A Gallery of Beta-Alanine & dl-Alpha-Alanine Photomicrographs
polarized light illumination)
This article shows images of the two
isomers of the amino acid alanine.
Amino acids are the building
blocks from which proteins are formed. Proteins then, consist of a series
of amino acids linked together in a particular order specified by a
gene’s DNA sequence. These proteins may act as structural
components in a cell, as signaling molecules, or as enzymes (catalysts)
which speed up chemical reactions in the cell.
There are twenty amino acids which can combine to form the 50 000 to 100 000 different proteins in the human body. Nine of the twenty amino acids must be obtained from foods, since the body cannot synthesize them. The other eleven amino acids, to which group alanine belongs, can be produced by the body by using the aforementioned nine. In the body, alanine is synthesized in muscle cells.
The structural formula and molecular shape can be seen below. (Both illustrations were produced by HyperChem software.)
Since the colourless crystalline solid has high solubility in water, I was able to produce an evaporation specimen by dissolving a small quantity in distilled water, and then placing a couple of drops of the solution on a microscope slide. After the crystals that formed were completely dry, a drop of Permount was applied over them, and a cover-glass added. This produces a specimen that lasts for decades! Note that the two compounds discussed in this article may act as skin or eye irritants.
The first image in the article, and the one below, show crystal structures that formed on the slide during evaporation of the solvent. The background is gray, instead of the expected black when using crossed polarizers, because two quarter-wave plates were utilized to produce elliptically polarized light.
The use of such plates, (called compensators), can dramatically change the appearance of a particular visual field. For example, consider the three images that follow. The first uses two lambda/4 compensators, the second lambda/4 and lambda compensators, and the third uses no compensators.
Another example follows. No compensators were used when producing the first image, while two lambda/4 compensators formed the second and third images. The subtle difference in the last two images was caused by rotation of one of the compensators.
The use of these techniques can dramatically alter the appearance of a field.
Lambda/4 and lambda compensators were used in the three images below. Rotation of the lambda/4 compensator resulted in the different background colour in each photomicrograph.
If the crystals formed by evaporation are thicker than the “ideal”, under polarized light, they may appear as shades of gray or brown, as in the four examples below.
All but the second image below utilized elliptically polarized light instead of the normal plane polarized variety.
Alpha and beta-alanine are isomers. This means that they have the same formula, C3H7NO2, but the molecular structures are different. If you compare the molecular shapes, it is evident that the NO2 group in alpha-alanine is attached to the central carbon, whereas in beta-alanine, it is attached to the end carbon. This results in different chemical properties for the two compounds. (For example, alpha-alanine has a melting temperature of 314 degrees Celsius, while beta-alanine melts at 196 degrees Celsius.) It is not surprising then, that the crystals formed by alpha-alanine in evaporation specimens bear no resemblance to those of the beta compound.
Some organic molecules have a mirror image twin that is structurally different. The twin molecules are referred to as the right-hand (“d”) form, and the left-hand (“l”) form. The “dl” in the name above tells the user that the bottle contains both the left, and right hand forms of the molecule.
By comparing the images below with the previous ones, it is clear that alpha-alanine is less photogenic than its beta sibling. The first four images were taken using two lambda/4 compensators, and the last image used lambda/4 and lambda compensators.
I would enjoy photomicrographing the other nineteen amino acids. Unfortunately, finding pure samples is a problem!
The images in the article were photographed using a Nikon Coolpix 4500 camera attached to a Leitz SM-Pol polarizing microscope. Crossed polars were used in all polarized light images. Compensators, ( lambda and lambda/4 plates ), were utilized to alter the appearance in some cases. A 2.5x, 6.3x, 16x or 25x flat-field objective formed the original image and a 10x Periplan eyepiece projected the image to the camera lens.
comments to the author Brian
Johnston are welcomed.
Published in the May
2007 edition of Micscape.
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