Transverse Relaxation of Scalar-Coupled Protons
In a preliminary communication (B. Baishya, T. F. Segawa, G. Bodenhausen, J. Am. Chem. Soc. 2009, 131, 17538–17539), we recently demonstrated that it is possible to obtain clean echo decays of protons in biomolecules despite the presence of homonuclear scalar couplings. These unmodulated decays allow one to determine apparent transverse relaxation rates R2app of individual protons. Herein, we report the observation of R2app for three methyl protons, four amide HN protons, and all 11 backbone Hα protons in cyclosporin A. If the proton resonances overlap, their R2app rates can be measured by transferring their magnetization to neighboring 13C nuclei, which are less prone to overlap. The R2app rates of protons attached to 13C are faster than those attached to 12C because of 13C–1H dipolar interactions. The differences of these rates allow the determination of local correlation functions. Backbone HN and Hα protons that have fast decay rates R2app also feature fast longitudinal relaxation rates R1 and intense NOESY cross peaks that are typical of crowded environments. Variations of R2app rates of backbone Hα protons in similar amino acids reflect differences in local environments.