Southpaw's pharmacology primer part I: What does the "L" of L-arginine stand for?
Some members have asked me about what “L” of arginine means, and about isomers in general, so I will post a general reply here.
Southpaw’s pharmacology primer part I: What does the “L” of L-arginine stand for?
Certain organic molecules are called stereoisomers, meaning they can exist with a left handed twist, or its mirror image, which is a right handed twist. The molecule must be asymmetrical on all four sides to be a stereoisomer. Have patience, I will explain below:
Lets say a car is an organic molecule. It is NOT a symmetrical object. It has a front, a back, a left and a right side to it. We can switch a set of objects located on either side of this car/molecule, such as the tires. If we rotate the tires left to right, or front to back, to the casual observer, the car looks the same, but in organic chemistry, we have a totally different “molecule”, because the tires are in a different place. Therefore non symmetrical objects like automobiles can have lefthanded and righthanded isomers. We can call these isomers “L”, for levo, or left, and “D”, for dexter, or right. So if you are an organic chemist, you can switch the tires and think you drive a different car to work every day!
Re: L-arginine. Arginine is an amino acid, and it so happens that all amino acids found in nature on this planet are lefthanded, so calling Arginine “L-Arginine” (or any other Amino Acid) is redundant in this special case. It is possible to create D-Arginine in a laboratory I suppose, but it would presumably be biologically inactive. Nature is extremely selective about its isomer preferences, and the activity index of the wrong handed isomer may plummet from 100 right down to zero. Isomers that bind receptors very actively on the left side may be as inert on the right as a handful of sand, or vice versa.
Example two: Lets say a rail road car sitting on a track is an organic molecule. A railroad car has no “front” or “back” because it can roll indefinitely in either direction, depending upon which way it is being pushed or pulled. If we swich the wheels left for right on a railroad car, we can pick up the car, rotate it 180 degrees, put it back on the track and be right back where we started, because railroad cars are symmetrical. Therefore, there can be no lefthanded or righthanded “boxcar” molecules because they are symmetrical, not asymmetrical like our automobile example above. Accordingly, not all organic molecules have stereoisomers; in fact, only some do.
See Part II for more.