31 research outputs found
Polymorphism, Structure, and Nucleation of Cholesterol·H2O at Aqueous Interfaces and in Pathological Media: Revisited from a Computational Perspective
No full textWe revisit the important issues of polymorphism, structure, and nucleation of cholesterol center dot H2O using first-principles calculations based on dispersion-augmented density functional theory. For the lesser known monoclinic polymorph, we obtain a fully extended H-bonded network in a structure akin to that of hexagonal ice. We show that the energy of the monoclinic and triclinic polymorphs is similar, strongly suggesting that kinetic and environmental effects play a significant role in determining polymorph nucleation. Furthermore, we find evidence in support of various O-H center dot center dot center dot O bonding motifs in both polymorphs that may result in hydroxyl disorder. We have been able to explain, via computation, why a single cholesterol bilayer in hydrated membranes always crystallizes in the monoclinic polymorph. We rationalize what we believe is a single-crystal to single-crystal transformation of the monoclinic form on increased interlayer growth beyond that of a single cholesterol bilayer, interleaved by a water bilayer. We show that the ice-like structure is also relevant to the related cholestanol center dot 2H(2)O and stigmasterol center dot H2O crystals. The structure of stigmasterol hydrate both as a trilayer film at the air-water interface and as a macroscopic crystal further assists us in understanding the polymorphic and thermal behavior of cholesterol center dot H2O. Finally, we posit a possible role for one of the sterol esters in the crystallization of cholesterol center dot H2O in pathological environments, based on a composite of a crystalline bilayer of cholesteryl palmitate bound epirmrially as a nucleating agent to the monoclinic cholesterol center dot H2O form
Interaction-Driven Giant Orbital Magnetic Moments in Carbon Nanotubes
No full textCarbon nanotubes continue to be model systems for studies of confinement and interactions. This is particularly true in the case of so-called "ultraclean" carbon nanotube devices offering the study of quantum dots with extremely low disorder. The quality of such systems, however, has increasingly revealed glaring discrepancies between experiment and theory. Here, we address the outstanding anomaly of exceptionally large orbital magnetic moments in carbon nanotube quantum dots. We perform low temperature magnetotransport measurements of the orbital magnetic moment and find it is up to 7 times larger than expected from the conventional semiclassical model. Moreover, the magnitude of the magnetic moment monotonically drops with the addition of each electron to the quantum dot directly contradicting the widely accepted shell filling picture of single-particle levels. We carry out quasiparticle calculations, both from first principles and within the effective-mass approximation, and find the giant magnetic moments can only be captured by considering a self-energy correction to the electronic band structure due to electron-electron interactions.QN/van der Zant LabQN/Steele La
Polymer-Rich Dense Phase Can Concentrate Metastable Silica Precursors and Regulate Their Mineralization
No full textMultistep mineralization processes are pivotal in the
fabrication
of functional materials and are often characterized by far from equilibrium
conditions and high supersaturation. Interestingly, such ‘nonclassical’
mineralization pathways are widespread in biological systems, even
though concentrating molecules well beyond their saturation level
is incompatible with cellular homeostasis. Here, we show how polymer
phase separation can facilitate bioinspired silica formation by passively
concentrating the inorganic building blocks within the polymer dense
phase. The high affinity of the dense phase to mobile silica precursors
generates a diffusive flux against the concentration gradient, similar
to dynamic equilibrium, and the resulting high supersaturation leads
to precipitation of insoluble silica. Manipulating the chemistry of
the dense phase allows to control the delicate interplay between polymer
chemistry and silica precipitation. These results connect two phase
transition phenomena, mineralization and coacervation, and offer a
framework to achieve better control of mineral formation
Formation of 3D Cholesterol Crystals from 2D Nucleation Sites in Lipid Bilayer Membranes: Implications for Atherosclerosis
No full text
Cholesterol-Enriched Macrophages Shed Cholesterol Microdomains Into the Extracellular Matrix
No full text
Polymorphism, Structure, and Nucleation of Cholesterol.H2O at Aqueous Interfaces and in Pathological Media: Revisited from a Computational Perspective
No full textWe revisit the important issues of polymorphism, structure, and nucleation of cholesterol monohydrate, using first principles calculations based on dispersion-augmented density functional theory. For the lesser known monoclinic polymorph, we obtain a new, fully extended H-bonded network, comprising the sterol hydroxyl groups and water molecules in a structure akin to that of hexagonal ice. We show that the energy of the monoclinic and triclinic polymorphs is similar, strongly suggesting that kinetic and environmental effects play a significant role in determining polymorph nucleation. Furthermore, we find evidence in support of various O-H…O bonding motifs, in both polymorphs, that may result in structural disorder. We then rationalize what we believe is a single-crystal to single-crystal transformation of the monoclinic form on increased interlayer growth beyond that of a single cholesterol bilayer, interleaved by a water bilayer, and show that the ice-like structure is also relevant to the related cholestanol dihydrate (2H2O) crystal. Finally, we posit a possible role for cholesterol esters in the crystallization of cholesterol.H2O in pathological environments, with a composite of a bilayer of cholesteryl palmitate bound epitaxially as a nucleating agent to the monoclinic form of cholesterol.H2O
Guanine crystal formation by the unicellular organism Phacotus lenticularis is part of a cellular stress response.
No full textOrganic crystals, and in particular guanine crystals, are widely used by multicellular organisms for manipulating light and producing structural colors. Many single celled eukaryotic organisms also produce organic crystals, and guanine is the most abundant type produced. Their functions are thought to be related to the fact that guanine is nitrogen rich. Here we studied a freshwater unicellular eukaryotic alga, Phacotus lenticularis, and found that when the growth medium is depleted in phosphorus, the alga stops reproducing and produces intracellular birefringent particles inside vesicles. Cryo-SEM showed that these particles are faceted and are located within membranes inside the cell. Using Raman spectroscopy, we showed that these particles are β-guanine crystals. 3D tomograms produced using cryo-soft-X-ray-microscopy quantitatively documented the increase in cell volume and distribution of guanine crystals within the cells with increasing time of phosphorous deprivation. The tomograms also showed additional morphological changes in other cellular organelles, namely starch granules, chloroplasts, nuclear DNA and membranes. The combined observations all indicate that under phosphorous depletion, the algal cells undergo a massive stress response. As guanine crystal formation is part of this response, we conclude that guanine crystals are formed in response to stress, and this is not related to nitrogen availability. Upon addition of phosphate to the P-depleted media, the algal cells, with their guanine crystals, resume reproduction. From this we conclude that the guanine crystals somehow contribute to the recovery from stress
Formation of 3D Cholesterol Crystals from 2D Nucleation Sites in Lipid Bilayer Membranes: Implications for Atherosclerosis
No full textAtherosclerosis is
the major precursor of cardiovascular disease.
The formation of cholesterol crystals in atherosclerotic plaques is
associated with the onset of acute pathology. The cholesterol crystals
induce physical injury in the plaque core, promoting cell apoptosis
and triggering an increased inflammatory response. Herein we address
the question of how cholesterol crystal formation occurs in atherosclerosis.
We demonstrate that three-dimensional (3D) cholesterol crystals can
undergo directed nucleation from bilayer membranes containing two-dimensional
(2D) cholesterol crystalline domains. We studied crystal formation
on supported lipid bilayers loaded with exogenous cholesterol and
labeled using a monoclonal antibody that specifically recognizes ordered
cholesterol arrays. Our findings show that 3D crystals are formed
exclusively on the bilayer regions where there are segregated 2D cholesterol
crystalline domains and that they form on the domains. This study
has potentially significant implications for our understanding of
the crucial step in the mechanism by which atherosclerotic lesions
form
