## 4 Results and Discussion
#### 4.1.1 The Number of the Possible Conformers  
The L-dopa molecule (Figure 1) has seven tops. To find out its total number of possible conformers, the total energy vs. dihedral angle curves were obtained for different tops. The total energy vs. dihedral angle plots for the seven tops are shown in Figure 2a-c and Figure S1. The L-dopa molecule is a benzene derivative with three substituents at the sites C\({}_{6}\), C\({}_{1}\) and C\({}_{3}\). The substituents at the sites C\({}_{6}\) and C\({}_{1}\) are the OH groups and the substituent at the site C\({}_{3}\) is a 2-aminopropenoic acid group. The 2-aminopropenoic acid skeleton has five top axes, namely C\({}_{3}\)-C\({}_{14}\), C\({}_{14}\)-C\({}_{17}\), C\({}_{17}\)-N\({}_{22}\), C\({}_{17}\)-C\({}_{19}\) and C\({}_{19}\)-O\({}_{20}\). The top axes C\({}_{3}\)-C\({}_{14}\), C\({}_{17}\)-C\({}_{19}\) and C\({}_{19}\)-O\({}_{20}\) have two-fold barriers (Figure S1a-c) and the top axis C\({}_{14}\)-C\({}_{17}\) has a three-fold barrier (Figure S2d). When the atom H\({}_{24}\) faces towards the atom N\({}_{22}\), the rotational barrier for the NH\({}_{2}\) group about the C\({}_{17}\)-N\({}_{22}\) axis is two-fold; however, when the atom H\({}_{24}\) is directed away from the atom N\({}_{22}\), the barrier about the same axis is one-fold. Hence, the rotation of the NH\({}_{2}\) group about the C\({}_{17}\)-N\({}_{22}\) axis produces three different stable configurations. As a result, the total number of the stable configurations of 2-aminopropenoic acid skeleton due to the rotation about the five top axes C\({}_{3}\)-C\({}_{14}\), C\({}_{14}\)-C\({}_{17}\), C\({}_{17}\)-N\({}_{22}\), C\({}_{17}\)-C\({}_{19}\) and C\({}_{19}\)-O\({}_{20}\) is \(2\times 2\times 2\times 3\times 3=72\). The rotations of both OH groups give rise to three different stable configurations. Out of these three configurations, there are two configurations in which the H-atoms of both the OH groups are oriented in the same sense of rotation, and in the third configuration both the H-atoms are oriented in the opposite sense of rotation. Hence, the total stable configurations arising due to the rotations about all the seven top axes is found to be 3 \(\times\) 72 = 216. Out of these 216 configurations, there are 108 pairs of configurations. In each pair, one configuration is the mirror image of the other configuration. The mirror images of the 108 configurations are generated by a rotation of 180\({}^{\circ}\) about the top axis C\({}_{3}\)-C\({}_{14}\). As a result, the total number of different stable configurations of the L-dopa molecule would be 216/2 = 108. The total energies and the energy differences of the first five lowest-energy conformers are given in Table 1. From this Table it is clear that the first higher-energy conformer is found at 388 K relative to the lowest-energy conformer. Different molecular parameters of the lowest-energy monomeric conformer C-I and its lowest-energy dimeric conformer D\({}_{1}\) are collected in Table S1. The optimized structures of the first six lowest-energy conformers of the L-dopa molecule are shown in Figure S2. The energy differences of the five conformers (C-II to C-VI) are found to be below 1000 K relative to the lowest-energy conformer C-I.  
Figure 1: Optimized structure of the lowest-energy conformer (C-I) of L-dopa.  
Figure 2: (**a**–**c**). Variation of the total energies with the dihedral angles for the rotations of the OH groups about their respective axes.