Understanding the self-assembly of amino ester-based benzene-1,3,5-tricarboxamides using molecular dynamics simulations

KK Bejagam and RC Remsing and ML Klein and S Balasubramanian, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 19, 258-266 (2017).

DOI: 10.1039/c6cp06742e

Amino ester-based benzene-1,3,5-tricarboxamides (BTAs) are widely studied experimentally for their facile self-assembly, which leads to strong three-fold hydrogen bonded supramolecular polymers. Understanding the supramolecular assembly of these BTAs is complicated by the presence of two types of dimers, based on the nature of the intermolecular hydrogen bonding pattern: amide-amide (AA) and amide-carboxylate (AC). AA dimers form three hydrogen bonds between the two molecules, are typical of BTA stacks, and act as a basic building block of assembly. In contrast, AC hydrogen bonding results in six hydrogen bonds between two molecules, and this face-to-face orientation results in a dimer that is more stable than the AA one, however, unfavorable for further assembly. We perform atomistic molecular dynamics (MD) simulations of three derivatives of BTA in order to rationalize the large body of experimental data for these systems, specifically the relative stabilities of AA and AC dimers and oligomers. We find that at zero Kelvin, the AC dimer is more stable than the AA dimer by roughly 20 kcal mol(-1). MD simulations of three BTA derivatives (BTA-Met, BTA-Nle, and BTA-Phe) under realistic conditions show that BTA-Met and BTA-Phe can aggregate to form longer assemblies via additional stabilization offered by weak CH center dot center dot center dot S and CH center dot center dot center dot pi hydrogen bonds, respectively. However, the aggregation of BTA-Nle, which is devoid of such functionalities, is limited to that of a dimer. We then employ umbrella sampling to show that oligomers of BTA-Met and BTA-Phe are stable over those of dimers and demonstrate that this results from such weak interactions.

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