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Molecules with Multiple Chiral Centers02:25

Molecules with Multiple Chiral Centers

Molecules that possess multiple chiral centers can afford a large number of stereoisomers. For instance, while some molecules like 2-butanol have one chiral center, defined as a tetrahedral carbon atom with four different substituents attached, several molecules like butane-2,3-diol have multiple chiral centers. A simple formula to predict the number of stereoisomers possible for a molecule with n chiral centers is 2n. However, there can be a lower number where some of the stereoisomers are...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)01:16

Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)

Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
Angle of Twist - Elastic Range01:13

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Consider a cylindrical shaft with a length denoted by L and a consistent cross-sectional radius referred to as r. This shaft undergoes a torque at the free end. The highest shearing strain within the shaft is directly proportional to the twist angle and the radial distance from the shaft axis. When the shaft behaves elastically, this shearing strain can be articulated using variables such as the applied torque, radial distance, the polar moment of inertia, and the modulus of rigidity. By...
Angle of Twist: Problem Solving01:13

Angle of Twist: Problem Solving

An electric motor applies a torque of 700 N·m to an aluminum shaft, triggering a stable rotation. Two pulleys, B and C, are subjected to torques of 300 N·m and 400 N·m, respectively. The modulus of rigidity is provided as 25 GPa. With the knowledge of the length and diameter of each segment, the twist angle between the two pulleys can be computed. First, a section cut is made between pulleys B and C, and the cut cross-section is analyzed using a free-body diagram. Given that the torque exerted...

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Fabricating van der Waals Heterostructures with Precise Rotational Alignment
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Las hélices polipeptídicas en secuencias de péptidos híbridos.

Kuppanna Ananda1, Prema G Vasudev, Anindita Sengupta

  • 1Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560 012, India.

Journal of the American Chemical Society
|November 25, 2005
PubMed
Resumen

Los investigadores describen nuevas hélices polipeptídicas utilizando residuos alfa, beta y gamma. Estas secuencias híbridas, incluidas (alfa alfhabeta) n y (alfa gamma) n, exhiben giros helicoidales expandidos únicos y estructuras repetitivas en conformaciones cristalinas.

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Área de la Ciencia:

  • La bioquímica es la bioquímica.
  • Biología Estructural Biología estructural.
  • Química orgánica es la química orgánica.

Sus antecedentes:

  • Las hélices polipeptídicas son fundamentales para la estructura y función de las proteínas.
  • Las secuencias híbridas que incorporan diversos tipos de aminoácidos ofrecen nuevas posibilidades de conformación.
  • Comprender estas estructuras es clave para diseñar nuevos biomateriales y terapias.

Objetivo del estudio:

  • Para describir una nueva clase de hélices polipeptídicas formadas por secuencias híbridas.
  • Para determinar las conformaciones moleculares de péptidos sintéticos que contienen residuos alfa, beta y gamma.
  • Explorar el potencial para giros helicoidales expandidos y nuevas estructuras de enlaces de hidrógeno.

Principales métodos:

  • Se utilizó la cristalografía de rayos X para determinar las conformaciones moleculares de péptidos sintéticos.
  • El análisis de las estructuras cristalinas reveló estructuras helicoidales repetitivas con unidades específicas enlazadas con hidrógeno (por ejemplo, C14 y C12).
  • Los ángulos de torsión de la columna vertebral determinados experimentalmente se utilizaron para generar hélices híbridas energéticamente favorables.

Principales resultados:

  • Se identificó una nueva clase de hélices polipeptídicas en secuencias híbridas (alpha alphabeta) n y (alphagamma) n.
  • El péptido 1 exhibió una estructura helicoidal repetitiva con unidades enlazadas con hidrógeno C14.
  • El péptido 2 demostró una estructura helicoidal repetitiva con unidades C12 enlazadas con hidrógeno.
  • Se proporcionaron parámetros de conformidad para las hélices C11 a C15 en secuencias híbridas.

Conclusiones:

  • Las secuencias híbridas que contienen residuos alfa, beta y gamma pueden formar nuevas hélices polipeptídicas con giros expandidos.
  • El estudio proporciona información sobre la diversidad estructural y los parámetros conformacionales de estas hélices híbridas.
  • Estos hallazgos podrían informar el diseño de péptidos con propiedades estructurales y funcionales a medida.