Calculation of the Conformation of cyclo-Hexaglycyl. 2. Application of a Monte-Carlo Method

Abstract

A Monte-Carlo method is used in a conformational energy analysis of cyclo-hexaglycyl. This method simulates, in an approximate way, the real process in which the molecule changes its conformation in solution. If the conformation is trapped in a local potential energy minimum for a given finite number of steps of the Monte-Carlo run, the minimum is defined as a good one; otherwise it is designated as a poor one. It is shown that such poor minima exist for this molecule and that the Monte-Carlo method can alter the conformation awaq from such minima. It is also found that many good minima (at least 81 and surely more) exist for this molecule. A nearly complete list of mini- mum-energy conformations (MEC's) with low conformational energy is obtained. All conformations with and with- out symmetry, that exist in a hemihydrate crystal of this molecule. are each found to be very similar to one of the good-minimum low-energy MEC's. The Monte-Carlo method is also used to examine the nature of the MEC's with symmetry that were located in a previous paper. Fourteen of the 24 MEC's with symmetry are found to be good mini- ma. The other ten either are poor minima or correspond to a saddle point in the full six-dimensional conformation space.