Spectroscopy Part 1 Notes

X-ray crystallography

  • Purpose: determine the three-dimensional structure of proteins by analyzing crystal diffraction patterns.
  • Basic idea: X-rays diffracted by electrons in a crystal produce a pattern of spots; the spacing and intensity of spots relate to the arrangement of atoms within the crystal.
  • Simple reference example: sodium chloride crystals form a simple cubic lattice; diffraction patterns reveal spacing between atoms in the lattice.
  • How it works for proteins:
    • A protein crystal is exposed to an X-ray beam of known wavelength.
    • Diffracted X-rays are detected on film or a detector; numerous diffraction spots are produced.
    • By analyzing the diffraction pattern, one can reconstruct the three-dimensional electron density map and build a model of the protein structure.
  • Strengths: provides high-resolution structural information; powerful for well-ordered crystals.
  • Limitations: requires crystallization of the molecule; not all proteins crystallize easily; potential artifacts from crystal packing.
  • Example discussed: myoglobin from sperm whale
    • Left: diffraction pattern from the crystal.
    • Process yields an electron density map and a final 3D structure.
  • Key takeaway: X-ray diffraction can yield detailed 3D structures but is limited by crystallization requirements.

Nuclear Magnetic Resonance (NMR) spectroscopy

  • Use: determine structures and dynamics of macromolecules in solution (no crystal required).
  • What NMR can reveal: conformational changes, folding processes, interactions with ligands or other molecules.
  • Fundamental physics (brief):
    • Certain atomic nuclei have spin, giving rise to magnetic moments.
    • In a strong external magnetic field, nuclei occupy energy levels; RF pulses induce transitions between levels.
    • The resulting NMR signal reflects the chemical environment of the nuclei.
  • Nuclei with useful spin for NMR in biomolecules:
    • Hydrogen, carbon, nitrogen, fluorine, phosphorus (H, {C, N, F, P} as common examples).
    • These spins generate a magnetic dipole in the field.
  • Experimental setup (conceptual):
    • A strong magnetic field is applied to a solution containing the macromolecule.
    • A short pulse of electromagnetic energy (RF) is transmitted at right angles to the field to flip spins.
    • After excitation, signals are detected and electronically averaged to improve signal-to-noise.
    • The resulting spectrum shows chemical shifts corresponding to the chemical environment of each nucleus.
  • Key concepts in the spectrum:
    • Chemical shift: the position on the spectrum that indicates the local electronic environment of a nucleus.
    • Spectrum visualization: peaks at specific chemical shifts correspond to different nuclei in the molecule (e.g., hydrogens in different environments).
    • Example in the lecture: globin from a marine worm (illustrative NMR spectrum showing multiple peaks).
  • Practical note: NMR provides structural information for molecules in solution, enabling observation of dynamics and interactions not easily captured in crystal structures.

Cryo-electron microscopy (cryo-EM)

  • Context: a relatively new technique that can determine structures at near-atomic resolution.
  • Nobel Prize context (2017): three scientists were awarded the Nobel Prize for Chemistry for developments in cryo-EM (the lecture mentions Joachim Frank and Jacques Dubochet; a third laureate is not named in the transcript).
  • Key ideas:
    • Conventional electron microscopy often required fixation and dehydration, which can alter native structures and introduce artifacts.
    • Cryo-EM preserves native-like states by rapidly freezing samples (vitrification) to immobilize water without forming ice crystals.
    • Early EM images were two-dimensional and fuzzy; image processing methods (e.g., by Joachim Frank) helped reconstruct sharp 3D images.
  • Sample preparation differences:
    • Conventional EM: fixation, staining, dehydration, chemical processing; native structures often altered.
    • Cryo-EM: flash freezing (vitrification) preserves structure; minimal chemical modification.
  • Resolution and applications:
    • Cryo-EM can achieve near-atomic resolution (angstrom-scale) for biomolecules and complexes.
    • It requires smaller amounts of sample compared to conventional EM for biomolecules/complexes.
    • Particularly useful for large biomolecular assemblies and flexible complexes.

Spectroscopy: overview and scope

  • Etymology and purpose: spectroscopy literally means to “see ghosts” in Latin; in Greek it means to see. It studies how matter interacts with different frequencies of the electromagnetic spectrum.
  • Core idea: different light frequencies provide different information about a sample; spectra are produced by interactions of light with the sample and analyzed to infer properties.
  • Classic example: light through a prism producing a spectrum, illustrating dispersion by wavelength.
  • What a spectrum measures:
    • The light emitted or transmitted after interaction with the sample.
    • Can reveal absorption, emission, scattering events, and changes in the sample.
  • Common spectral regions (in order of increasing wavelength): X-ray, ultraviolet, visible, infrared, near-infrared, microwaves, etc.
  • Basic energy-level physics (conceptual):
    • Absorption moves a molecule from a ground state to an excited state during illumination.
    • Relaxation pathways include internal conversion (vibrational relaxation) and emission processes (fluorescence or phosphorescence).
    • Triplet states (T1, T2) involve unpaired electrons and can give rise to phosphorescence.
  • Emission types (three main types mentioned):
    • Fluorescence: emission from a singlet excited state to a singlet ground state; rapid decay after excitation.
    • Phosphorescence: emission from a triplet state; typically longer-lived than fluorescence.
    • Raman scattering: inelastic scattering involving vibrational transitions of the molecule.
  • Diagrammatic concepts:
    • Absorption (blue in the diagram) involves promoting electrons to higher energy levels.
    • Emission occurs when the molecule returns to lower energy states, often at different wavelengths than the excitation.
  • Energy and measurement relations:
    • Energy of a photon: E = h\nu = \frac{hc}{\lambda} where \nu is frequency and \lambda is wavelength.
    • Spectral axis choices: wavelength or frequency.
    • Intensity axis choices: absorbance or emission intensity.
    • Relationship to concentration via Beer's law (see Absorption section).

Absorption spectroscopy (UV-Vis) and Beer's law

  • Concept: measures how much light is absorbed by a sample as a function of wavelength; relates to electronic transitions in molecules.
  • Typical experimental setup:
    • Light source and excitation filter to select wavelength.
    • Dichroic mirror and objective to direct light to the sample.
    • Emission/filter optics for detection of transmitted or emitted light.
    • Detector records the light after interaction with the sample.
  • Absorption spectrum interpretation:
    • Axis: wavelength or frequency (often converted to energy terms).
    • Axis: absorbance (A) or molar extinction coefficient (\epsilon) vs concentration and path length.
  • Beer-Lambert law (key relationship):
    • A = \epsilon c l
    • Where A is absorbance, \epsilon is the molar extinction coefficient (units: M^{-1} cm^{-1}), c is concentration, and l is path length (cm).
    • Typical cuvette path length: l = 1 \text{ cm}.
    • If expressed in terms of transmitted light: A = -\log{10} T with (T = I/I0).
  • Assumptions and limitations:
    • Light intensity is proportional to concentration and path length under ideal (clear, non-turbid) conditions.
    • Turbidity, scattering, and sample heterogeneity violate the simple Beer-Lambert behavior.
    • Scattering, reflection, refraction, adsorption, and polarization can alter measured signals.
  • Practical considerations for measurements:
    • Polarization control can modify how light interacts with the sample.
    • Instrumental arrangement: light source, excitation filter, dichroic mirror, objective, sample in excited state, emission collection via an emission filter.
  • Experimental applications and examples:
    • NAD/NADH kinetics:
    • Absorbance at 260 nm shows both NAD and NADH; indistinguishable at this wavelength.
    • Absorbance at 340 nm differentiates NADH (elevated when NADH is present) from NAD.
    • Given extinction coefficient for NADH at 340 nm: \epsilon_{NADH}^{340} = 6{,}220\ \text{M}^{-1}\ \text{cm}^{-1}.
    • Hemoglobin oxidation state monitoring (Soret peak):
    • Soret region peak around 400-420\ \text{nm}; sharp, intense absorbance feature.
    • Useful to distinguish deoxyhemoglobin vs oxyhemoglobin based on peak characteristics.
    • DNA quantification and purity:
    • Measure absorbance at 260\ \text{nm} to quantify DNA concentration.
    • Measure at 280\ \text{nm} to assess purity and detect contaminants.
    • Extinction coefficients differ for double-stranded DNA, single-stranded DNA, and RNA.
  • Summary point: UV-Vis absorption spectroscopy, via Beer's law, is a fundamental tool for concentration measurements and studying biochemical states, with caveats when samples scatter or are turbid.

Photoluminescence: fluorescence, phosphorescence, and chemiluminescence

  • Definition: luminescence is light emission from a material after photon absorption, occurring without heat generation.
  • Main types mentioned:
    • Fluorescence: emission from a singlet excited state to a singlet ground state.
    • Phosphorescence: emission from a triplet state; longer-lived emission.
    • Chemiluminescence: light emission resulting from a chemical reaction rather than electronic excitation by light.
  • Fluorescence specifics:
    • Typical lifetime: on the order of 10^{-8} - 10^{-5}\ \text{s}, i.e., very fast.
    • After excitation is removed, fluorescence decays rapidly as the system relaxes to the ground state.
    • Quantum yield: fraction of excited molecules that return to ground state via fluorescence; ranges from 0 to 1 (0 means no fluorescence; 1 means all excited molecules fluoresce).
  • Spectral measurement concepts:
    • Excitation spectrum (left): shows how efficiently different excitation wavelengths can raise the molecule to an excited state.
    • Emission spectrum (right): shows wavelengths of light emitted by the sample after excitation.
    • The emission peak typically shifts to longer wavelengths compared to the excitation peak (Stokes shift).
  • Uses and applications:
    • Qualitative analysis: compare peak shapes and positions to standards to identify molecules,
    • Quantitative analysis at low concentrations: fluorescence intensity is proportional to concentration.
    • Practical applications: DNA quantification, antibody/antigen detection, drug pharmacokinetics, antimicrobial testing, food and water analysis, soil analysis, etc.
    • Limitations: not used for treatment or direct clinical diagnosis; fluorescence can indicate presence of certain species (e.g., antibodies) but is not a standalone diagnostic therapy tool.
  • Instrumental setup (conceptual):
    • Excitation beam interacts with sample; detector placed at right angles to minimize scattered excitation light; emission filters tuned to collect specific wavelengths.

Raman scattering (brief)

  • Mentioned as a type of light scattering due to vibrational transitions, providing vibrational fingerprints of molecules.
  • Not elaborated in depth in the transcript, but noted as part of the emission/scattering spectrum toolbox.

Practical spectroscopy examples and considerations

  • Soret peak in heme proteins:
    • A characteristic absorption feature near 400-420\ \text{nm} useful for analyzing oxidation state and interactions of heme proteins.
  • DNA/nucleic acid assessments:
    • Absorbance at 260\ \text{nm} for quantity.
    • Absorbance at 280\ \text{nm} for purity—helps identify contaminants in nucleic acid preparations.
    • Different extinction coefficients for dsDNA, ssDNA, and RNA.
  • NAD/NADH kinetics in enzyme studies:
    • Using different wavelengths to distinguish oxidized vs reduced forms.
    • Example data: at 260\ \text{nm}, NAD and NADH show similar absorbance; at 340\ \text{nm} NADH shows a distinct signal enabling kinetic analysis.
  • Biological redox indicators:
    • NADH and deoxy/deoxyhemoglobin provide functional readouts for metabolic and oxygenation states, respectively.

Interconnections and real-world relevance

  • Applications span basic science, clinical diagnostics, and forensic analysis, as mentioned in the lecture:
    • Basic science: structural biology (X-ray, NMR, cryo-EM) and spectroscopy to understand molecular function.
    • Clinical diagnostics: spectroscopic assays for biomolecule quantification and state (e.g., DNA purity, Hb oxygenation).
    • Forensic analysis: spectral fingerprints for identifying substances.
  • Complementary nature of techniques:
    • X-ray crystallography provides detailed static structures for crystallizable proteins.
    • NMR offers structural and dynamic information in solution and can capture conformational changes.
    • Cryo-EM enables near-native state structures of biomolecules/complexes without crystallization.
    • Spectroscopy (UV-Vis and PL) provides rapid, sensitive readouts of concentration, state, and reactions, and can monitor kinetics in real time.

Key equations and constants (summary)

  • Photon energy: E = h\nu = \dfrac{hc}{\lambda}
  • Beer-Lambert law (absorbance): A = \epsilon c l
  • Absorbance in terms of transmittance: A = -\log{10} T ,\quad T = \dfrac{I}{I0}
  • Extinction coefficient: \epsilon$$ (units: M^{-1}\,cm^{-1})
  • Relationship between absorbance and concentration/path length via Beer's law for quantitative analysis
  • Fluorescence considerations:
    • Quantum yield: fraction of excited molecules that emit a photon on returning to the ground state.
  • Note on spectra:
    • X-axis: wavelength or frequency; Y-axis: absorbance or emission intensity (or molar extinction coefficient in some plots).

Practical implications and caveats

  • Crystallization bias: X-ray crystallography can produce structures that are influenced by crystal packing; not every protein crystallizes well.
  • NMR strengths: allows observation of macromolecules in solution and can capture dynamic states, but may be limited by size (larger proteins are more challenging).
  • Cryo-EM advancements: dramatically reduces need for crystallization; preserves native-like states; high resolution but requires careful sample preparation and data processing.
  • Spectroscopy caveats: turbidity and scattering can distort Beer-Lambert behavior; polarization and optical alignment can affect measurements; careful controls and calibrations are essential.
  • Ethical and practical relevance: spectral and structural data underpin clinical diagnostics, forensic analyses, and pharmaceutical development; data interpretation must consider limitations, potential artifacts, and proper context for conclusions.

Summary takeaways

  • There are three main structural biology approaches to protein structure: X-ray crystallography, NMR, and cryo-EM, each with unique strengths, methods, and limitations.
  • Spectroscopy provides complementary information about electronic transitions, energy levels, and dynamic processes, spanning absorption, emission (fluorescence, phosphorescence), and scattering (Raman).
  • Beer-Lambert law links absorbance to concentration and path length; care must be taken when samples scatter or are turbid.
  • Photoluminescence reveals rapid (fluorescence) or longer-lived (phosphorescence) emission processes, with quantitative and qualitative uses in biology, medicine, and environmental analysis.
  • Real-world examples (NAD/NADH, hemoglobin, DNA) illustrate how spectral features delineate chemical state, concentration, and quality of samples.
  • Cryo-EM represents a powerful advance in visualizing biomolecules with minimal disruption to native structure, complementing X-ray and NMR.