Physical methods used in mineral bio-chemistry

I. Scale of time
In biology, to understand the nature of the metallic centre which contains a transition metal, it is often useful to obtain some informations about electronics states and magnetic states. Properties correlated with the coordination number and the molecular geometry of the metal center. These properties have an influence on the reactions which might happens in this environment. So, electronics ans magnetic phenomenas are the base of a lot spectroscopic techniques.
For kinetics studies, we must choose the appropriate technique.
II. Method using RX
A. RX Diffraction

One thing very important is in RX the wavelength which is about the order of the interatomic system: 1 A. 
This method is crucial for the determination of molecular structure.
For these, on a crystal organized comes an incident beam of RX which is diffracted and go until a screen where we can see diffraction tasks. Each tasks will be appropriate to an atom thanks to a CIF file.

The angular position of the diffracted beams is a function of the geometry about the unit cell
characteristic of the crystalized substance. 
Intensity of the diffracted beam is function of the element nature: more there are electrons, more the intensity increases , also about the position of the atomes in this unit cell.

B. XANES and EXAFS

These two methods are based on the absorption of RX.

-XANES means X Ray absorption near edge structure: informations about the local order: we can identify the metal by excitation of electron towards an empty orbital. Look at the energy level of the beginning of the absorption we can obtain the state oxidation of the molecular ion also information about the electronic structure.
-EXAFS means Extended X ray absorption fine structure and allows the gain of the environment of the compounds and about the coordination number of the metal because the electrons meet the coordinated ligands .
Disadvantages: the distance must be low (4A) and we have an inaccuracy on the nature of coordinated ligands about + or - 1 e-.

III. PRE

This is the same technique of RMN but for paramagnetic metals. RMN is used for diamagnetic metals. The aspect of the signal gives the aspect of the coordination environment of the metal.
We can obtain a factor "g" and we have three situations:
1) x=y=z, we have only one valor of g, the geometry is cubic.
2) x=y but z is different. The symmetry is axial and we have two valors for g
3) the three components are different , the symmetry is rhombic and there are three g.
"g" is very important, allowing to calcul the number of electron :
if g=2 for example , we know that (2S+1=g=2) and s=1/2, so there is one electron alone.

IV) NMR

This is the technique of choice for characterized diamagnetic compounds

V) Mossbauer

This spectroscopy is very known for compounds with iron II. We obtain thanks to the shift the oxidation state of iron.




Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s