A theoretical estimate of the energy barriers between stable conformations of the proline dimer.

Semi-empirical energy calculations for an internal Pro-Pro dimer are presented that take into account the nature of the flexibility of the proline ring due to its puckering. Calculations show that three stable conformations are available for the dimer: the cis (w = O", + = 160'); the trans (w = 180", $ = 160", also referred to as trans'); and the cis' (w = 180°, tb = -40') conformations. The best conformational pathways between these stable conformations are determined. Calculations also show that the barrier for cis'-trans' conversion is of the same order of magnitude as that for cis-trans conversion.