If I remember correctly the single letter comes from the sound of the first letter in the name. However, some have the the same first letter as others and so they were assigned the next available letter that didn't conflict with another. Not sure if I am remembering that correctly, you may wanna google it :)

Dear Diane

For e.g., Why "D" for aspartate and "E" for glutamate?

That's what I was trying to say :P

Dear Joachim Pimiskern

Thanks a lot for providing good explanation