30 K. LINDERSTR0M-LANG [2 



considering especially the additional stabilization of the helix by non-polar 

 bonds discussed below. In fact it mainly serves to show the necessity of 

 introducing a third constant besides y and kz in the description of the 

 kinetic curves. It will be observed that Eq. (12) is of the form Bxat (Eq. 

 (5)), so that the whole set of curves in Figs. 1 and 2 is determined by Eq. (13) 

 introducing the following expression for ä:2[QH] : 



^2[QH] = 6-2. 10i«[OH-]+4-6. 102[OH3+]+0-72 



(14) 



It appears therefore from Figs, 1 and 2 that the exchange is 'catalysed' 

 both by hydroxyl ions, hydrogen ions, and water ([HgO] is put equal to 1). 



In the present stage of our investigation we shall refrain from going more 

 fully into the mechanism of the exchange reaction proper, but a few words 

 may be said about Eq. (14) which ought to apply generally to the exchange 

 of deuterium atoms in ND groups which are exposed to water (k = 1). At 

 the rate minimum, pH'--'2-8, Â:2[QH] assumes the value 1-7 corresponding 

 to a half-time of 0-4 min., and the rate is therefore so high that a quantita- 

 tive measurement by our method is impossible. 



Table 1 



DEUTERIUM EXCHANGE OF GLYCYLGLYCYLGLYCINE AT 0°C 



(«„ = 5) 



In general the process of dissolution of the lyophilized, deuterated pep- 

 tide or protein takes about 15-30 seconds, and the first 15 /xl. sample for 

 deuterium oxide estimation is taken about 1-2 minutes after the start of 

 the kinetic run. Our experiments with the short peptide DL-leucyltriglycine^ 

 could therefore only demonstrate that all six of the exchangeable hydrogen 

 atoms in this substance exchanged within one minute at 0°. The pH was, 

 however, in the neighbourhood of 6 in this case, and in view of the strong 

 dependence upon pH found for the exchange of PDLA, a reinvestigation 

 of the exchange of short peptides at pH 2-5-3'5 and 0°C was indicated. 

 Such a reinvestigation has been carried out by Dr. Wierzchowski in our 

 laboratory using glycylglycylglycine (GGG) as test substance. Table 1 shows 

 the results of two typical kinetic runs carried out in essentially the same 

 way as the PDLA experiments. It appears from this table that the exchange 



