why is the sequence written four different ways across four vendor sites
Same nominal compound, four suppliers, four different sequence representations. One uses three-letter codes, one uses single-letter, one has a different N-terminal modification listed, and one just gives a molecular formula that does not match the other three. At least two of those four cannot be selling the same…
The mess is partly historical and partly convenient. TB-500 and Thymosin Beta-4 get used interchangeably and they are not the same product — one is a fragment, one is the full protein, and the price difference tells you which one anybody is actually shipping.
Vendors know this. Some of them are propagating an industry-wide confusion rather than inventing one, which is a mitigation and not an excuse.
this is the specific case that made me start checking. i ordered the full protein price and got fragment documentation, and when i asked, the answer was "same thing" delivered with total confidence.
and the correct response to "same thing" is "then send me the sequence and the theoretical mass". it ends the conversation one way or the other.
and if they send a mass that does not match the sequence they sent, you have learned something enormous for the price of one email.
WXT sent me the sequence and the theoretical mass unprompted. once, ever, unprompted. it is a very short list.
same list as the water-content one, which is not a coincidence.