Aromatic ring flips in differently packed ubiquitin protein crystals from MAS NMR and MD

•A specific isotope labeling scheme and fast magic-angle spinning NMR provide insight into ring flips.•One of the two phenylalanines, Phe4, undergoes 10–20 ns flips.•Crystal contacts (pi stacking) lead to marked upfield shift in one chain of one crystal.•The other phenylalanine, Phe45, is at least 1–2 orders of magnitude slower, and is broadened beyond detection.•The behavior is similar in three crystals, suggesting mostly intramolecular influence on ring flips.