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Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei in a...
Hemoglobin01:24

Hemoglobin

Hemoglobin is a globular protein made up of four subunits. Two of these subunits are alpha chains, and the other two are beta chains. Each subunit contains a molecule of heme, which has an iron atom and can bind to oxygen. When an oxygen molecule binds to one heme group, it changes the shape of hemoglobin, making it easier for the other heme groups to bind oxygen as well.
When all four heme groups are bound to oxygen, the resulting molecule is called oxyhemoglobin. As a result, arterial blood...
Polyprotic Acids03:38

Polyprotic Acids

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids. Their reactions with water are:
¹H NMR of Labile Protons: Deuterium (²H) Substitution00:48

¹H NMR of Labile Protons: Deuterium (²H) Substitution

This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is the use of deuterium. Amongst the three isotopes of hydrogen, deuterium (2H) has a nucleus composed of one proton and one neutron. When the D2O solvent is added to a pure dry ethanol solution, its labile proton is substituted with deuterium.
¹H NMR of Labile Protons: Temporal Resolution01:10

¹H NMR of Labile Protons: Temporal Resolution

Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.
The –OH proton in alcohols typically appears in the range of δ 2 to 5 ppm but can vary depending on the specific...
Protein and Protein Structure02:15

Protein and Protein Structure

Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. They may serve in transport, storage, or membranes; or they may be toxins or enzymes. Their structures, like their functions, vary greatly. They are all, however, amino acid polymers arranged in a linear sequence.
A protein's shape is critical to its function. For example, an enzyme can...

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A Study of the Complexation of Mercury(II) with Dicysteinyl Tetrapeptides by Electrospray Ionization Mass Spectrometry
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Published on: January 8, 2016

Protonated heme.

Barbara Chiavarino1, Maria Elisa Crestoni, Simonetta Fornarini

  • 1Dipartimento Studi di Chimica e Tecnologia delle Sostanze, Biologicamente Attive, Università di Roma "La Sapienza", P.le A. Moro 5, 00185 Roma, Italy.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|October 17, 2006
PubMed
Summary

Protonated heme ions exhibit unique gas-phase chemistry, with the most stable form involving protonation on a vinyl group. This structure displays dual reactivity, influenced by electron transfer from iron, impacting its behavior in H/D exchange and ligand addition reactions.

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Area of Science:

  • * Inorganic Chemistry
  • * Physical Chemistry
  • * Biochemistry

Background:

  • * Heme proteins are crucial in biological systems, and understanding their fundamental chemistry, including protonation states, is vital.
  • * Gas-phase studies offer a unique perspective on molecular structure and reactivity, free from solvent effects.
  • * Microperoxidase (MP11) serves as a model system for studying heme chemistry.

Purpose of the Study:

  • * To investigate the gas-phase structure and reactivity of protonated heme ions.
  • * To identify the primary site of protonation in heme and characterize the resulting species.
  • * To elucidate the electronic and chemical properties of protonated heme, particularly its dual reactivity.

Main Methods:

  • * Electrospray ionization and collision-induced dissociation to generate protonated heme ions from microperoxidase.
  • * Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry for gas-phase ion studies.
  • * Hydrogen/deuterium (H/D) exchange reactions to probe protonation sites.
  • * Density functional theory (DFT) calculations to evaluate potential isomers and their stability.

Main Results:

  • * H/D exchange experiments indicated labile hydrogens on the propionyl substituents of protoporphyrin IX.
  • * DFT calculations identified protonation at the beta carbon of a vinyl group as the most stable isomer for protonated heme, with a high proton affinity (PA) of 1220 kJ mol(-1).
  • * The electronic structure analysis revealed significant radical character in the vinyl group and partial Fe(III) character in the iron center, indicating a mixed Fe(II)/Fe(III) state.
  • * Protonated heme exhibited dual reactivity: behaving like Fe(III) in H/D exchange and showing decreased ligand addition reactivity (except for NO) compared to typical Fe(II) or Fe(III) states.

Conclusions:

  • * The most stable protonated heme ion in the gas phase results from protonation on a vinyl group, leading to a unique electronic structure with mixed iron character.
  • * This protonated heme species displays a complex reactivity profile, influenced by both its formal protonation state and the delocalization of charge and spin.
  • * The findings provide fundamental insights into heme chemistry and its potential for diverse biological roles, particularly concerning iron's redox activity and ligand binding.