Self-assembled nanostructures of peptide amphiphiles (PAs) with molecular structures C 16 K 2 and C 16 K 3 (where C indicates the number of carbon atoms in the alkyl chain and K is the lysine in the head group) were studied by a combination of theoretical modeling, transmission electron and atomic force microscopes, and acid−base titration experiments. The supramolecular morphology of the PAs (micelles, fibers, or lamellas) was dependent on the pH and ionic strength of the solution. Theoretical modeling was performed using a molecular theory that allows determining the equilibrium morphology, the size, and the charge of the soft nanoassemblies as a function of the molecular structure of the PA, and the pH and salt concentration of the solution. Theoretical predictions showed good agreement with experimental data for the pH-dependent morphology and size of the nanoassemblies and their apparent pK a s. Two interesting effects associated with charge regulation mechanisms were found: first, ionic strength plays a dual role in the modulation of the electrostatic interactions in the system, which leads to complex dependencies of the aggregation numbers with salt concentration; second, the aggregation number of the nanostructures decreases upon increasing the charge per PA. The second mechanism, charge regulation by size regulation, tunes the net charge of the assemblies to decrease the electrostatic repulsions. A remarkable consequence of this behavior is that adding an extra lysine residue to the charged region of the PAs can lead to an unexpected decrease in the total charge of the micelles. 59 Antimicrobial properties are also highly dependent on the 60 charge of the PAs: a recent study has shown that cationic PAs 61 can inhibit the formation of bacterial films, while anionic ones 62 show no antimicrobial activity at all. 25 In a related biomedical 63 application, the performance of vaccines prepared from PA 64 nanostructures was found to be strongly dependent on their 65 morphology, size, and charge. 26 The importance of nanostruc-66 ture morphology and charge transcends the biological uses of 67 PAs and spans nanotechnology applications as well. For 68 example, Stupp's group has developed a biomineralization 69 strategy for PA nanofibers that requires a negative surface

The assembly of synaptic protein-DNA complexes by specialized proteins is critical for bringing together two distant sites within a DNA molecule or bridging two DNA molecules. The assembly of such synaptosomes is needed in numerous genetic processes requiring the interactions of two or more sites. The molecular mechanisms by which the protein brings the sites together, enabling the assembly of synaptosomes, remain unknown. Such proteins can utilize sliding, jumping, and segmental transfer pathways proposed for the single-site search process, but none of these pathways explains how the synaptosome assembles. Here we used restriction enzyme SfiI, that requires the assembly of synaptosome for DNA cleavage, as our experimental system and applied time-lapse, high-speed AFM (HS-AFM) to directly visualize the site search process accomplished by the SfiI enzyme. For the single-site SfiI-DNA complexes, we were able to directly visualize such pathways as sliding, jumping, and segmental site transfer. However, within the synaptic looped complexes, we visualized the threading and site-bound segment transfer as the synaptosome-specific search pathways for SfiI. In addition, we visualised sliding and jumping pathways for the loop dissociated complexes. Based on our data, we propose the site-search model for synaptic protein-DNA systems.

Cytochrome P-450 (CYP) enzymes and P-glycoprotein (P-gp) play an important role in the oral bioavailability and first-pass-metabolism (FPM) of many drugs. Rasagiline is a selective, monoamine oxidase-B inhibitor and it undergoes significant FPM in the liver prior to excretion by CYP1A2. Hesperetin and naringenin are naturally occurring flavanones and are reported as modulators of CYP enzymes and P-gp. The objective of the present investigation was to evaluate the effect of hesperetin and naringenin on the pharmacokinetics (PK) of rasagiline in rats. Rats were treated orally with rasagiline (2 mg/kg) alone and co-administered with hesperetin and naringenin (12.5 and 25 mg/kg) for 15 consecutive days. Blood samples were collected from tail vein on the 1st day in a single dose PK study (SDS) and on 15th day in the multiple dose PK study (MDS). Hesperetin and naringenin co-administration significantly enhanced the area under the curve (AUC), maximum plasma concentration (Cmax) and elimination half life (t1/2) of rasagiline with a concomitant reduction in clearance (CL/F) in both SDS and MDS. Rasagiline concentrations were significantly increased when co-administered with hesperetin and naringenin in the brain. No significant difference was found in rasagiline transport from mucosal to serosal side in the presence of hesperetin and naringenin ex vivo (rat everted gut sacs used). Our findings suggested that hesperetin and naringenin enhanced the systemic exposure of rasagiline might be through the inhibition of CYP1A2 but not P-gp. Further studies are needed on CYP1A2 and P-gp over expressed cells to confirm this interaction at cellular level.

Interactions between distant DNA segments play important roles in various biological processes, such as DNA recombination. Certain restriction enzymes create DNA loops when two sites are held together and then cleave the DNA. DNA looping is important during DNA synapsis. Here we investigated the mechanisms of DNA looping by restriction enzyme SfiI by measuring the properties of the system at various temperatures. Different sized loop complexes, mediated by SfiI–DNA interactions, were visualized with AFM. The experimental results revealed that small loops are more favorable compared to other loop sizes at all temperatures. Our theoretical model found that entropic cost dominates at all conditions, which explains the preference for short loops. Furthermore, specific loop sizes were predicted as favorable from an energetic point of view. These predictions were tested by experiments with transiently assembled SfiI loops on a substrate with a single SfiI site.

Chromatin structure is dictated by nucleosome assembly and internucleosomal interactions. The tight wrapping of nucleosomes inhibits gene expression, but modifications to histone tails modulate chromatin structure, allowing for proper genetic function. The histone H4 tail is thought to play a large role in regulating chromatin structure. Here we investigated the structure of nucleosomes assembled with a tail-truncated H4 histone using Atomic Force Microscopy. We assembled tail-truncated H4 nucleosomes on DNA templates allowing for the assembly of mononucleosomes or dinucleosomes. Mononucleosomes assembled on nonspecific DNA led to decreased DNA wrapping efficiency. This effect is less pronounced for nucleosomes assembled on positioning motifs. Dinucleosome studies resulted in the discovery of two effects- truncation of the H4 tail does not diminish the preferential positioning observed in full-length nucleosomes, and internucleosomal interaction eliminates the DNA unwrapping effect. These findings provide insight on the role of histone H4 in chromatin structure and stability.

The synaptic protein-DNA complexes, formed by specialized proteins that bridge two or more distant sites on DNA, are critically involved in various genetic processes. However, the molecular mechanism by which the protein searches for these sites and how it brings them together is not well understood. Our previous studies directly visualized search pathways used by SfiI, and we identified two pathways, DNA threading and site-bound transfer pathways, specific to the site search process for synaptic DNA-protein systems. To investigate the molecular mechanism behind these site search pathways, we assembled complexes of SfiI with various DNA substrates corresponding to different transient states and measured their stability using a single-molecule fluorescence approach. These assemblies corresponded to specific-specific (synaptic), non-specific-non-specific (non-specific), and specific-non-specific (pre-synaptic) SfiI-DNA states. Unexpectedly, there was an elevated stability in pre-synaptic complexes assembled with specific and non-specific DNA substrates has been found. To explain these surprising observations, a theoretical approach that describes the assembly of these complexes and compares the predictions with the experiment is developed. The theory explains this effect by utilizing entropic arguments, according to which, after the partial dissociation, the non-specific DNA template has multiple possibilities of rebinding, effectively increasing the stability. Such difference in the stabilities of SfiI complexes with specific and non-specific DNA explains the utilization of threading and site-bound transfer pathways in the search process of synaptic protein-DNA complexes discovered in the time-lapse AFM experiments.

Background Metoprolol is a substrate of CYP3A4, 2B6, CYP2D6, CYP2C9, and p-glycoprotein (p-gp). Hesperetin was reported as an inhibitor of cytochrome P-450 (CYP) enzymes and p-gp. The objective of this study was to investigate the effect of hesperetin on the pharmacokinetics of metoprolol in rats and in vitro models. In in vivo studies, male Wistar rats were treated with metoprolol (30 mg/kg) once a day for 15 consecutive days alone and in combination with hesperetin (25, 50, and 100 mg/kg). Blood samples were withdrawn from the tail vein on the 1st day in the single-dose pharmacokinetic study and on the 15th day in the repeated-dose pharmacokinetic study. In in vitro studies, metoprolol was incubated in the presence or absence of hesperetin and traditional p-gp inhibitors using rat-everted gut sacs. Reverse phase-high-performance liquid chromatography (RP-HPLC) was used to determine the amounts of metoprolol in the plasma and incubated samples (RP-HPLC). Results The Cmax, AUC, and half-life (t1/2) of metoprolol significantly increased by twofold compared to the metoprolol group in rats pre-treated with hesperetin. The clearance and volume of distribution both decreased significantly. Metoprolol transport was dramatically increased in the presence of hesperetin and quinidine (standard p-gp inhibitor) in in vitro study. Conclusion The present study results revealed that hesperetin significantly increased the absorption of metoprolol in rats and everted gut sacs in vitro might be due to the inhibition of CYP and p-gp.

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