SAS Switched Angle

Switched Angle Spinning (SAS) Probes

One of the more promising recent additions to the arsenal of NMR tools has been the use of long-range constraints from residual dipolar couplings in partially aligned solutions. Partial macromolecule alignment has been obtained by using dilute liquid-crystal solutions of disc-shaped bicelles, but this alone is not sufficient for the needed dynamical control over the alignment.

New High-field SAS probe

for residual dipolar couplings.

high-field-saas-probe-photo

Analyses and experiments indicate novel Switched Angle Spinning SAS ) techniques should provide the needed dynamic control over the bicelle alignment. When a sample containing discoidal bicelles of negative magnetic anisotropy is spun at the Magic Angle, their interaction with B0 vanishes and their orientation becomes random. For sample spinning at angles less than 54.7° , they align with their normals perpendicular to the spinning axis, while spinning at greater angles causes their normals to align with the spinning axis. The dynamic control over the spinning axis provided by a SAS probe may provide the protein alignment control needed for more effective utilization of the bond angle information inherent in the residual dipolar coupling.

Our PROTONMAX ( inner cross coil) MAS probes have allowed VAS (Variable Angle Spinning) and SAS (Switched Angle Spinning) in wide-bore probes since 1997. The two-coil MAS technology offers an order of magnitude reduction in decoupler heating. In response to the new applications for SAS, we have developed a SAS probe with more durable, fatigue-resistant leads (for up to 500,000 flips before replacement), faster switching and improved 1H S/N. Precise computer control of angle setting (via a servo motor) results in magic angle setting reproducibility during SAS of >0.015°, with a 60 ms settling time. Tuning can be H/X or H/X/Y with 3, 4, or 5 mm spinners. The first of these new SAS probes was delivered in June 2003. VAS (Variable Angle Spinning, 0° to 90°, manual control) and SAS (Switched Angle Spinning, 0° to 90°, with servo/computer control) are currently available only on the widebore XC MAS probe.

Download a SAS probe technical poster presented at ENC 2006, Asilomar, CA.

2H quadrupolar
splitting for D2O in bicelles.

high-field-saas-chart

A) spectrum for sample rotating at θ=80°.

B) spectrum on rotating
sample obtained with SAS from θ=80° to θ=10°.

Laura Holte, Doty Scientific, Inc.

 

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