Cornelia Bohne

New Insights into Peptide–Silver Nanoparticle Interaction: Deciphering the Role of Cysteine and Lysine in the Peptide Sequence

We studied the interaction of four new pentapeptides with spherical silver nanoparticles. Our findings indicate that the combination of the thiol in Cys and amines in Lys/Arg residues is critical to providing stable protection for the silver surface. Molecular simulation reveals the atomic scale interactions that underlie the observed stabilizing effect of these peptides, while yielding qualitative agreement with experiment for ranking the affinity of the four pentapeptides for the silver surface.

Guest binding dynamics with cucurbit[7]uril in the presence of cations.

The binding dynamics of R-(+)-2-naphthyl-1-ethylammonium cation (NpH(+)) with cucurbit[7]uril (CB[7]) was investigated. Competitive binding with Na(+) or H(3)O(+) cations enabled the reaction to be slowed down sufficiently for the kinetics to be studied by fluorescence stopped-flow experiments. The binding of two Na(+) cations to CB[7], i.e., CB[7]·Na(+) (K(01) = 130 ± 10 M(-1)) and Na(+)·CB[7]·Na(+) (K(02) = 21 ± 2 M(-1)), was derived from the analysis of binding isotherms and the kinetic studies. NpH(+) binds only to free CB[7] ((1.06 ± 0.05) × 10(7) M(-1)), and the association rate constant of (6.3 ± 0.3) × 10(8) M(-1) s(-1) is 1 order of magnitude lower than that for a diffusion-controlled process and much higher than the association rate constant previously determined for other CB[n] systems. The high equilibrium constant for the NpH(+)@CB[7] complex is a consequence of the slow dissociation rate constant of 55 s(-1). The kinetics results showed that formation of a complex between a positively charged guest with CB[n] can occur at a rate close to the diffusion-controlled limit with no detection of a stable exclusion complex.

Laser photolysis studies of photochromic processes in spirooxazines: solvent effects on photomerocyanine behavior☆

Abstract The photochemistry of spirooxazines I and II was examined using laser flash photolysis techniques. Both compounds lead to the formation of a strongly absorbing and long-lived, ring-opened photomerocyanine (PMC). In non-polar solvents the PMCs undergo photo-isomerization induced by visible light irradiation which ultimately regenerates the starting materials. In the case of II an additional transient (believed to be a charge-transfer intermediate) is also formed. In spite of the rich transient phenomena observed, no new products were detected even after extensive laser irradiation. The remarkable fatigue resistance of these compounds makes them excellent candidates for commercial applications.

Supramolecular Complexation and Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylic Acid with 4-Aminoprolinol Derivatives as Chiral Hydrogen-Bonding Templates

The photochirogenesis of 2-anthracenecarboxylic acid (AC) complexed to a hydrogen-bonding template (TKS159) was investigated to obtain mechanistic information on how chirogenesis is achieved for the dimerization of AC. Complexation of AC to TKS159 leads to the shielding of one of the two surfaces of the prochiral AC molecule. The two diastereomeric AC-TKS complexes, i.e., re-AC-TKS and si-AC-TKS, were characterized by changes in the UV-vis, fluorescence, and circular dichroism spectra and excited-state lifetimes. The ee is not simply determined by the diastereomeric ratio of the re- and si-AC-TKS complexes but also depends on the relative lifetimes of the diastereomeric complexes. The relative population of the re and si complexes was calculated from the enantiomeric excess (ee) for the products, taking into account the relative lifetimes of the two complexes. These studies established a protocol that can be used to reveal the mechanism for photochirogenesis by investigating the ground state and the excited state behavior of supramolecular systems.

Chlorophyll: an efficient detector of electronically excited species in biochemical systems

Micelle-solubilized chlorophyll efficiently detects electronically excited species generated in enzymatic systems. In most, if not all, systems the chemiexcited species is formed in the triplet state; chlorophyll fluorescence is observed as result of energy transfer. Red emission can also be elicited from chlorophyll in chloroplasts or bound to microsomes.

Intramolecular H-Atom Abstraction in γ-Azido-Butyrophenones: Formation of 1,5 Ketyl Iminyl Radicals

Photolysis of gamma-azidobutyrophenone derivatives yields 1,4 ketyl biradicals via intramolecular H-atom abstraction. The 1,4 ketyl biradicals expel a nitrogen molecule to form 1,5 ketyl iminyl biradicals, which decay by ring closure to form a new carbon-nitrogen bond. The 1,5 ketyl iminyl biradicals were characterized with transient spectroscopy. In argon/nitrogen-saturated solutions, the biradicals have lambda(max) approximately 300 nm and tau = 15 micros. DFT-TD calculations were used to support the proposed mechanism for formation of the 1,5 ketyl iminyl radicals.

Effect of the Guest Size and Shape on Its Binding Dynamics with Sodium Cholate Aggregates

The binding dynamics of the guests acenaphthene, phenanthrene, fluorene, and acenaphthenol with sodium cholate aggregates were studied using laser flash photolysis and fluorescence. The location of the guests in the bile salt aggregate is determined by the guests hydrophobicity, where acenaphthene, phenanthrene, and fluorene bind to the primary aggregates, while acenaphthenol binds to the secondary bile salt aggregates. The residence time of the guests in the primary aggregates and the access of ionic species from the aqueous phase to the guest in the aggregate depend on the size and the shape of the guest. These results show that bile salt aggregates are adaptable supramolecular host systems.

PROBING BILE SALT AGGREGATES BY FLUORESCENCE QUENCHING

Abstract— The combination of steady‐state and time‐resolved quenching experiments was employed to study the aggregation behavior of sodium cholate at concentrations below 50 mAf. Naphthalene, anthracene and pyrene were used as fluorescent probe molecules, and protection by the aggregates from aqueous quenchers, as well as the onset of aggregation at low sodium cholate concentrations, was dependent on the shape of the probes. Protection from aqueous quenchers was inferred by comparing the efficiency for dynamic quenching in the absence and presence of sodium cholate and was best for naphthalene followed by pyrene and anthracene. Static quenching was observed, suggesting that probe molecules are located in an aggregate environment that also contains iodide. The incorporation of pyrene at low sodium cholate concentrations, as well as the small degree of static quenching observed, suggest that the shape complementarity, i.e. hydrophobic surface and packing, between pyrene and sodium cholate is optimum for aggregate formation.

Photophysical studies on the supramolecular photochirogenesis for the photocyclodimerization of 2-anthracenecarboxylate within human serum albumin.

The mechanism for the chirogenesis in the photocyclodimerization of 2-anthracenecarboxylate (AC) bound to human serum albumin (HSA) was investigated using time-resolved fluorescence measurements in the presence of HSA inhibitors and/or an AC singlet excited state quencher. The photophysical studies were correlated with product studies to explain the high enantiomeric excess (ee) observed for the chiral photoproducts. AC binds to HSA in five different binding sites with decreasing affinities. AC bound to the sites with the highest affinity (sites 1 and 2) is unreactive, and the AC can be displaced from these sites by the use of known inhibitors. Time-resolved fluorescence studies isolated a singlet excited state AC bound to a site which exhibited moderate protection from interactions with species in the aqueous phase. This site was assigned to binding site 3, where the chiral photoproducts are formed with a high ee based on the correlation of the photophysical studies with product studies in the presence of a quencher. These results show that the use of inhibitors for multiple binding site proteins is useful to uncover the properties of binding sites for which guest binding has only moderate affinity and where the photophysical characterization of these binding sites is not possible in the absence of inhibitors.

Explaining the highly enantiomeric photocyclodimerization of 2-anthracenecarboxylate bound to human serum albumin using time-resolved anisotropy studies.

The mechanism for the high enantiomeric excess (ee) (80-90%) observed in the photocyclodimerization of 2-anthracenecarboxylate (AC) in the chiral binding sites of human serum albumin (HSA) was studied using fluorescence anisotropy. A long rotational correlation time of 36 ns was observed for the excited states of the ACs bound to the HSA site responsible for the high ee, suggesting that the ACs have restricted rotational mobility in this site. The ACs in this site have the same prochiral face protected by the protein, and this protection is responsible for the high ee observed. These insights provide a strategy for the rational design of supramolecular photochirogenic systems.