Conformational profile of 1-aminocyclopropanecarboxylic acid

1-Aminocyclopropanecarboxylic acid (Ac(3)c) is a constrained alpha, amino a cid residue that exhibits peculiar conformational characteristics. The aim of the present study is to provide a deeper understanding of these features to be used as guidance to decide when to choose Ac(3)c as a building block for the design of peptide and protein surrogates. The whole Ramachandran p lot of the Ace-Ac(3)c-NCH3 dipeptide was investigated at the Hartree-Fock l evel using a 6-31G(d) basis set and the most favorable structures were asse ssed on this surface by energy minimization. These results were subsequentl y used as a reference to generate specific molecular mechanics parameters f or Ac(3)c compatible with the parm94 set of the AMBER force field. The effe ct of water as a solvent on the conformational profile of the dipeptide was also assessed using the Miertus-Sorocco-Tomasi self-consistent reaction-fi eld model at the Hartree-Fock level using a 6-31G(d) basis set and using th e AMI semiempirical method. The conformational profile of the Ac(3)c dipept ide can be characterized by two symmetric low-energy regions for values of phi around +/- 80 degrees with a wide range of values for mu ranging from - 40 to 180 degrees, with the lower areas located at low values of psi. Solve nt effects do nor alter the features of the conformational map, but a shift of the two absolute minima to (phi. psi) values near (+/- 90 degrees, 0 de grees) can be observed. These results are in accord with all experimental e vidence and with the known tendency of Ac(3)c to induce beta-turn conformat ions in peptides.