In z94o I.IN1)ERSTROM-I-ANG 2 put forward the s t imulat ing hypothesis that the appearance of increased amounts of free sulfhydryl groups upon protein dena tura t ion might be due t the opening of thiazoline rings possibly present m the native protein. Renewed interest in thiazolines as possible s tructural uni ts in proteins was generated by (TALVIN'S a report that GSH, when dissolved in z2 N HCI at 25 ~ sh.w~ properties of a thiazolinv and, like 2-nwthvlthiazoline, develops a new ultraviolet absorption l)eak at about 260 mix. Although ('ALVlN's experiments have been repeatedly confirmed 4,G, all past attempts to isolate a thiazoline structure from GSIt were unsuccessful. "l'he present report describes the preparat ion and properties of thiazolines derived from GSH. GSIt , 5oo mg, (1.63 mmoles) was dissolved in 3 ml 12 N HC1. Maximum cvcl izat i .n was reached in 48 h at 25" as demonstra ted by ultraviolet absorption at 268 mix. The solution was concentrated to dryness in vacuo o v e r P,,O s at 25 ~'. A white hygroscopic powder (512 mg) (I) was obtained. In sinailar manner , I.oo g (3.25 n:molesl (;SH was dissolved in 5 ° ml anh. methanol saturated with h.vdrogen ch lor ide . . \ f t e r 48 h at 25 the solution was concentrated to dryness in vacuo over P,,()a at 3 ° . .\ hygroscopic solid (1.o 5 g) was isolated (II). A s imi lar preparat ion was made in anh. ethanol saturated with hydrogen chloride**. The conclusion that the solid products (I and II) isolated from GStl do in fact contain thiazolines, although labile in aqueous or alcoholic solution, is supported by the fact that when dissolved in 12 N HC1, in 12 N perchloric acid, or in methanulic ,,r ethanolic hydrogen chloride they immediately exhibit max imum molar ext inct ion at 2()S mix and do not require the reaction period of 48 h n . rmalh" required for coinplete cvclization. Fur thermore, ultraviolet absorption spectra of the anhv(trous solid hvdrochh>rides in KBr discs demonstrates comph'te s tabil i ty of the anh. th iaz . l ines derived from C, SH. However, Products I and II are not simply the thiazolinvs produced by cvclization of the tr ipeptide GSH, but are shown to be complex mixtures. At least four distinct products are formed with 12 N HCI'**, anti four different products, the by reaction in methanolic or ethanolic hydrogen electrophorctic data derived from the components as a result of t rea tment of GSI-[ corresponding esters, arc produced chloride. "l'abk, I summarizes the of Products I and II. About zo % . f the initial GSH is completely hvdrohzed by I2 N HC1 to tile free anaino acid components, while the major products of cleavage (about 9 ° °,,) are glycine and 7<.glutanlyl-L-cystcine (Glu CySH). (PRE..\UX :xYI~ I.ON'IUE: reported the liberati,m ,,f glycine upon t rea tment of (;SH with I2 N H('I under similar conditions.) The prl~ducts of reaction of methanolic or ethanolic hydrogen chloride are the analogous nwthvl or ethyl ester hvdrochlorides. However these are not the only eomt)onents for we