Proton magnetic resonance study of conformational dynamics, coordianted internal motions, and chemical shifts of tocinamide.

All proton magnetic resonance lines of the NH-CHalpha-CH2beta fragments of five amino acid residues of the heterodetic peptide, tocinamide, have been analyzed, including reassigment of certain NH, CHalpha, and CH2beta resonances. The 1H NMR spectral parameters evaluated from this analysis include (a) all chemical shifts, (b) (3JNHCH), (3Jalpha beta), and (2Jbeta beta) values, and (c) temperature dependencies of these coupling constants and chemical shifts--the latter includes aliphatic and aromatic CH protons. The (3Jalpha beta) coupling constants yielded Calpha-Cbeta rotamer populations, Pi, for five residues and these were used to explore conformational dynamics and coordinated internal motions of tocinamide. The rotamer populations and their temperature dependencies established, (1) that all Calpha-Cbeta bonds exhibit extensive internal rotation; (2) that rotamer preferences exist for every residue; (3) that the 162 conformations possible for completely free rotation around every bond of the Calpha1-Cbeta1-S1-S6-Cbeta6-Calpha6 disulfide fragment can be reduced to two or three if the concept of coordinated internal motion is introduced. We reject the possibility of a frozen conformation for the Cys1-Cys6 fragment in the eclipsed, staggered, or nonclassical rotamer states and propose the following possibilities: (a) three classical rotamers populated in the ratio 60:25:15, (b) three nonclassical rotamers not differing by +/- 15 degrees from the classical values of 180 degrees, + 60 degrees, and -60 degrees, or (c) two nonclassical but noneclipsed rotamers with a third rotamer essentially zero, and (4) that the Calpha-Cbeta rotamer populations of the Tyr2-Cys1-S-S-Cys6 fragment are consistent with coordinated internal motions of the Calpha-Cbeta bonds of all three residues. The existence of temperature coefficients, deltadelta/deltaT, POF +/- 2 ppb/degrees C for aliphatic protons can be accounted for, in part, by postulating intrinsic rotamer chemical shifts which are temperature independent. Deltadelta/deltaT values should therefore be useful for investigating conformational dynamics. A further consequence of this interpretation is that greater care should be exercised in using deltadelta/deltaT values to assign hydrogen-bonded amide protons, especially when deltadelta/deltaT approximately -2 to -4 ppb/degrees C.