This article describes results on using steered molecular dynamics (SMD) simulations and experimental single molecule force spectroscopy (SMFS) to investigate the relationship between hydrogen bonding and mechanical stability of a series of homodimeric β-sheet mimics. The dimers consisting of 4, 6, and 8 H-bonding sites were modeled in explicit chloroform solvent and the rupture force was studied using constant velocity SMD. The role of solvent structuring on the conformation of the dimers was analyzed and showed no significant contribution of chloroform molecules in the rupture event. The simulated stability of the dimers was validated by force data obtained with atomic force microscopy (AFM)-based SMFS in toluene. The computational model for the 8H dimer also offered insight into a possible mismatched dimer intermediate that may contribute to the lower than expected mechanical stability observed by single molecule AFM force studies. In addition, atomic level analysis of the rupture mechanism verified the dependence of mechanical strength on pulling trajectory due to the directional nature of chemical bonding under an applied force. The knowledge gained from this basic study will be used to guide further design of modular polymers having folded nanostructures through strategic programming of weak, non-covalent interactions into polymer backbones.
The increase of information in biology makes it difficult for researchers in any field to keep current with the literature. The MEDLINE database of scientific abstracts can be quickly scanned using electronic mechanisms. Potentially interesting abstracts can be selected by matching words joined by Boolean operators. However this means of selecting documents is not optimal. Nonspecific queries have to be effected, resulting in large numbers of irrelevant abstracts that have to be manually scanned To facilitate this analysis, we have developed a system that compiles a summary of subjects and related documents on the results of a MEDLINE query. For this, we have applied a fuzzy binary relation formalism that deduces relations between words present in a set of abstracts preprocessed with a standard grammatical tagger. Those relations are used to derive ensembles of related words and their associated subsets of abstracts. The algorithm can be used publicly at http:// www.bork.embl-heidelberg.de/xplormed/.
Light harvesting complexes 2 (LH2) from Rhodospirillum (Rs.) molischianum and Rhodopseudomonas (Rps.) acidophila form ring complexes out of eight or nine identical subunits, respectively. Here, we investigate computationally what factors govern the different ring sizes. Starting from the crystal structure geometries, we embed two subunits of each species into their native lipid-bilayer/water environment. Using molecular dynamics simulations with umbrella sampling and steered molecular dynamics, we probe the free energy profiles along two reaction coordinates, the angle and the distance between two subunits. We find that two subunits prefer to arrange at distinctly different angles, depending on the species, at about 42.5 • for Rs. molischianum and at about 38.5 • for Rps. acidophila, which is likely to be an important factor contributing to the assembly into different ring sizes. Our calculations suggest a key role of surface contacts within the transmembrane domain in constraining these angles, whereas the strongest interactions stabilizing the subunit dimers are found in the C-, and to a lesser extent, N-terminal domains. The presented computational approach provides a promising starting point to investigate the factors contributing to the assembly of protein complexes, in particular if combined with modeling of genetic variants.
Most of the currently known light-harvesting complexes 2 (LH2) rings are formed by 8 or 9 subunits. As of now, questions like "what factors govern the LH2 ring size?" and "are there other ring sizes possible?" remain largely unanswered. Here, we investigate by means of molecular dynamics (MD) simulations and stochastic modeling the possibility of predicting the size of an LH2 ring from the sole knowledge of the high resolution crystal structure of a single subunit. Starting with single subunits of two LH2 rings with known size, that is, an 8-ring from Rs. moliscianum (MOLI) and a 9-ring from Rps. acidophila (ACI), and one with unknown size (referred to as X), we build atomic models of subunit dimers corresponding to assumed 8-, 9-, and 10-ring geometries. After inserting each of the dimers into a lipid-water environment, we determine the preferred angle between the corresponding subunits by three methods: (1) energy minimization, (2) free MD simulations, and (3) potential of mean force calculations. We find that the results from all three methods are consistent with each other, and when taken together, it allows one to predict with reasonable level of confidence the sizes of the corresponding ring structures. One finds that X and ACI very likely form a 9-ring, while MOLI is more likely to form an 8-ring than a 9-ring. Finally, we discuss both the merits and limitations of all three prediction methods.
Cholesterol is thought to be important for the structure and assembly of lipid rafts, and its interaction with other membrane lipids has been a topic of great interest. This study focuses on the interactions between 1,2-dimyristoyl-snglycero-3-phosphoethanolamine (DMPE) and dihydrocholesterol (Dchol) in Langmuir monolayers using fluorescence microscopy (FM), beta-cyclodextrin (CD) desorption assays, and grazing incidence x-ray diffraction (GIXD). Similar to our previous results for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/Dchol monolayers [Biophys. J. 2007, 93, 2038-2047], FM and CD assays show 2 regimes for the DMPE/Dchol system. Short-ranged lateral ordering was observed using GIXD that was also consistent with our recent work on sphingomyelin (SM)/Dchol monolayers [Phys. Rev. Lett. 2009, 103, 028103]. We investigate how the smaller headgroup of DMPE affects the surface morphology, Dchol chemical activity, and lateral structure compared to monolayers of Dchol with DMPC or SM.
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