Specific configurations of hydrogen bonding. I. Hydrogen bonding and conformational preferences of N-acylamino-acids, peptides and derivatives

From a reexamination of the X-ray studies of the crystal structures of 27 N-acylamino acids, peptides and their derivatives and 30 linear peptides, it is concluded that specific formation of short intermolecular hydrogen bonds (2,5 to 2.6 A) from the carboxyl OH to the N-acyl oxygen is an important feature for N-acylamino acids. For N-acyl-N-amides, the formation of hydrogen bonds 2.7 to 2.9A long between N(acyl-H...O(amide) is strongly preferred. The dihedral angle delta between the N-acyl and carboxyl groups or adjacent amide groups shows a preference for values near 20 degrees or 90 degrees for N-acylamino acids and 90 degrees for N-acyl-N-amides.