Investigate the structure of the amino acid alanine.

•	Examine and compare two crystal structures of alanine (refcodes LALNIN23 and ALUCAL05). To display a structure type it’s reference code (refcode) in the Entry box above the structure list on the left hand side of the WebCSD page:

 

•	What is the relationship between these two structures, are they identical?

 

left: Alanine (LALNIN23); right: Alanine (ALUCAL05)

 

To manipulate structures in the 3D viewer:

 

1. Structures can be rotated by moving the cursor in the display area while keeping the left-hand mouse button pressed down.

 

2. To zoom in and out move the cursor up and down in the display area while keeping both the shift key and the left-hand mouse button pressed down.

 

3. To translate structures move the cursor in the display area while keeping both the left-hand mouse button and the keyboard Ctrl key pressed down.

 

4. At any stage the display area can be returned to the default view by hitting the r button on the keyboard.

 

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Although the two structures initially look identical, after careful inspection, we can see they are not the same. The structures are non-superimposable. However we orientate the structures we cannot directly overlay one onto the other such that all four substituents align. Note that if you have full access to the Cambridge Structural Database then you will be able to display both structures together and manipulate them independently within Mercury. Try to overlay one structure onto the other. However the structures are orientated all four substituents cannot be aligned.

•	In fact, the two structures are mirror-images of each other. If we reflected LALNIN23 in a mirror, we would get a structure that is identical to ALUCAL05.

 

•	Two structures that are not identical, but are mirror images of each other are called enantiomers. Structures that are not superimposable on their mirror image and can therefore exist as two enantiomers are called chiral.

•	Enantiomers are identical in all physical properties except for the direction in which they rotate the plane of polarised light. Compounds that are able to rotate the plane of polarized light are said to be optically active.