J . Am. Chem. Soc. 1991, 113, 1406-1408
1406
(M+), 759 (MH+); high-resolution FAB mass spectrum, 758.2043 (C32H31N6016 requires 758.2031 amu).
Acknowledgment. This work was supported by research Grant C H E 84-1 6336 from the National Science Foundation and by an unrestricted grant from Eli Lilly and Co. N M R data were obtained on a QE-300 spectrometer supported by N I H Grant PHS 1532135. Mass spectra were obtained on a VG ZAB-1 H F spectrometer supported by N I H Grant GM 27079. W e thank Dr. Viswanathan Sasisekharan, Fogarty Scholar-in-Residence, 1989-1990, National Institutes of Health, for valuable discussion. Registry No. 4a, 120172-85-2; 5a, 120172-86-3; 5b. 120172-91-0; 6a,
120172-87-4; 6c, 120172-89-6; 7a, 120172-88-5;7b, 120172-90-9;7c, 120204-15-1; 8a, 131154-63-7; 8b, 120172-94-3; Sc, 120172-93-2; 10, 25855-37-2;11, 42520-09-2;12, 120172-82-9;13, 69180-46-7;14, 120204-13-9;15, I 20204-14-0;16, 106160-98-9;17, 1061 39-36-0;1%. I201 72-83-0;19, 120172-84-1;tri-0-acetyladenosine, 7387-57-7; 2',3',5'-tri-O-acetyIcytidine,56787-28-1; 2-aminopyridine, 504-29-0.
Supplementary Material Available: Final atomic positional parameters, anisotropic thermal properties, torsion angles, bond lengths and andes, and graphics from X-ray structure determinations Of 7b, 8b* 14* and l5 (12' pages); Observed and structure factors (21 pages). Ordering information is given on any current masthead page.
Amide Chemical Shifts in Many Helices in Peptides and Proteins Are Periodic I. D. Kuntz,* P.A. Kosen, and E. C. Craig Contribution from the Department of Pharmaceutical Chemistry, University of California. San Francisco, California 94/43-0446. Received August 16, 1990
Abstract: A survey of the literature data on the NMR chemical shifts of amide hydrogens in peptides and proteins finds many cases in which resonances in helical structures show a periodicity of ca. 3-4 residues/cycle. Some helices do not show such behavior, nor do a carbon hydrogen resonances. The simplest explanation is that the helical curvature found by high-resolution X-ray crystallographic studies for some helices in proteins is a fairly general phenomenon for protein and peptide structures in solution.
Introduction There have been several that the mean chemical shift of amide protons is approximately 0.2 ppm upfield in helical structures and approximately 0.3 ppm downfield in @ sheet structures. W e add the further observation that, in many cases, the helical component is periodic in character, with a dominant repeat near 3.6 residues. The most striking example is a coil-coil peptide: the leucine zipper6 (Figure 1). The periodicity is obvious for residues 10-31 of the leucine zipper, and, on analysis, accounts for 65% of the sequential variation in chemical shift for these amide protons. Other examples abound in proteins and peptides (Table I; see also ref 5, Figure 1). Methods and Results W e used three methods to extract the periodic contribution to the chemical shifts. Simple plots of chemical shift vs sequence5 allow a visual assessment. For such plots, we used the criterion that the spacing of three maximum of three minima corresponded to periodicity of N helices (i; i + 3 or i 4; i + 7). For sequences that met this standard, we estimated the peak-to-peak value of the sinusoidal component (Table I). The second method used a linear prediction method to calculate the dominant periodicity over an arbitrary window' (Table I). Finally, Fourier coefficients can
+
( I ) Dalgarno, D. C.; Levine, B. A.; Williams, R. J. P. Biosci. Rep. 1983, 3, 443-452. (2)Sziligyi, L.; Jardetzky, 0. J . Mogn. Reson. 1989, 83, 441-449. (3)Zuiderweg, E. R. P.; Nettesheim, N. G.; Mollison, K. W.; Carter, G. W. Biochemistry 1989, 28, 172-185. (4)Guiles, R. D.; Altman, J.; Lipka, J.; Kuntz, I . D.; Waskell, L. Biochemistry 1990, 29, 1276-1289. (5)Williamson. M. P.Biopolymers 1990, 29, 1423-1431. (6)Oas. T.,G.; McIntosh, L. P.; O'Shea, E. K.; Dahlquist, F. W.; Kim, P. S.Biochemistry 1990,29, 289 1-2894.
0002-7863/91/l513-1406$02.50/0
be extracted by using the formula given by Eisenberg et aL8 In Table 1 we give the average amplitude (ca. 40% of the peak-topeak value for sinusoids) and the fraction of the total oscillatory signal that occurs with a period of 3.6 residues/cycle. On the basis of the data in Table I, we draw the following observations: ( I ) significant oscillations of the amide proton chemical shift are seen for three-fourths of the helices assigned in the data set; (2) the average helical variation is approximately 0.4 ppm in amplitude, with the maximal variations approaching 1 ppm; (3) in most cases, the downfield protons are associated with hydrophobic side chains; (4) the chemical shifts of a carbon protons do not show much periodicity, although a weak out-ofphase component (
