NMR Lecture 3 Flashcards
What is the mechanism of spin-spin coupling? What is the difference between the origins of spin-spin coupling and dipolar coupling?
The coupling mechanism is based on the magnetic dipole-dipole interactions between the magnetic moments associated with the nuclear spins. It’s due to the influence of bonding electrons on the magnetic field running between the two nuclei.
J-Coupling: specific interaction between (in)directly bonded nuclei in a molecule; dipole-dipole interactions between the magnetic moments associated with the nuclear spin; results in the splitting of NMR signals into multiplets where the number of peaks in the multiplet is determined by the number of equivalent neighboring nuclei. SUPER dependent on distance and becomes weaker as the distance between the interacting nuclei increases.
Dipolar Coupling: magnetic interactions between 2 magnetic dipoles, not necessarily with nuclear spins. Dependent on distance but it decreases more slowly than J coupling, so it can be observed over longer ranges. It can lead to line broadening in NMR spectra but does not result in splitting.
What is the “roof effect?”
Typically observed in solid-state NMR experiments. It refers to the suppression of NMR signals from certain regions of a sample, resulting in a sloping baseline resembling the shape of a roof.
Know how to draw the peak splitting patterns for nuclei with strong couplings.
J-coupling: splitting patterns use the n+1 rule, where n is the number of equivalent neighboring protons.
- doublet: n=1, when a proton has one equivalent neighboring proton, 2 peaks of equal intensity.
- triplet: n=2, when a proton has two equivalent neighboring protons, split into 3 peaks of 1:2:1 ratio
- quartet: n = 3, when a proton has three equivalent neighboring protons, split into 4 peaks of 1:3:3:1 ratio
- quintet: n=4, when a proton has four equivalent neighboring protons, split into five peaks in 1:4:6:4:1 ratio
PASCALS TRIANGLE DESCRIBES PEAK RATIO
What is Karplus curve? What structural information can be derived from the measurement of 3J(HNHalpha)?
The Karplus curve is graph for the relationship between three-bond scalar coupling constants (3J) and dihedral angles in organic molecules. 3J(HNHalpha) is associated with the dihedral angle between the amide proton and the alpha carbon of an amino acid residue in a peptide.
The coupling constant is sensitive to the values of phi and psi angles in the backbone. By comparing the values with the Karplus curve, it is possible to deduce the dihedral angles, helping to characterize the secondary structure of the peptide or protein.
What interaction of 3hJ(NC) coupling be used to identify?
It’s a three-bond nitrogen-carbon coupling. This provides information about the spatial arrangement of nitrogen and carbon nuclei in a molecule.
Know the definition of spin decoupling. What are homonuclear coupling and heteronuclear coupling, respectively? What advantages does this technique have?
Spin coupling is used in NMR spectroscopy to simplify the interpretation of NMR spectra by eliminating or decoupling specific couplings between nuclear spins.
Homonuclear coupling: scalar coupling between nuclei of the same type
- advantages: helps establish the sequential connectivity of protons in a chain, aiding in the determination of molecular structure. useful for studying conformations of organic molecules; contributes to NOE enhancements
Heteronuclear coupling: scalar coupling between nuclei of different types
- advantages: aids in the structural elucidation of complex molecules, provides connectivity information between different types of nuclei and assists in building up the structural puzzle. By decoupling certain nuclei, it is possible to focus on the signals of interest, simplifying the interpretation of the spectrum.
What sample condition is normally used to measure residual dipolar coupling?
Liquid Crystalline Media: Pf1 phage
Polymer Gels
Phospholipid bicelles
Membrane mimetics
What is NMR relaxation?
NMR relaxation: process by which the nuclear magnetization of a sample returns to its equilibrium state after being perturbed by an external radiofrequency pulse
What is Nuclear Overhauser Effect (NOE)?
NOE describes the enhancement of signals for certain nuclear spins due to their proximity to other spins. It’s based on the transfer of nuclear spin polarization from one nucleus to another through dipolar interactions.
Know how to draw the energy diagram for transient NOE.
Contains:
1- equilibrium state
2 - perturbation
3 - coherence transfer
4- relaxation processes
5 - NOE buildup
6 - transient NOE
7 - Detection
