Myungshim Kang

π–π Stacking Mediated Chirality in Functional Supramolecular Filaments

While a great diversity of peptide-based supra-molecular filaments have been reported, the impact of an auxiliary segment on the chiral assembly of peptides remains poorly understood. Herein we report on the formation of chiral filaments by the self-assembly of a peptide-drug conjugate containing an aromatic drug camptothecin (CPT) in a computational study. We find that the chirality of the filament is mediated by the π‒π stacking between CPTs, not only by the well-expected intermolecular hydrogen bonding between peptide segments. Our simulations show that π‒π stacking of CPTs governs the early stages of the self-assembly process, while a hydrogen bonding network starts at a relatively later stage to contribute to the eventual morphology of the filament. Our results also show the possible presence of water within the core of the CPT filament. These results provide very useful guiding principles for the rational design of supramolecular assemblies of peptide conjugates with aromatic segments.

Molecular simulation of the concentration-dependent interaction of hydrophobic drugs with model cellular membranes.

We report here the interactions between a hydrophobic drug and a model cellular membrane at the molecular level using all-atom molecular dynamics simulations of paclitaxel, a hydrophobic cancer drug. The calculated free energy of a single drug across the bilayer interface displays a minimum in the outer hydrophobic zone of the membrane. The transfer free energy shows excellent agreement with reported experimental data. In two sets of long-time simulations of high concentrations of drug in the membrane (12 and 11 mol %), the drugs display substantial clustering and rotation with significant directional preference in their diffusion. The main taxane ring partitions in the outer hydrophobic zone, while the three phenyl rings prefer to be closer to the hydrophobic core of the membrane. The clustering of the drug molecules, order parameters of the lipid tails, and water penetration suggest that the fluidity and permeability of the membrane are affected by the concentration of drugs that it contains. Furthermore, at the high-concentration limit, the free energy minimum shifts closer to the hydrophobic core and the central barrier to cross the membrane decreases. Moreover, the transfer free energy change substantially increases, suggesting that increasing concentration facilitates drug partitioning into the membrane.

Mechanistic insights into phosphopeptide--BRCT domain association: preorganization, flexibility, and phosphate recognition.

Promiscuous proteins are commonly observed in biological systems, for example, in modular domains that recognize phosphopeptides during signal transduction. This promiscuous recognition is of fundamental interest in chemistry and biology but is challenging when designing phosphopeptides in silico for cell biology studies. To investigate promiscuous recognition and binding processes of phosphopeptides and the modular domain, we selected a domain essential in breast cancer-the breast-cancer-associated protein 1 (BRCA1) C-terminal (BRCT) repeats as our model system. We performed molecular dynamics simulations and detailed analyses of the dihedral space to study protein fluctuation and conformational changes with phosphopeptide binding. We also studied the association processes of phosphorylated and unphosphorylated peptides using Brownian dynamics with a coarse-grained model. We found that the BRCT domain is preorganized for phosphopeptide binding but has a moderate arrangement of side chains to form complexes with various types of phosphopeptides. Phosphopeptide binding restricts the system motion in general, while the nonpolar phosphopeptide becomes more flexible in the bound state. Our analysis found that the BRCT domain utilizes different mechanisms, usually termed lock and key, induced-fit, and population-shift/conformational-selection models, to recognize peptides with different features. Brownian dynamics simulations revealed that the charged phosphate group may not always accelerate peptide association processes, but it helps the phosphopeptide orient into binding pockets accurately and stabilizes the complex. This work provides insights into molecular recognition in the promiscuous protein system.

Effect of Nucleotide State on the Protofilament Conformation of Tubulin Octamers

At the molecular level, the dynamic instability (random growth and shrinkage) of the microtubule (MT) is driven by the nucleotide state (GTP vs GDP) in the β subunit of the tubulin dimers at the MT cap. Here, we use large-scale molecular dynamics (MD) simulations and normal-mode analysis (NMA) to characterize the effect of a single GTP cap layer on tubulin octamers composed of two neighboring protofilaments (PFs). We utilize recently reported high-resolution structures of dynamic MTs to simulate a GDP octamer both with and without a single GTP cap layer. We perform multiple replicas of long-time atomistic MD simulations (3 replicas, 0.3 μs for each replica, 0.9 μs for each octamer system, and 1.8 μs total) of both octamers. We observe that a single GTP cap layer induces structural differences in neighboring PFs, finding that one PF possesses a gradual curvature, compared to the second PF which possesses a kinked conformation. This results in either curling or splaying between these PFs. We suggest that this is due to asymmetric strengths of longitudinal contacts between the two PFs. Furthermore, using NMA, we calculate mechanical properties of these octamer systems and find that octamer system with a single GTP cap layer possesses a lower flexural rigidity.

Molecular Dynamics Simulations of Supramolecular Anticancer Nanotubes

We report here on long-time all-atomistic molecular dynamics simulations of functional supramolecular nanotubes composed by the self-assembly of peptide-drug amphiphiles (DAs). These DAs have been shown to possess an inherently high drug loading of the hydrophobic anticancer drug camptothecin. We probe the self-assembly mechanism from random with ∼0.4 μs molecular dynamics simulations. Furthermore, we also computationally characterize the interfacial structure, directionality of π-π stacking, and water dynamics within several peptide-drug nanotubes with diameters consistent with the reported experimental nanotube diameter. Insight gained should inform the future design of these novel anticancer drug delivery systems.