Thymalin

The names differ by one letter and describe different molecules. Thymalin, as supplied here, is the dipeptide Glu-Trp, 333.34 g/mol, CAS 38101-59-6. Thymulin, once called facteur thymique sérique (FTS), is a nonapeptide, pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn, with a pyroglutamate N-terminus. Bach and colleagues characterised it as a serum thymic factor (Nature, 1977), and its activity was later shown to depend on bound zinc. Both come from the thymus and sit in the immunology literature, but they are chemically unrelated. Check which molecule a paper used before carrying a protocol or citation from one to the other.

Thymalin, as supplied here, is the thymus-derived dipeptide L-glutamyl-L-tryptophan, written Glu-Trp and in one-letter code EW, with the molecular formula C₁₆H₁₉N₃O₅, a molecular weight of 333.34 g/mol and CAS number 38101-59-6. It is the synthetic dipeptide also referred to as timogen (also written thymogen). The name carries a precise history: thymalin originally denoted a polypeptide complex isolated from calf thymus extracts in the Soviet immunomodulator research programme by Morozov and Khavinson (Biokhimiya, 1981), and the Glu-Trp dipeptide was later separated from that complex by reversed-phase HPLC, synthesised and taken forward as a discrete immunomodulatory molecule (Morozov and Khavinson, Int J Immunopharmacol, 1997; Khavinson et al., Biol Bull Rev, 2021).

Research on thymus peptides has examined their reported effects on T-cell function and immune restoration in aged animal models. The English-language review by Khavinson and colleagues (Biol Bull Rev, 2021) summarises the reported immunoprotective effects of the thymus peptides: restoration of the number and ratio of T and B lymphocytes and their subpopulations, an increase in their functional activity, and increased phagocytosis in immunodepressed states. Anisimov and Khavinson (Biogerontology, 2010) review the same programme's geroprotective work across the thymus and pineal preparations. For the dipeptide specifically, the defining animal study is Anisimov, Khavinson and Morozov (Biogerontology, 2000), who reported that L-Glu-L-Trp slowed the ageing rate, increased survival and decreased spontaneous tumour incidence, roughly 1.5-fold, in outbred female rats.

The chemistry worth knowing before experimental design is concentrated in the tryptophan residue. The indole side chain absorbs UV light strongly, which makes the dipeptide convenient to follow spectrophotometrically, and it oxidises readily in solution, which is the degradation route to design against in handling and storage.

It is worth being direct about two things. First, the evidence base: the primary data originate overwhelmingly from one affiliated research cluster, much of it is Russian-language, the human evidence is small and does not meet modern regulatory standards, and thymalin holds no marketing authorisation in the UK, the EU or the United States. Second, the naming: thymulin, the distinct zinc-dependent nonapeptide characterised by Bach and colleagues (Nature, 1977), is a different molecule with a different literature, and the two names are routinely confused.