Physical methods in bioinorganic chemistry

Question 1

XAS

Answer 1

X-ray Absorption Spectroscopy
Involves absorbing X-rays by ejecting electrons
The X-rays can be absorbed at the K-edge (1s), L-edge (2s, 2p), or M-edge (n=3)
Absorption is element specific
Gives information about the oxidation state

Question 2

EXAFS

Answer 2

Extended X-ray Absorption Fine Structure Spectroscopy
Part of XAS
Accurate distance to neighbouring atoms (+/- 0.01 A)
Ideally Z > 14, lighter elements are difficult to distinguish
Inaccurate number of neighbouring atoms, but can be fitted by a structural model
Amorphous solid samples, no crystals needed

Question 3

ESR

Answer 3

Electron Spin Resonance Spectroscopy
Selective for paramagnetic metal environment
Very sensitive - can detect 1 uM Fe(III) in a high-spin configuration
Provides info on electronic structure (g tensor) and coupling to nuclei with I>0
Allows classification of metalloenzymes

Question 4

Effects of paramagnetic complexes on NMR spectra

Answer 4

Line-broadening
Alter chemical shift of neighbouring nuclei depending on distance/orientations
Increased relaxation around the metal centre

Question 5

Mößbauer spectroscopy

Answer 5

A form of nuclear resonance vibrational spectroscopy
The nuclei of certain atoms (esp 57Fe) can undergo energy level transformations associated with the absorption/emission of a gamma ray
These are influenced by the surrounding electronic/magnetic environment

Question 6

UV/Vis spectroscopy

Answer 6

3 types of electronic transitions:

1. Ligand to ligand e.g. pi—>pi* in porphyrin ligand in haem (Fe)
2. Ligand to metal charge transfer (LMCT) - usually very intense e.g. Fe/S cluster S—>Fe
3. d–>d transitions (less intense, but geometry dependent)

Question 7

Vibrational spectroscopy

Answer 7

= IR spectroscopy
Generally complex spectra for most proteins
Resonance Raman spectroscopy probes vibrations around the metal
Difference spectroscopy with isotopically-labelled substrates e.g. 18O

Question 8

X-ray diffraction

Answer 8

Uses the Bragg equation d = lambda/2sintheta
Max value of theta gives resolution of > 150 pm in proteins
Cannot resolve single atoms
Long time for measurements, several crystals and high X-ray intensity needed (synchotron)
Protein crystals contain a large amount fo water so cannot be cooled below 0 degrees C - adding heavy atoms e.g. Hg, Pt can solve problem (isomorphous replacement)