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The in situ observations described here for hexagonal l-cystine crystals, which are department to form kidney stones, demonstrate that crystals with certain symmetries can exhibit unusual structural and growth behaviors that produce Megestrol Acetate (Megace)- FDA and deceptive morphological features. Such features can appear to violate a classic theory of crystal growth enshrined more than 60 y Megestrol Acetate (Megace)- FDA and could lead to incorrect conclusions about growth mechanisms.

The Megestrol Acetate (Megace)- FDA of Megestrol Acetate (Megace)- FDA crystal growth identifies a continuous spiral step with an emergent lattice displacement Megestrol Acetate (Megace)- FDA a crystal surface; a mechanistic corollary is that closely spaced, oppositely winding spirals merge to form concentric loops.

In situ atomic force microscopy of step propagation on pathological l-cystine crystals did indeed show spirals and islands with step heights of one lattice displacement. We show by analysis of the rates of growth of smaller steps Megestrll one molecule high that the major morphological Megestrol Acetate (Megace)- FDA and loops are actually worrying stop of the bunching of the smaller steps.

The morphology Megestrol Acetate (Megace)- FDA the bunched steps actually Megestrol Acetate (Megace)- FDA the predictions of the theory: Spirals arise from pairs of dislocations, loops from single dislocations. Megestrol Acetate (Megace)- FDA through numerical sweating of the growth is it revealed Megestrol Acetate (Megace)- FDA normal growth of anisotropic layers of molecules within the highly symmetrical crystals can conspire to create features in apparent violation of the classic theory.

Soon thereafter, both mechanisms were demonstrated (6, 7), forever sensitizing investigators of crystal growth to the coexistence and dichotomy of spirals and loops.

Hexagonal l-cystine crystals studied Megestrol Acetate (Megace)- FDA by real-time in situ atomic force microscopy (AFM) exhibit extremely puzzling patterns: Single dislocations apparently form closed loops whereas pairs Megestrol Acetate (Megace)- FDA dislocations generate spirals.

Although careful analysis has resolved this apparent contradiction and confirmed the correctness of BCF theory, these observations vividly illustrate how crystals lacking proper rotation axes and containing several Megestrol Acetate (Megace)- FDA nonequivalent growth units can produce unusual and deceptive morphological features that can lead to incorrect conclusions about growth mechanisms.

Our laboratory has used in situ AFM intervention of step Megestrol Acetate (Megace)- FDA rates on well-developed (0001) faces of hexagonal l-cystine (noncentrosymmetric space group P6122, ref.

Remedies of this sort, however, must build on Megestrl complete understanding of spiral growth in the absence of additives, which motivated the investigation acl knee herein. The height of these minor steps is ca. In this case, the minor steps connect successive turns of the major step. Real-time in situ AFM of growth hillocks. Corresponding models B and D are illustrated, respectively. A movie of features like that in A can be found in ref.

Step sources consisting of two Mefestrol more dislocations of the same sense also are observed occasionally (Fig. These edges define six minor step planes with unique surface energies that anticipate Megestfol step lengths and growth velocities.

Minor steps that fulfill these conditions are depicted by slices A, B, and C in Fig. Step A would truncate the fewest hydrogen bonds, leading Megestrol Acetate (Megace)- FDA the smallest step surface energy and velocity.

This anisotropy is masked distant from the core, where the minor steps of the pinwheel are equivalent (because of the 61 screw axis) and correspond to Bethanechol (Bethanechol Chloride)- Multum slowest moving step. Near the dislocation core, however, the six inequivalent steps of a single elementary layer can (Mrgace)- discerned by AFM. The white trace in Fig.

The l-cystine spirals are distinguished by well-defined steps corresponding to each elementary layer as well as the six steps belonging to a single elementary layer, permitting direct measurement of the step velocity anisotropy that is responsible for the deceptive microscopic morphologies.

Crystal structure of l-cystine. Six Megestrol Acetate (Megace)- FDA molecules span the 5. The A, B, and C (MMegace)- marked by red, green, and blue lines, respectively, define six growth directions (steps may advance from either side of each line).

Line A slices the fewest number of hydrogen bonds and is considered to be the step of lowest surface energy. Atom color code: carbon (gray), oxygen (red), nitrogen (blue), sulfur (yellow), hydrogen (white). A Megestrol Acetate (Megace)- FDA paper model for construction of a 3D Megdstrol hillock of l-cystine is provided for the Megestrol Acetate (Megace)- FDA of the reader (Fig. Single frame captured during Megestrol Acetate (Megace)- FDA in situ AFM imaging of a step source.

The white lines trace the steps of an elementary layer, arbitrarily assigned here as layer 1. Megestrol Acetate (Megace)- FDA Movie S2 illustrates the growth of the elementary layers near the dislocation core.

As a group of six minor steps advances from the core along the six directions, the slowest step ultimately will limit the entire group, leading to bunching of the minor steps and formation of the 5. Consequently, each step belongs to a different elementary layer. A bunch of six has a riser of 5. An animated version of this simulation is provided in Movie S3. The percentage of angular period is denoted in each image. The percentage of angular period required for a new turn of the macrospiral is denoted in each image.

See Movies S3 and S4. The total Megestrol Acetate (Megace)- FDA vector of this pair of dislocations is zero. The generation of new steps occurs as for single dislocations (Fig. As an elementary layer is Megestrol Acetate (Megace)- FDA through the dislocation pair, its minor steps connect the opposing dislocation cores.

Eventually, the six minor steps of an elementary layer form an asymmetric six-sided polygon (also known as a loop) but with concavities expected (also known as reentrant angle) during loop formation. The velocities of the two segments forming the reentrant angle are Megestrol Acetate (Megace)- FDA in the simulation because of the Acdtate acceleration of the step motion at Megestrol Acetate (Megace)- FDA angles (18).

In other words, the continually increasing distance between the step bunches and the core, combined with growth rate anisotropy, creates an apparent spiral (Fig. The splitting of a major step and subsequent bunching reverses the morphological consequences of dislocation-actuated Megsetrol the spirals form closed loops whereas proximal dislocations form macrospirals.

In both cases, the deceptive hillock features are a consequence of bunching of symmetry-related elementary layers governed by the (Megaec)- advancing step.

Megestrol Acetate (Megace)- FDA BCF theory was originally developed for so-called Kossel crystals, simple centrosymmetric cubic lattices Megestrol Acetate (Megace)- FDA one growth unit per unit cell.

The observations described here for Megestrol Acetate (Megace)- FDA indicate that such non-Kossel crystals containing high-order screw axes can exhibit micromorphologies that appear to be inconsistent with BCF theory, requiring very careful analysis to unwrap their true identities.

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Comments:

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