Thymosin Alpha-1: The Peptide With No 280 Nanometre Signal
Thymosin alpha-1 is an unusual peptide to analyse, and the reason is what it does not contain. There is no tryptophan in it, no tyrosine and no phenylalanine, which means the detection method most laboratories reach for first returns almost nothing.
Sequence and composition
Thymosin alpha-1 is a 28-residue peptide with an acetylated N-terminus: Ac-Ser-Asp-Ala-Ala-Val-Asp-Thr-Ser-Ser-Glu-Ile-Thr-Thr-Lys-Asp-Leu-Lys-Glu-Lys-Lys-Glu-Val-Val-Glu-Glu-Ala-Glu-Asn, with a free carboxyl terminus.
It is C129H215N33O55, average mass approximately 3108.3 Da, CAS 62304-98-7. It was originally isolated from thymic tissue fractions, and the synthetic material supplied for research is sequence-identical to that published fragment.
The acetyl group is part of the identity
The N-terminal acetylation is not an optional modification. The naturally occurring peptide is acetylated, and a synthetic preparation lacking the acetyl group is a different molecule, 42 daltons lighter. That 42 dalton difference is the first thing an accurate mass measurement should be checked against, because an incomplete acetylation step during synthesis produces exactly that species and it can carry through purification.
Why 280 nm fails
Ultraviolet absorbance at 280 nm in peptides comes almost entirely from tryptophan and tyrosine, with a small contribution from cystine. Thymosin alpha-1 has none of them. Its absorbance at 280 nm is effectively zero, and a chromatogram recorded at that wavelength will show a flat baseline where the main peak should be.
Detection therefore has to be at 214 nm or nearby, where the amide bond itself absorbs. Every peptide responds at 214 nm, which is the point, but the trade-off is that mobile-phase components also absorb there. Acetonitrile, trifluoroacetic acid and any dissolved impurity in the water contribute baseline drift across a gradient, so a 214 nm method needs a cleaner solvent system than a 280 nm method does.
A strongly acidic peptide
Count the charged side chains: six glutamates and three aspartates against four lysines. The net charge at neutral pH is strongly negative and the isoelectric point sits near 4.2.
Two consequences follow. Under the acidic conditions of a standard trifluoroacetic acid gradient, the carboxylates are largely protonated and the peptide is less soluble than its composition suggests, which is why it often dissolves poorly in water alone and readily in dilute base or buffered solution near neutral pH. And because the peptide is anionic rather than cationic at working pH, it does not show the silanol tailing that basic peptides do.
Deamidation is the degradation route to watch
The sequence ends in asparagine, and it contains asparagine adjacent to nothing helpful. Asparagine deamidates to aspartate and isoaspartate under aqueous conditions, faster at elevated pH and temperature, adding 0.984 daltons.
On a 3108 dalton peptide, a one dalton shift is not resolvable on a low-resolution instrument, and it is at the edge of what a routine method distinguishes even on a high-resolution one because the isotope envelope of the parent overlaps it. The chromatographic consequence is more useful: deamidated species usually elute slightly differently and appear as a shoulder or a resolved late peak. A certificate showing a persistent shoulder on a thymosin alpha-1 peak is describing something real.
Thymosin alpha-1 and thymalin are not the same
Thymalin is a peptide preparation derived from thymic tissue and is not a single defined 28-residue sequence. The two are listed near each other and are sometimes described interchangeably in supplier catalogues, which is a naming error rather than a chemical relationship. A defined sequence and a calculated mass belong to thymosin alpha-1; a fraction does not have either.
Thymosin beta-4 is also unrelated despite the shared word. It is a 43-residue actin-binding peptide with a different sequence, a different mass and a different origin.
Mass spectrometry at 3108 daltons
At this size, electrospray produces a charge envelope rather than a single ion. The doubly, triply and quadruply protonated species appear near m/z 1555.2, 1037.1 and 778.1, and deconvolution software reconstructs the neutral mass from that series. Reading a deconvoluted mass is not the same as reading a raw spectrum, and a certificate should indicate which it reports.
What confirms a lot
A deconvoluted neutral mass near 3108.3 Da, a chromatogram recorded at 214 nm with the wavelength stated, and attention to the 42 dalton and one dalton neighbours that incomplete acetylation and deamidation produce. A 280 nm chromatogram of this peptide is not evidence of anything.
Related naming problems
The word thymosin refers to origin rather than to a structural family, which is why thymosin alpha-1 and thymosin beta-4 share a name and nothing else. That second confusion, and the separate question of what distinguishes thymosin beta-4 from the fragment marketed as TB-500, is covered in thymosin beta-4 versus the TB-500 fragment.
