NMR Spectroscopy and X-Ray Crystallography Practice Flashcards

These vocabulary flashcards cover the fundamental principles of NMR Spectroscopy, chemical shifts, shielding, protein NMR parameters, and the basics of X-Ray Crystallography as detailed in the lecture transcript.

Nuclear Magnetic Resonance (NMR) Spectroscopy

A technique that involves the change or resonance in nuclear spin energy in the presence of an external magnetic field, acting on nuclei with non-zero spin ($I \neq 0$) such as $^1H$, $^{13}C$, and $^{31}P$.

Nuclear Spin Quantum Number ($I$)

A value that determines the number of spin states ($2I + 1$); nuclei with even protons and even neutrons have $I = 0$ (non-spinning), while those with odd numbers have integer or half-integer spin.

Gyromagnetic Ratio ($\gamma_H$)

A constant for specific nuclei, such as $26,753\,\text{radians gauss}^{-1}\,s^{-1}$ for protons, used to calculate energy differences in a magnetic field.

Larmor Frequency

The specific frequency ($\nu$) at which a nucleus resonates, calculated relative to the speed of light ($c = 3 \times 10^8\,m/s$) and wavelength ($\lambda$); example frequencies include $200\,MHz$ or $300\,MHz$.

Tetramethylsilane (TMS)

The standard reference sample in NMR with a chemical shift defined as $\delta = 0\,ppm$; it contains 12 equivalent protons and 4 equivalent carbons that absorb at the same frequency.

Chemical Shift ($\delta$)

The resonant frequency of a nucleus relative to a standard, calculated as $\delta = \frac{\nu_{\text{sample}} - \nu_{\text{ref}}}{\nu_{\text{ref}}} \times 10^6$ and measured in parts per million ($ppm$).

Shielding Effect

Occurs when high electron density around a nucleus reduces the effective magnetic field it feels, causing an upfield shift toward lower frequency and lower $ppm$.

Deshielding Effect

Occurs when electronegative atoms decrease electron density, increasing the effective magnetic field felt by the nucleus and causing a downfield shift toward higher frequency and higher $ppm$.

Spin-Spin Splitting ($n+1$ Rule)

The division of an NMR signal into multiples based on the number of neighboring protons ($n$); resulting patterns include doublets, triplets, and quartets with relative intensities defined by Pascal's Triangle.

Intensity of Signals (Integration)

The area under an NMR peak which is proportional to the relative number of equivalent protons giving rise to that specific signal.

COSY (Correlation Spectroscopy)

A 2D NMR technique used to show proton-proton coupling through bonds, helping to identify which hydrogens in a molecule are close enough to influence each other's signals.

NOESY (Nuclear Overhauser Effect Spectroscopy)

A 2D NMR technique that detects through-space interactions between protons within a distance of approximately $< 0.5\,nm$, used for determine the 3D folding of proteins.

Deuterium Exchange

A process where amide protons ($NH$) are replaced by deuterium in $D_2O$; the rate is fastest in exposed $\beta$-sheets and slowest in H-bond protected $\alpha$-helices.

Coupling Constant ($J_{NH-H\alpha}$)

A value measured in Hertz used to distinguish secondary structures; for an $\alpha$-helix it is approximately $4.8\,Hz$ and for a $\beta$-strand it is approximately $8.5\,Hz$.

X-Ray Crystallography

A method for determining the 3D arrangement of atoms in a crystal by measuring the angles and intensities of a diffracted X-ray beam.

Unit Cell

The smallest and simplest volume element that is completely representative of the whole crystal lattice.

Bragg’s Law

The fundamental equation for X-ray diffraction: $n\lambda = 2d\sin(\theta)$, where $d$ is the distance between electron density regions and $\theta$ is the angle of diffraction.

Electron Density Map

A 3D representation reconstructed using a Fourier Transform of the intensities of an X-ray diffraction pattern, used to build an atomic model.

HSQC (Heteronuclear Single Quantum Coherence)

A 2D NMR technique showing correlations between different types of nuclei, such as $^1H$-$^{15}N$ or $^1H$-$^{13}C$, connected by a single bond.

R-factor

A measure used in crystallography calculated as $\frac{\sum w ||F| - k|F_c||}{\sum w |F|}$ to refine and validate the structural model against experimental data.