Conformational versatility among crystalline solids of L-phenylalanine derivatives

In this study, we present a new N-derivative of L-phenylalanine with 2-naphthaldehyde (PN), obtained by the Schiff base formation procedure and its subsequent reduction. This compound was crystallized as a zwitterion {2-[(naphthalen-2-ylmethyl)azaniumyl]-3-phenylpropanoate, C₂₀H₁₉NO₂}, as an anion in a sodium salt (catena-poly[[diaquasodium(I)-di-μ-aqua] 2-[(naphthalen-2-ylmethyl)amino]-3-phenylpropanoate monohydrate], {[Na(H₂O)₄](C₂₀H₁₈NO₂)·H₂O}_n), as a cation in a chloride salt [(1-carboxy-2-phenylethyl)(naphthalen-2-ylmethyl)azanium chloride acetic acid monosolvate, C₂₀H₂₀NO₂⁺·Cl^-·CH₃COOH], and additionally acting as a ligand in the pentacoordinated zinc compound aquabis{2-[(naphthalen-2-ylmethyl)amino]-3-phenylpropanoato-κO}zinc(II), [Zn(C₂₀H₁₈NO₂)₂(H₂O)] or [Zn(PN)₂(H₂O)], denoted (PN-Zn), with the amino acid derivative in its carboxylate form. Interestingly, both enantiomers of the zinc complex co-exist within the crystalline structure, one constructed by the ligands with the L (or S) configuration and the other with the ligands having the D (or R) configuration, represented as L,L-PN-Zn and D,D-PN-Zn, respectively. Also, in the structure of the zwitterion, the racemate L,D is observed. These results imply that chirality inversion of the amino acid derivative synthesized from enantiomerically pure L-phenylalanine is taking place, a phenomenon known as oscillatory transenantiomerization. The analysis of these crystal structures reveals that they are primarily stabilized through electrostatic interactions assisted by hydrogen bonds. An interesting finding is that the conformation of PN varies along this family: it is unfolded in the zwitterionic and cationic forms, and folded in the anionic form. To evaluate such conformational differences, we propose the use of a dimensionless Shape Factor quantity defined as the Structural Aspect Ratio (SA_R), computed from the geometrical features of the parallelepiped that tightly encloses a conformer constructed by rigid spheres. This parameter provides a simple but useful tool to distinguish conformational differences, providing insights that complement traditional structural analyses. The study of the structural features, conformational diversity, chirality and supramolecular properties of these compounds is also supported by density functional theory (DFT) calculations.