ (n =1,4) both in neutral solution and as a function of added protons and hydroxide ions. The species distributions in neutral solution correspond reasonably well with those expected for real trivalent metal ions at neutral pH. However, the change in the species distributions as a function of either added protons or hydroxide ions is much less than expected with very large concentrations of protons or hydroxide ions required to shift the species equilibria in either direction. The influence of added protons and hydroxide ions on the species distributions appears to be proportional to the average charge of the hydrolysis couples, being highest for the 3+/2+ couple and lowest for the 1+/0 and 0/1- couples. Proton exchange rates vary with proton/hydroxide ion concentration giving a minimum at intermediate values ([H+]≈ 0.166) with increasing rates at both lower and higher pH.     

EXCITED STATES OF DNA and ITS COMPONENTS AT ROOM TEMPERATURE–V. SPECTRAL, POLARIZATION and QUANTUM YIELD STUDIES OF CYTIDYLYL-(3',5')-ADENOSINE

Abstract— The corrected normalized emission spectrum, quantum yield, and emission anisotropy are reported for partially-stacked CpA excited at 266 nm. Utilizing parameters determined separately for cytidine and AMP, the emission spectrum of CpA is quantitatively resolved into components characteristic of the two monomers plus a red shifted emission characteristic of complex formation. The results fit accurately to a simple stacking model in which monomer-like fluorescence originates only from the unstacked fraction and "complex" fluorescence from the stacked fraction. It is shown that the stacked fraction absorbs in a manner qualitatively distinct from the unstacked fraction and the "complex" emission is really the excited state of a ground state complex. This emission, which peaks at 440 nm and contains at least two species can be resolved into three Gaussian components. It is suggested that the emitting species may originate in distinct stacked conformations.     

Proton exchange kinetics from the bound waters on the oxo-centered rhodium(III) trimer [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+: a variable pH and temperature 1H NMR study

Proton exchange from the bound to the bulk waters on the oxo-centered rhodium(III) trimer, [Rh(3)(micro(3)-O)(micro-O(2)CCH(3))(6)(OH(2))(3)](+)(abbreviated as Rh(3)(+)), was investigated over the temperature range of 219.1-313.9 K using a (1)H NMR line-broadening technique. By solving the modified Bloch equations for a two-site chemical exchange, lifetimes (tau) for proton transfer at pH = 2.7, 3.6, and 7.0 ([Rh(3)(+)]= 26 mM, T= 298 K) were determined to be 0.3 (+/-.08) ms, 2 (+/-0.3) ms, and 0.2 (+/-0.2) ms, respectively. From the temperature dependence of the rate, the activation parameters were determined to be DeltaH(++)= 16.2 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 123 (+/-2) J mol(-1) K(-1), DeltaH(++)= 14.9 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 141 (+/-2) J mol(-1) K(-1), and DeltaH(++)= 45 (+/-2) kJ mol(-1) and DeltaS(++)=- 22 (+/-5) J mol(-1) K(-1) for pH = 2.7, 3.6 and 7.0, respectively. All results are reported for a mixed solvent system [acetone : 250 mM NaClO(4)(aq)(3:1)], which was necessary to depress the freezing point of the solution so that the (1)H NMR signal due to bound water could be observed. The pK(a) of Rh(3)(+) was measured to be 8.9 (+/-0.2) in the mixed solvent, which is near the pK(a) for an aqueous solution (8.3 (+/-0.2)). Surprisingly, the lifetimes for protons on Rh(3)(+) are close to those observed for the Rh(OH(2))(6)(3+) ion, in spite of the considerable difference in structure, Br?nsted acidity of the bound waters and average charge on the metal ion.     

Water exchange in fluoroaluminate complexes in aqueous solution: a variable temperature multinuclear NMR study

An 17O, 19F, and 27Al NMR study of fluoroaluminate complexes (AlFn(H2O)6-n((3-n)+), n = 0, 1, and 2) in aqueous solution supports the idea that for each substitution of a bound water molecule by a fluoride anion, the exchange rate of bound water with free water increases by about 2 orders of magnitude. New rate coefficients for exchange of inner-sphere water molecules in AlF(H2O)5(2+) are kex(298) = 230(+/-20) s(-1), DeltaH(dagger) = 65(+/-3) kJ mol(-1), and DeltaS(dagger) = 19(+/-10) J mol(-1) K(-1). The corresponding new values for the AlF2(H2O)4(+) complex are: kex(298) = 17 100(+/-500) s(-1), DeltaH(dagger) = 66(+/-2) kJ mol(-1), and DeltaS(dagger) = 57(+/-8) J mol(-1) K(-1). When these new results are combined with those of our previous study,(4) we find no dependence of the solvent exchange rate, in either AlF(H2O)5(2+) or AlF2(H2O)4(+), on the concentration of fluoride or protons over the range of SigmaF = 0.06-0.50 M and [H(+)] = 0.01-0.44 M. A paramagnetic shift of 27Al resonances results from addition of Mn(II) to the aqueous solution as a relaxation agent for bulk waters. This shift allows resolution of the AlFn(H2O)6-n((3-n)+) species in 27Al NMR spectra and comparison of the speciation determined via thermodynamic calculations with that determined by 27Al, 19F, and 17O NMR.