The role of (15)N CSA and CSA/dipole cross-correlation in (15)N relaxation in solid proteins

Accelerating 15N and 13C R1 and R1ρ relaxation measurements by multiple pathway solid-state NMR experiments

Magic angle spinning (MAS) Solid-state NMR is a powerful technique to probe dynamics of biological systems at atomic resolution. R₁ and R_1ρ relaxation measurements can provide detailed insight on amplitudes and time scales of motions, especially when information from several different site-specific types of probes is combined. However, such experiments are time-consuming to perform. Shortening the time necessary to record relaxation data for different nuclei will greatly enhance practicality of such approaches. Here, we present staggered acquisition experiments to acquire multiple relaxation experiments from a single excitation to reduce the overall experimental time. Our strategy enables one to collect ¹⁵N and ¹³C relaxation data in a single experiment in a fraction of the time necessary for two separate experiments, with the same signal to noise ratio.

Access to Cα backbone dynamics of biological solids by 13C T1 relaxation and molecular dynamics simulation

We introduce a labeling scheme for magic angle spinning (MAS) solid-state NMR that is based on deuteration in combination with dilution of the carbon spin system. The labeling strategy achieves spectral editing by simplification of the HαCα and aliphatic side chain spectral region. A reduction in both proton and carbon spin density in combination with fast spinning (≥50 kHz) is essential to retrieve artifact-free (13)C-R1 relaxation data for aliphatic carbons. We obtain good agreement between the NMR experimental data and order parameters extracted from a molecular dynamics (MD) trajectory, which indicates that carbon based relaxation parameters can yield complementary information on protein backbone as well as side chain dynamics.

(15)N-(15)N proton assisted recoupling in magic angle spinning NMR

We describe a new magic angle spinning (MAS) NMR experiment for obtaining (15)N-(15)N correlation spectra. The approach yields direct information about the secondary and tertiary structure of proteins, including identification of alpha-helical stretches and interstrand connectivity in antiparallel beta-sheets, which are of major interest for structural studies of membrane proteins and amyloid fibrils. The method, (15)N-(15)N proton assisted recoupling (PAR), relies on a second-order mechanism, third spin assisted recoupling (TSAR), used previously in the context of (15)N-(13)C and (13)C-(13)C polarization transfer schemes. In comparison to (15)N-(15)N proton-driven spin diffusion experiments, the PAR technique accelerates polarization transfer between (15)N's by a factor of approximately 10(2)-10(3) and is furthermore applicable over the entire range of currently available MAS frequencies (10-70 kHz).

Asymmetric doublets in MAS NMR: coherent and incoherent mechanisms

It has been long noted that J-resolved doublets observed in solid-state MAS experiments are asymmetric. The asymmetry has been attributed to a coherent interference effect involving dipolar and CSA interactions. Recently, Bernd Reif and co-workers suggested that under fast MAS conditions the coherent portion of the effect is suppressed and it becomes possible to observe an incoherent mechanism reminiscent of TROSY. The researchers were able to observe the characteristic TROSY-type patterns in (15)N-(1)H(N) spectra of heavily deuterated protein samples (Chevlekov, Diehl, and Reif, previous article in this issue). In the present computer simulation study, we seek to obtain a unified picture of this phenomenon, including both coherent and incoherent aspects. The chosen model focuses on the (15)N-(1)H(N) pair from a polycrystalline sample subject to magic angle spinning. To mimic local dynamics, we assume that the corresponding peptide plane jumps between two orientations. The simulations demonstrate that this simple model reproduces both coherent and incoherent behavior, depending on the MAS speed and the time scale of local dynamics. Furthermore, semianalytical expressions can be derived for both coherent and incoherent (Redfield) limits. Of particular interest is the possibility to use solution-style Redfield results to probe internal protein motions, especially slower motions on the nanosecond time scale. Our simulations show that the differential relaxation measurement permits accurate determination of (15)N dipolar-CSA cross correlations already at moderately high MAS speed (ca 15 kHz).