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UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given...
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Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational and vibrational energy levels.  Each energy level is separated by an energy gap, with the gaps between adjacent electronic, vibrational, and rotational levels varying significantly. The three types of energy levels in a diatomic molecule are shown in Figure 1.
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In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this...
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Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed to π → π* electronic excitations. Generally, a UV–vis absorption spectrum is recorded as a plot of absorbance vs wavelength. The wavelength of maximum absorbance, which manifests as a peak in the absorption spectrum, is denoted as λmax.
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Infrared spectroscopy is primarily used to determine the types of bonds and functional groups. In carboxylic acid derivatives, a typical carbonyl bond absorption is observed around 1650–1850 cm−1. For esters, the absorption is recorded at around 1740 cm−1, while acid halides show the absorption at about 1800 cm−1. Another acid derivative, the acid anhydrides, exhibit two carbonyl absorption around 1760 cm−1 and 1820 cm−1, arising from the symmetrical and...
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In IR spectroscopy of carboxylic acids, the C=O bond shows a characteristic band between 1710 and 1760 cm⁻¹, and the O–H bond exhibits a broad band between 2500 and 3300 cm⁻¹.
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Los datos espectroscópicos para el compuesto II de la ascorbato peroxidasa (APX) apoyan fuertemente una forma de hierro (IV) -oxo, no de hierro (IV) -hidroxo. Esto resuelve los resultados experimentales contradictorios con respecto al intermedio crucial de la enzima.

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Área de la Ciencia:

  • La bioquímica
  • Química computacional
  • Espectroscopia

Sus antecedentes:

  • La ascorbato peroxidasa (APX) es crucial para la defensa de las plantas.
  • El compuesto II en APX presenta un hierro clave (IV) intermedio.
  • Existen datos estructurales y espectroscópicos contradictorios con respecto a su estado de protonación.

Objetivo del estudio:

  • Para resolver el estado de protonación del intermediario de hierro en el compuesto APX II.
  • Para conciliar las discrepancias entre los hallazgos cristalográficos y espectroscópicos.

Principales métodos:

  • Cálculos de mecánica cuántica y mecánica molecular orientados a la espectroscopia (QM/MM).
  • Extensa exploración del espacio conformacional.
  • Cálculos de agrupación acoplada (DLPNO-CCSD) para su validación.
  • Análisis de las espectroscopias de emisión Mössbauer, XAS, NRVS, ópticas y de rayos X.

Principales resultados:

  • Las distancias Fe-O para las formas oxo e hidroxo se encuentran dentro de rangos distintos y no superpuestos.
  • Los cálculos QM/MM asignan de manera única todas las observaciones espectroscópicas a una forma de hierro.
  • Un grupo hidroxi terminal es inconsistente con los datos espectroscópicos.
  • Las distancias cristalográficas de Fe-O solo se alinean con las especies hidroxo.

Conclusiones:

  • La formulación de hierro (IV) O del compuesto APX II está fuertemente respaldada por datos espectroscópicos.
  • Las discrepancias ponen de relieve posibles problemas con la preparación de muestras en los estudios.
  • El modelo de hierro-hidroxo es incompatible con la evidencia espectroscópica.