Conformational studies of cyclic peptide structures in solution from ¹H‐Nmr data by distance geometry calculation and restrained energy minimization

The three‐dimensional structure of a cyclic bouvardin analogue, cyclo (‐Pro‐MeTyr‐Ala‐MeTyr‐MeTyr‐D‐Ala‐) has been determined by distance geomtry calculation and restrained energy minimization from nmr data. The preparation of the input for the distance geometry calculations, the modification of the amino acid library, and the analysis of the structures were done with the aid of a recently developed software package, GEOM. A great variety of different initial structures were explored to check the uniqueness of the determined solution structure. Calculations with 500 different initial structures and two different strategies led to a uniquely determined backbone conformation with a root mean square deviations value of 0.4 Å. The backbone structure consists of two β‐turns, a β‐II turn at Pro¹‐MeTyr², and a β‐VI turn at MeTyr⁴‐MeTyr⁵. The efficiency of the two calculation strategies were compared in order to propose an optimal means for performing distance geometry calculations with cyclic structures.