th  - 1987 - 51st ENC Conference

WF6
MOTIONAL PROPERTIES FROM FORBIDDEN PEAK INTENSITIES IN DOUBLE
QUANTUM SPECTRA OF MACROMOLECULES.
Lewis E. Kay*, T.A. Holak, and J.H. Prestegard
Department of Chemistry, Yale University, New Haven CT., 06511
Typical n quantum filtered one and two dimensional spectra
consist of peaks associated only with spins having Kesolved
scalar couplings to at least (n-l) equivalent or nonequivalent
spins. Consider, for example, a methyl group connected to a
tertiary carbon forming an A 3 spin system. On the basis of a
simple theoretical formalism for multl-pulse experiments all
peaks stemming from the single resonance associated with this
system should be removed by a multiple quantum filter. Multiple
quantum peaks associated with degenerate spin systems of this
type do, however, appear in spectra of macromolecules. 1"3 Despite
the potential pitfalls associated with interpretation of these
additional resonances, their appearance, when properly
understood, can provide useful information concerning the
dynamics of the spin system involved and ultimately the
structural properties of the parent molecule. This is due to the
fact that the appearance of such forbidden peaks is a direct
consequence of cross correlation effects between spins.
We will present an investigation of the motional properties
of an X 3 spin system found in mlcelles of deoxycholate using
forbidden peak intensities from a one dimensional analogue of the
double quantum 2D experiment. Applications to the AX 3 spin
systems of alanine residues in proteins will also be discussed
with an emphasis on how an understanding of methyl group dynamics
can aid in interpretation of NOE cross peak intensities involving
these groups.
I. Muller, N; Bodenhausen, GI Wuthrich, K: Ernst, R.R.
J. Magn Reson. 1985, 65, 531.
2. Muller, N; Ernst, R.R; Wuthrich, K. J. Am. Chem. Soc. 1986,
108, 6482.
3. Rance, M; Wright, P.E; Chem. Phys. Letters 1986, 124, 572.