The Isoelectric Points of Threonine and Some Related Compounds

In connection with attempts in this laboratory to increase the yields of threonine in isolations from various prtiteins, it recently became essential to determine its isoelectric point. During the time since the isolation and identification of threonine by Rose and coworkers (1) and its subsequent synthesis by West and Carter (2), the isoelectric point of the pure compound has not been reported. Employing an electrometric method similar to that described by Hitchcock (3) in recalculating SGrensen’s data (4) for glycine, we have carried out determinations of the apparent dissociation constants and of the isoelectric points of E-threonine, dl-allothreonine, dl-O-methylthreonine, dE-0-methylallothreonine, and dl-a-amino-n-butyric acid. The last named ,compound has been studied by other workers and serves as a reference substance. The introduction of a “negative” hydroxyl group in cu-amino-n-butyric acid should increase the degree of dissociation of either or both acid groups. The data to bc reported in this paper indicate that the dissociation of the adjacent substituted ammonium group in the resulting a!-amino-p-hydroxyn-butyric acid molecule is most affected. (See Table II.) Further evidence in support of these results may be noted by comparing the dissociation const,ants of alanine (5) with those of serine (6) and by a similar comparison in the case of lysine and hydroxylysine (7). Czarnetsky and Schmidt (8) reported data for a compound postulated to be a hydroxyaminobutyric acid which they isolated from casein by the procedure of Schryver and Ruston (9). The identity of their preparation with a pure a-amino-@-hydroxy-n-butyric acid is unlikely, however, since the shifts in dissociation constants noted when their compound is compared with a-amino-n-butyric acid are in the opposite direction to that cited above.