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The interpretation of the nuclear magnetic resonance (NMR) spectra of proteins in structural terms is predicated on a quantitative knowledge of the spectral changes which accompany the polymerization of amino acids and their arrangement in specific secondary and tertiary structures. Despite some elegant work,'-4 the decade which elapsed since the high-resolution NMR spectra of amino acids5' 6 and proteins7' 8 were first observed and qualitatively accounted for9 has left unanswered some of the most important questions, such as (1) the extent to which chemical shifts observed upon the incorporation of an amino acid into a peptide chain reflect the formation of a peptide bond at the N and the C terminal, respectively; (2) the effects of a given residue on the shifts of its nearest and next-nearest neighbors along the peptide chain; and (3) the side-chain dependence of the shifts observed on titration5' 6, 10 and helix-coil transition."-"5 It is the purpose of this and subsequent papers to show that the chemical shifts accompanying the incorporation of a given amino acid into a polypeptide chain can be systematized in terms of a few, relatively simple rules. The findings offer little hope for the use of NMR for the analysis of the primary sequence of large peptides, but underscore its usefulness in the study of secondary and tertiary structure. Experimental.-Most of the peptides were purchased and, if spectroscopically pure, were used without purification. All spectra were recorded on a Varian DP-60 high-resolution spectrometer, as previously described.'5 The measurements were made in DC1, D20, or NaOD solution at 30'C, unless otherwise specified. Sodium 2,2-dimethyl-2-silopentane5-sulfonate (DSS) was used as an internal and hexamethyl-disiloxane (HMS) as an external reference. All shifts are reported in counts per second (cps) or parts per million (ppm) from DSS. The concentration of each peptide was 0.05 M. The ionic form of each solute was identified by pD measurements using a glass electrode with a Beckman model G pH meter. The empirical correction pD = pH + 0.4 has been applied to all measurements. Solutions of peptide cations showed pD values of 0.9-1.0, those of zwitterions 5.4-6.0, and those of anions 12-13. In the case of peptides containing either more than one acidic or basic group, the pD value of the solution was adjusted so as to correspond to the fully ionized form. Results and Discussion.-Additivity of shifts: The chemical shifts of glycineCH2 protons in peptides containing a glycine residue are summarized in Table 1. The data show several interesting features: (1) All chemical shifts of the CH2 group of glycine tend to cluster about six mean values, as follows: (a) glycylamino acid cations, 231.8 cps from DSS; (b) glycyl-amino acid zwitterions, 228.8 cps; (c) glycyl-amino acid anions, 199.0 cps; (d) aminoacyl-glycine cations, 242.5 cps; (e) aminoacyl-glycine zwitterions, 227.2 cps; and (f) aminoacyl-glycine anions, 225.0 cps. This immediately suggests that the shifts resulting from dif-

SYSTEMATIC ANALYSIS OF CHEMICAL SHIFTS IN THE NUCLEA MAGNETIC RESONANCE SPECTRA OF PEPTIDE CHAINS, I. GLYCINE-CONTAINING DIPEPTIDES