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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
Atomic Nuclei: Magnetic Resonance01:05

Atomic Nuclei: Magnetic Resonance

The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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.
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.

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Video Experimental Relacionado

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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
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Mapeo de las conformaciones de las etiquetas de espín de la resonancia paramagnética de electrones mediante el método

Mikolai I Fajer1, Hongzhi Li, Wei Yang

  • 1Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306, USA.

Journal of the American Chemical Society
|October 24, 2007
PubMed
Resumen

Desarrollamos un nuevo enfoque de escalado simulado para simulaciones eficientes de etiquetas de espín de proteínas. Este método predice con precisión el comportamiento de las etiquetas de giro y ayuda a interpretar las mediciones de EPR para la conformación y dinámica de las proteínas.

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Área de la Ciencia:

  • Química computacional es la química computacional.
  • La biofísica es la biofísica.
  • Modelado molecular y modelado molecular.

Sus antecedentes:

  • La simulación del comportamiento de la etiqueta de giro es crucial para comprender la dinámica de las proteínas.
  • El muestreo conformacional preciso es esencial para el modelado molecular confiable.

Objetivo del estudio:

  • Desarrollar un enfoque computacional eficiente para simular el comportamiento de las etiquetas de giro adheridas a la columna vertebral de las proteínas.
  • Mejorar el muestreo conformacional local para una mayor precisión en la dinámica molecular.
  • Validar el método con datos experimentales y explorar su aplicación en la interpretación de las mediciones de EPR.

Principales métodos:

  • Desarrolló el enfoque de escalado simulado (SS), acoplando un parámetro de escalado potencial de caminata aleatoria con dinámica molecular dentro de un marco híbrido de Monte Carlo.
  • Asegurado que el método conserva el equilibrio detallado termodinámico para cálculos precisos de la energía libre relativa.
  • Valida el enfoque utilizando la estructura cristalina de rayos X de la lisozima T4 con etiqueta de espín.

Principales resultados:

  • El enfoque SS permite cruces eficientes de barreras entre conformaciones, mejorando el muestreo.
  • Los potenciales de fuerza media (PMF) para los ángulos de torsión de la etiqueta de giro fueron consistentes en varios entornos de proteínas (superficie, semiburiado, enterrado).
  • Un modelo de disolvente implícito mostró un excelente acuerdo con el tratamiento de disolvente explícito, ofreciendo eficiencia computacional.

Conclusiones:

  • El enfoque SS desarrollado simula con precisión el comportamiento de la etiqueta de giro y proporciona información sobre la conformación y la dinámica de las proteínas.
  • El método es eficaz en diversos entornos de proteínas y apoya la interpretación de los datos de resonancia paramagnética electrónica (EPR).
  • Los modelos de disolventes implícitos son alternativas computacionalmente viables para el modelado de etiquetas de giro.