Oct. 5 , 1962
3773
COMMUNICATIONS TO THE EDITOR
CONFORMATIONAL ASPECTS OF POLYPEPTIDES. VII. REVERSAL OF THE HELICAL SENSE OF POLY-I,-ASPARTATE ESTERS'
t 500
sir:
+ 300
We wish to report a reversal in the helical sense of poly-p-benzyl-L-aspartate in chloroform resulting from the introduction of a nitro group in the para position of the aromatic ring in the side chain. In order t o ascertain the sense of the helical confornlation of poly-p-p-nitrobenzyl-L-aspartate, copolymers of p-p-nitrobenzyl-L-aspartate with pbenzyl-L-aspartate were prepared. The dependence of the bo coefficient of the Moffitt equationr2 the A, values from the Drude equation and [a1546 on copolymer composition were determined in chloroform. Figures 1, 2, and 3 summarize the data obtained. I t can be seen that a distinct transition occurs between 35 and 40% nitro residue content in each of these figures. Unfortunately, copolymers with nitro residue contents greater than 66% are insoluble in chloroform. Blout3 and Bradbury4 have shown that copolymers of poly-y-benzyl- glutamate and polyp-benzyl-L-aspartate exhibit an irregular progression of optical rotatory properties. This is attributed to a difference in the sense of the helix of the homopolymers. Furthermore, Fasman5 showed that copolymers of poly-L-tyrosine and poly-L-glutamic acid (although the respective homopolymers have bo values of +494 and -560) show a linear change of rotatory properties. He concludes that both homopolypeptides have the same sense of helical conformation. Moreover, Schellman ,E assuming a mixture of poly-y-benzyl-Lglutamate and poly-P-benzyl-L-aspartate helices, calculated that A, values of copolymers of these amino acids change from 141 mp a t 0% glutamate to 279 mp a t 8OyOglutamate. At higher glutamate concentrations, the Drude dispersion plots become nonlinear. Figure 1 reveals a region of rapid change in bo with percentage nitro residue content; (all data refer to mole per cent.) the bo changes from +354 a t 20% nitro residue content to -1016 a t 54% nitro residue content. An even more striking transition is shown in Fig. 2 . The specific rotation, [ a ] 5 4 6 , is not only non-linear, but i t changes in direction after 40% nitroresidue content. We found that the Ac values from the Drude equation range from 153 mp a t 0% nitro residue content to 284 mp a t 66yo nitro residue content. This is in agreement with Schellman's predictions for copolymers of opposite helical sense. We have also prepared copolymers of p-p-nitrobenzyl-L-aspartate with y-benzyl-L-glutamate. (1) Previous paper in this series: 31. Goodman and I. Listowsky, J . A m . Chem. Soc., 8 4 , 3770 (1962). ( 2 ) W. hloffitt and J. T. Yang, Proc. N a f l . Acad. Sci., 42, 596 (1966). (3) E . R. Blout, Tetrahedron, 13, 123-133 (1901); E. R. Blout and R. H. Karlson, J . A m . C h e m . SOL, 80, 1259 (1958); and R. H. Karlson, K . S. Norland, G . D. Fasman and E. R. Blout, ibid., 82, 2268 (1960). (4) E. M. Bradbury, L. Brown, A. R. Downie, A. Elliott, W. E. Hanby and T. R. R . MacDonald, A'arum, 183, 1736 (1939). ( 5 ) G. D. Fasman, i b i d , 193, 681 (1962). (rj) J. A. Srhrllman and C. G. Srhellman, J . Poly. S c i . , 49, 120 (1961)
t 100 0
-100
- 300 b0
- 500
-1100
-1300
t -1500
PO
0
40
80
80
100
Per cent. p-nitrobenzyl-L-aspartate residues. Fig. 1.-The bo values for the wave length range 435 to 589 mp of copolymers of 8-p-nitrobenzyl-L-aspartate: P-benzyl-L-aspartate as a function of mole 7,of p-nitrobenzyl-L-aspartate residues in chloroform a t 25.0'.
5 4 6 -loo
1
0
20
40
60
W
100
Per cent. p-nitrobenzyl-L-aspartate residues. Fig. 2.-The specific rotation a t the wave length 546 mp of copolymers of p-p-nitrobenzyl-L-aspartate: P-benzyl+ aspartate as a function of mole 7,of p-nitrobenzyl-r~-nspartate residues in chloroform a t 25.0'.
Preliminary observations indicate that the [ a]bi6 as well as bo values vary regularly with copolymer composition in dimethylformamide. There are no regions of sharp transition. These results demonstrate that poly-/3-p-nitrobenzyl-L-aspartate and poly- y-benzyl-L-glutamate are most probably of the same helical sense. Since Blout3 and Bradbury4 have shown that poly-p-benzyl-Laspartate and poly- y-benzyl-L-glutamate are of opposite helical sense, i t follows that poly-p-p-
3774
COMMUNICATIONS TO THE EDITOR
Vol. 84
drochloric acid. p-Nitrobenzyl alcohol was isolated and characterized unambiguously. (10) Undergraduate research participant for 1961 under NSF Grant G15814. (11) Supported by grant RG 8974 from the National Institutes of Health.
DEPARTMENT OF CHEMISTRY MURRAYGOODMAN POLYTECHNIC INSTITUTE OF BROOKLYN CHARLES M. DEBER'O BROOKLYN, N. Y . ARTHURM. FELIX'~ RECEIVED JULY 2, 1962
*0°
tt
I 160 8
140
PHOTOCHEMICAL ISOMERIZATION OF SQUARE COMPLEXES
/
Sir : We wish to report photochemically effected isomerizations of platinum(I1) complexes. Both sunlight and a General Electric 275 watt sunlamp cause rapid isomerization of solutions of cis- and transbis-(triethy1phosphine)-dichloroplatinum(II),[(Et3P)2PtC12].1 Mixtures of isomers were analyzed
/
O
V
20
40
60
80
\
100
Per cent. p-nitrobenzyl-L-aspartate residues.
2.0
Fig. 3.-The values of As from the Drude equation for copolymers of 8-p-nitrobenzyl-L-aspartate: p-benzyl-L-aspartate as a function of mole 70of p-nitrobenzyl-L-aspartate residues in chloroform a t 25.0'.
L1
I
I
1.5
nitrobenzyl-L-aspartate and poly-p-benzyl-L-aspartate are of opposite helical sense. Thus, we conclude that the introduction of a p-nitro group into the 8-benzyl-L-aspartate molecule does indeed reverse the sense of the helix. The unusually large negative bo values (e.g., -1338 a t 66.0%, -1268 a t 62.0% and -11016 a t 54.0% nitro residue contents) may be attributed to the presence of groups in the side chain which produce Cotton effects in the near ultraviolet spectral region. Alternatively, the nitro side chain groups which can be assumed to form a rigid helix of their own may have a n additive effect on the bovalues. 0-p:Nitrobenzyl-L-aspartate was prepared7 by nitration of P-benzyl-L-aspartate using nitronium fluoroborates in glacial acetic acid, then neutralization with sodium bicarbonate.
+
HOAc
+A
.-
+i
g
-a 1.0
8
. I
u
8 0.5
200
250 300 350 Wave length (mfi). Fig. 1.-Spectra of (EtaP)sPtCl* isomers in methanol: trans, 1.592 x 1 0 - 4 ~ ,; cis, 1.8% x 1 0 - 4 ~ , -.
HZN-CH-COOH NOz'BFaI CH~COCH~C$I~ H2N-CH-COOH spectrophotometrically utilizing differences in the II 0 I electronic absorption spectra; with known mixCHZCOCH~CBH~--NO~ 1%. NO I
I1
0
The N-carb~xyanhydride~of 8-p-nitrobenzyl-Laspartate was prepared by phosgenation of the amino acid in dioxane. It was then polymerized using diethylamine initiator in dimethylforrnamide and isolated by precipitation with ethyl ether. Exclusive para-nitration was proved by hydrolysis of 0-p-nitrobenzyl-L-aspartate with 10% hy(7) Details of the synthesis will be published elsewhere. (8) S. Kuhn and G. Olah, J . Am. Chcm. Soc., 83,4570 (1961). (9) A. Berger and E. Katchalskl, ibid.. IS, 4084 (1951).
-
*
tures each isomer could be analyzed to filter was used in the experiments with sunlight; a solution of potassium hydrogen phthalate (5 g./l.) was used as a filter2 (sharp cut-off at 304 mp) with the sunlamp. The electronic absorption spectra are shown in Fig 1; they indicate that it is the low intensity, d-td transition at long wave length that is active in the isomerization. When the isomers were irradiated with unfiltered light from the sunlamp, a dark precipitate formed indicating decomposition. (1) Prepared by the procedure of K. A. Jensen, 2. onorg. allgem. Ckein., 229,238 (1936). (2) M. Kasha. J . Opt. SOC.Am., 88, 929 (1948).
