2363
COMMUNICATIONS TO THE EDITOR
On the Radiation Chemistry of Aqueous Nitrous Oxide Solutions
Sir: In their paper on the radiolysis of nitrous oxide solutions, Dainton and Walker’ commented on differences between their results and our earlier paper2 on the radiation-induced chain reaction between dissolved nitrous oxide and hydrogen. These differences, their comments, and some other aspects of their paper are considered here. Dainton and Walker’s values of G(N2) for the chain reaction were obtained from single-point determinations at one fixed dose. Simple computations will show that tho strikingly abrupt transition of their results into a “pH-independent” range above pH 13 and the seemingly linear dependence on (Hz) actually were due to serious overexposure, so that the extent of reaction was limited by complete consumption of the hydrogen initially in the aqueous phase plus an additional amount that diffused into the depleted solution from the gas space. In regard to other pH effects, we have confirmed our earlier statement2 that the depression in our yields at
0.15
v)
y
0.10
0
I
0 (r
0
3
0.05
0-
I
I
I
Figure 1. Curve A: Dependence of NZyield on absorbed dose for 7.9 X 10-6 M aqueous solutions of NzO; data from ref 1; dashed horizontal line indicates initial quantity of NzO in 15-cc sample. Curve B: Dependence of Hzyield on absorbed dose for pure water; data from ref 1; dashed line through origin indicates G( &) = 0.45.
pH 8-12 could be removed by the use of carbonatefree base, and we stand by our result indicating that no chain reaction occurs in HC104 at pH 0.4, not pH 1. Contrary to Dainton and Walker’s assertion, the yield in our Figure 1 (ref 2) is independent of (H2) to nearly 50% decomposition. Similar graphs for neutral and carbonate-free alkaline solutions were linear to over 80% depletion of Hz, indicating that the yield is essentially independent of (H2). The chain-terminating process in our mechanism for alkaline solutions2is revised to H R/I -t term, which is in better accord with the verbal argument. The chain length depends upon competition of the termination step with H N20 + OH N2 in neutral OH- + eaq- above pH 12. solution and with H Then G(N2) and G(-H2) should be independent of (H2) at any pH, as observed. The dependence on (N2O) should be unity in neutral solution, as found by Dainton and Walker,’ and zero at pH >12, for which neither paper presents evidence (discounting their fortuitous zero value obtained by overexposure). Dainton and Walker’s expression describing our mechanism in neutral solution should not contain k 9 . (H2O.J in the denominator of the last term, and their indicated method of converting to an expression for alkaline solutions is inapplicable. Dainton and Walker attribute differences in the results to our use of much higher doses than theirs. The fact is that our dosages were substantially lower. For example, the G value in our Figure 1 (ref 2) was obtained in the linear range up t o 8.25 X l O I 9 ev l.-l, less than one-tenth of the fixed dosage used in their chain-reaction studies. At the sensitivity of 70 cm2/ pmole of h y d r ~ g e nour , ~ analytical method will detect mole reported for their the lower limit of 3 X method; yet no hydrogen could be detected in our NzO solutions at pH >3 after irradiation without gas space. The data from which Dainton and Walker obtained single-point, “integral” G values for the nitrogen yield in very dilute N20 solutions and for the Hz yield in pure water at very low doses are re-presented in Figure 1 as the quantity of gas formed as a function of absorbed dose. Curve A shows the nonlinearity of N2 formation. The amounts of N2 formed at the higher doses consider-
+
+ +
+
(1) F. s. Dainton and D. C. Walker, Proc. Roy. SOC. (London), A285,339 (1965). (2) C. H.Cheek and J. W. Swinnerton, J . Phys. Chem., 68, 1429 (1964). (3) C.H.Cheek, V. J. Linnenbom, and J. W. Swinnerton, Radiation Res., 19, 636 (1963).
Volume 71, Number 7 Jum 1967
2364
COMMUNICATIONS TO THE EDITOR
ably exceed the initial amount of N2O in solution, so that once again the overexposure and the effect of the gas space are manifest. These results provide a highly questionable basis for their novel arguments concerning the dependence of G(N2) on dose, dose rate, and (N2O). Curve B shows that G(H2) in irradiated pure water does not exceed 0.45 between any two data points. Furthermore, the data strongly suggest that their high values of G(H2) at very low doses may be due to neglect of a substantial blank correction.
u. s. NAVALRESEARCHLABOR.4TORY
CONRAD H. CHEEK
WASHINGTON, D. C. 20390 RECEIVED FEBRUARY 13, 1967
Circular Dichroism of Some Polypeptides in Solvent Mixtures Containing Strong Organic Acids
Sir: Increasing concentrations of strong organic acids such as dichloroacetic acid (DCA) and trifluoroacetic acid (TFA) have been utilized to ellicit transitions of polypeptides in organic solvents. These transitions have been studied using various physical methods, e.g., optical rotatory dispersion, flow birefringence, viscometry, and Kerr effect, and generally have been interpreted as a-helix-random coil transformations.1-* However, the mechanism and the type of forces involved in the disruption of ordered structures of polypeptides are not yet clear. A strong solvation of the polypeptide chain by DCA and TFAa-5 and/or the large ability of these acids to form hydrogen bonds with the peptide groups' have been suggested to be the main causes of the phenomenon. Recently, a number of investigator^,^-'^ on the basis of near-infrared spectra in the overtone region of the amide group and on the basis of conductivity measurements, have concluded that some polypeptides, namely poly-y-benzyl-L-glutamate (PBLG), poly-L-alanine (PLA), and poly-L-leucine (PLL), dissolved in chloroform or ethylene dichloride (EDC) are protonated at the amide group in presence of DCA and TFA. On the basis of their results, these authors concluded that the transitions observed in bo could not reflect disruption of intramolecular peptide hydrogen bonds.l0 On the other hand, Stewart, et a1.,13 on the basis of nmr spectral data, concluded that in the case of PLA and PLL dissolved in CHClrTFA there is no evidence for the protonation of the peptide group, but rather their data suggest that TFA is hydrogen-bonded to the polypeptide chain. In their view, the interThe Journal of Physical Chemistry
molecular hydrogen bonding is the driving force converting the polypeptide from the helical to the random coil form. In order to clarify the effect of strong organic acids, we are carrying out systematic circular dichroism (CD) measurements in the region of the peptide chromophore on PBLG, PLA, PLL, poly-y-methyl-L-glutamate (PMLG), and poly-cmethionine (PLM) dissolved in various solvents in the presence of increasing amounts of DCA and TFA. We wish to communicate some preliminary results which, for the polyglutamate systems herein reported, rule out the protonation mechanism prior to the orderdisorder transition. The basis of our argument is the assignment of an n-?r* transition (a transition from a nonbonding orbital on the oxygen atom to a T* orbital delocalized over the entire peptide chromophore) to the 222-mp CD extremumI4 and the position that protonation at the acyl oxygen cannot occur without markedly altering this CD extremum. PBLG (Lot G-79, MW 350,000) and PMLG (Lot 6612, MW 290,000) were Pilot samples. All the solvents were purified by distillation over desiccants prior to use. The solutions were prepared in a drybox and run immediately to avoid possible hydrolysis.l6 The CD measurements were carried out on a Cary 60 spectropolarimeter with a prototype CD attachment, built by Cary Instruments. A 0.064-mm cell was used throughout. In Figure 1, circular dichroism spectra of PBLG dissolved in EDC and CHCl,, and of PMLG dissolved in trifluoroethanol (TFE) are reported. In the case of (1) P. Doty, PTOC. Intern. S p p . MCZCTO~WZ. Chem., 5 (1957); Tetrahedron suppl., 2,1951 (1957). (2) W. Moffitt and J. T. Yang, Proc. Natl. Acad. Sci., U.S.,42, 596 (1956). (3) J. T. Yang and P. Doty, J . Am. Chem. Soc., 79, 761 (1957). (4) P. Doty and J. T. Yang, ibid., 78, 498 (1956). (5) P. Doty, A. M. Holtzer, J. H. Bradbury, and E. R. Blout, ibid., 76, 4493 (1956). (6) G. E. Perlmann and E. Katchalski, ibid., 84, 452 (1962). (7) G. D. Fasman, Proceedinga of the International Symposium on Polyamino Acids, Polypeptides, and Proteins, Madison, Wis., 1961, pp 222-226. (8) H. Watanabe, K. Yoshioka, and A. Wada, BWpolpe78, 2, 91 (1964). (9) 8. Hanlon, S. F. Ruaso, and I. M. Klotz, J . Am. Chem. Soc., 85, 2024 (1963). (10) 9. Hanlon and I. M. Klotz, BhChentiStTy, 6, 37 (1965). (11) M.A. Stake and I. M. Klotz, ibid., 5, 1726 (1966). (12) 8. Hanlon, ibid., 5, 2049 (1966). (13) W. E. Stewart, L. Mandelkern, and R. E. Glick, ibid., 6, 143 (1967). (14) G. Holzwarth and P. Doty, J . Am. Chem. Soc., 87, 218 (1965). (15) M. V. Vol'kenshtein, A. T. Kol'tsov, and Zh. Marshal, Vysoh o l . Soedin., 4, 944 (1962).
