Why is Benzylpenicillin called Penicillin G and Phenoxymethylpenicillin Penicillin V?

This one took a lot of searching. My initial hunch was that the G and the V stood for amino acids. G for Glycine and V for Valine. I thought, maybe, if these amino acids were not substituents, at least they would be the precursors involved in synthesis of Penicillins. I had also heard the word "Penicillin Gold" somewhere suggesting that they could be acronyms as well.

After some searching around, there was a chance discovery of this page on some encyclopedia that said "The different forms of penicillin are distinguished from each other by adding a single capital letter to their names. Thus: penicillin F, penicillin G, penicillin K, penicillin N, penicillin O, penicillin S, penicillin V, and penicillin X"

Now I knew there are more letters and these are chosen just because they are in the alphabet and not because of anything special. So the question became, why these letters? Did they start with Penicillin A and go down all the way till Penicillin V and even X? Is there a list of all Penicillins? Who maintains this list?

After figuring out what Penicillin A and B was, I remembered Alexander Fleming. If Fleming discovered Pencillin, then we should start with him.

So, here's Fleming's 1929 paper where he describes the discovery of "mould broth filtrate" which for convenience he decided to call "penicillin" : On the Antibacterial Action of Cultures of a Penicillium, with Special Reference to their Use in the Isolation of B. influenzæ.

He wrote the fungus closely resembles P. rubrum. Some people "corrected" him later. Some then corrected the corrections.

Okay, so in 1929, there was only one Penicillin and it was Fleming's Penicillin.

Then, for almost 10 years nothing happened. That's when Ernest Chain and Howard Florey came into the picture. They figured out a way to get good Penicillin. As early as 1940, they discovered Pencillin resistance. An Enzyme from Bacteria able to Destroy Penicillin. If you want to read more about the interesting history of the discovery of penicillin, read this review.

What Chain and Florey synthesised apparently was different from what Fleming discovered and therefore they initially named it Penicillin A and filed a patent. Later, they renamed it to "notatin" to avoid confusion. They also wrote this brilliant article on how they used it on some patients.

Van Bruggen and others in 1943 described another compound from Penicillium which had bactericidal activity and was different from any of the Penicillins till then and named it Penicillin B.

It was soon clear that Penicillin A and Penicillin B were identical. This compound is now called Glucose Oxidase.

From then on, it was mostly about improving on the techniques and therefore most literature is on patent articles. Here is one where Penicillin F and Penicillin G is described. I have no clue why they skipped over C, D, and E.

Around this time, people started producing all kinds of Pencillins.

As I could not find the list anywhere, I decided I will make that list. Here it goes.

Penicillin A - Glucose Oxidase
Penicillin B - Glucose Oxidase
Penicillin C -
Penicillin D -
Penicillin E -
Penicillin F - C14H20N2O4S
Penicillin G - Benzylpenicillin
Penicillin H -
Penicillin I -
Penicillin J -
Penicillin K - Natural penicillin
Penicillin L -
Penicillin M -
Penicillin N - Natural penicillin
Penicillin O - Almecillin
Penicillin P -
Penicillin Q -
Penicillin R -
Penicillin S -
Penicillin T -
Penicillin U -
Penicillin V - Natural penicillin
Penicillin W -
Penicillin X - Natural penicillin
Penicillin Y -
Penicillin Z -

Please let me know if you find the missing items.