L-Proline Lab Report

Computational Methods

Of all the possible conformations of L-proline, only four can exist at ambient temperature,3, 33 and we chose the two most stable conformers for this study. The chosen conformers differ only in the puckering of pyrolidine ring of L-proline which is syn with the carboxyl group (Fig. 1a) in one conformer and anti to it (Fig. 1b) in the other. In the both chosen conformers the carboxylate group twisted toward the imino group. This geometry allows the possibility of intramolecular hydrogen bonding between the hydroxyl and the imino group of the pyrrolidine ring.

Coordination of metal cations with L-proline can be achieved in many possible modes,3, 27 and we chose the three coordination patterns recommended by the work of Fleming

(iii) Charge solvated 2 (CS2): a bi-dentate coordination between the metal cation and neutral L-proline through the nitrogen atom of the imino and the oxygen atom of the carbonyl groups Each structure was fully optimized by density functional theory (DFT) calculations using a nonlocal hybrid B3LYP (Becke-Lee-Parr) exchange correlation functional.34, 35 Then vibrational frequencies were calculated to verify the stationary structure for all the structures. The standard split-valence 6–311++G(d,p) basis set of atomic orbitals was used for all non metal elements; all the metal cations were described using the LANL2DZ relativistic pseudopotentials.36-38 The GAMESS suit of programs was used for all calculations.39

Since group IIB transition metals (Zn2+, Cd2+ and Hg2+) are d10 ions, their complexes have singlet ground states in a closed-shell system.

Thermodynamic calculations including enthalpy (∆H), entropy (∆S) and free energy (∆G) terms were also performed at the standard temperature of 298 K and then used for the determination of metal ion affinities (MIA) for each system, which is defined as the negative enthalpy variation (-∆H) of the following coordination process at 298

3 indicate that the trend in coordination strength is (SB) ˃ (CS2) ˃ (CS1) within all studied metal cations. These results indicate that all studied cations favor binding to L-proline in a (SB) pattern through both oxygens on the carboxylate group. For example the (SB) coordination pattern in the case of zinc cation is preferred by 24.1 and 5.2 kcal/mol from (CS1) and (CS2) coordination patterns respectively (Table 3).

Essentially the same order in coordination patterns of alkali and alkaline earth metal cations with L-proline were reported by Marino et al27 and Fleming et al3 respectively. They found that all of alkali and alkaline earth metal cations except Be2+ were coordinated with L-proline most strongly in the (SB) coordination pattern. Only Be2+ make coordinated structures with L-proline in (CS2) coordination pattern with higher stability than that of other coordination patterns. For five of six metal cations of these two groups, the stability order of coordination patterns is as (SB) ˃ (CS2) ˃ (CS1) but for Be2+ this order change to (CS2) ˃ (SB) ˃ (CS1).

The comparison of our results for [L-proline-Mn+] and previously reported works3,27,32 are shown in Fig. 4. In this comparison, only coordination patterns of (SB) and (CS2) were