title: Rotational-resonance distance measurements in multi-spin systems.
abstract: It is demonstrated that internuclear distances can be evaluated from rotational-resonance (RR) experiments in uniformly (13)C-labelled compounds. The errors in the obtained distances are less than 10% without the need to know any parameters of the spin system except the isotropic chemical shifts of all spins. We describe the multi-spin system with a simple fictitious spin-1/2 model. The influence of the couplings to the passive spins (J and dipolar coupling) is described by an empirical constant offset from the rotational-resonance condition. Using simulated data for a three-spin system, we show that the two-spin model describes the rotational-resonance transfer curves well as long as none of the passive spins is close to a rotational-resonance condition with one of the active spins. The usability of the two-spin model is demonstrated experimentally using a sample of acetylcholine perchlorate with labelling schemes of various levels of complexity. Doubly-, triply-, and fully labelled compounds lead to strongly varying RR polarization-transfer curves but the evaluated distances using the two-spin model are identical within the expected error limits and coincide with the distance from the X-ray structure. Rotational-resonance distance measurements in fully labelled compounds allow, in particular, the measurement of weak couplings in the presence of strong couplings.
authors.affiliation: Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Aswin
authors.initials: A
authors.lastname: Verhoeven
authors.forename: Philip T F
authors.initials: PT
authors.lastname: Williamson
authors.forename: Herbert
authors.initials: H
authors.lastname: Zimmermann
authors.forename: Matthias
authors.initials: M
authors.lastname: Ernst
authors.forename: Beat H
authors.initials: BH
authors.lastname: Meier