2D solid state NMR spectra of HIV-1 CA tubes. (A) Aliphatic region of a 2D 13C-13C spectrum of sample I, obtained with a spin diffusion mixing period τSD = 200 ms. The inset shows assignments of Met Cα-Cε crosspeaks. (B) Aromatic region of a 2D 13C-13C spectrum of sample II, obtained with τSD = 700 ms. (C) Aromatic/aliphatic region of a 2D 13C-13C correlation spectrum of sample II, obtained with τSD = 700 ms. Crosspeaks indicating long-range contacts from W184 to V181 and L189 are labeled. Intraresidue crosspeaks from the four other Trp residues (W23, W80, W117, and W133) are also resolved13, but for clarity are not labeled. Contour levels increase by factors of 1.2 in panels A–C.