Phosphatidylcholine “Wobble” in Vesicles Assessed by High-Resolution 13C Field Cycling NMR Spectroscopy

High resolution 13C NMR field cycling (covering 11.7 down to 0.002 T) relaxation studies of the sn-2 carbonyl of phosphatidylcholines in vesicles provide a detailed look at the dynamics of this position of the phospholipid in vesicles. The spin−lattice relaxation rate, R 1, observed down to 0.05 T is the result of dipolar and CSA relaxation components characterized by a single correlation time τc, with a small contribution from a faster motion contributing to CSA relaxation. At lower fields, R 1 increases further with a correlation time consistent with vesicle tumbling. The τc is particularly interesting since it is 2−3 times slower than what is observed for 31P of the same phospholipid. However, cholesterol increases the τc for both 31P and 13C sites to the same value, ∼25 ns. These observations suggest faster local motion dominates the dipolar relaxation of the 31P, while a slower rotation or wobble dominates the relaxation of the carbonyl carbon by the α-CH2 group. The faster motion must be damped with the sterol present. As a general methodology, high resolution 13C field cycling may be useful for quantifying dynamics in other complex systems as long as a 13C label (without attached protons) can be introduced.