Solution conformations of proline rings in proteins studied by NMR spectroscopy

Three different conformations of proline rings in a protein in solution, Up, Down and Twist, have been distinguished, and stereospecific assignments of the pyrrolidine beta-, gamma- and delta-hydrogens have been made on the basis of 1H-1H vicinal coupling constant patterns and intraresidue NOEs. For all three conformations, interhydrogen distances in the pairs alpha-beta 3, beta 3-gamma 3, beta 2-gamma 2, gamma 2-delta 2, and gamma 3-delta 3 (2.3 A) are shorter than those in the pairs alpha-beta 2, beta 2-gamma 3, beta 3-gamma 2, gamma 2-delta 3, and gamma 3-delta 2 (2.7-3.0 A), resulting in stronger NOESY cross peaks. For the Up conformation, the beta 3-gamma 2 and gamma 2-delta 3 spin-spin coupling constants are small (< 3 Hz), and weak cross peaks are obtained in a short-mixing-time (10 ms) TOCSY spectrum; all other vicinal coupling constants are in the range 5-12 Hz, and result in medium to strong TOCSY cross peaks. For the Down form, the alpha-beta 2, beta 2-gamma 3, and gamma 3-delta 2 vicinal coupling constants are small, leading to weak TOCSY cross peaks; all other couplings again are in the range 5-12 Hz, and result in medium to strong TOCSY cross peaks. In the case of a Twist conformation, dynamically averaged coupling constants are anticipated. The procedure has been applied to bovine pancreatic trypsin inhibitor and Cucurbita maxima trypsin inhibitor-V, and ring conformations of all prolines in the two proteins have been determined.