Theoretical Study of Intermolecular Interaction at the Lipid−Water Interface. 1. Quantum Chemical Analysis Using a Reaction Field Theory

Abstract

Reaction field calculation combined with the AM1 molecular orbital method is applied in order to understand why intermolecular hydrogen bonding is stabilized at the lipid−water interface. Here, we focus on the interaction between a guanidinium-functionalized lipid (1) and phosphate and between a diaminotriazine-functionalized lipid (2) and thymine. The interface is approximated by a double layer composed of two dielectrics. The lower dielectric medium with a dielectric constant of 2 corresponds to the lipid layer, and the higher dielectric medium with dielectric constant of 80 to the aqueous subphase. A pair of interacting molecules is placed on/near the interface, and the binding energy profile is obtained. For comparison, the calculation of binding energy is also performed for a homogeneous system defined as a single dielectric constant, corresponding to a normal solution system. It is shown that the calculation reproduces well the observed binding constants, when the position of the interface is appropriately displaced relative to the interacting molecules. One of the most important findings is that hydrogen bonding is remarkably strengthened even if the binding site is exposed to the aqueous subphase. The results for the binding profiles are interpreted from those for Mulliken population analysis.

Challenges and breakthroughs in recent research on self-assembly

“…The magnitude of binding constants should be related to the type of interface. Sakurai and coworkers proposed application of a quantum chemical calculation to molecular recognition at the air-water interface [655][656][657]. Calculations were performed based on a multi-dielectric model for the guanidinium-phosphate system situated at an interface.…”

Two-dimensional nanoarchitectonics based on self-assembly

“…In particular, molecular recognition through hydrogen bonding occurs efficiently at this interface, as demonstrated in molecular recognition of bio-related substances such as peptides [149][150][151][152], nucleotidic compounds [153][154][155], and sugars [156,157] or as simulated in theoretical calculations [158][159][160]. Therefore, molecular-level patterns can be designed and prepared through molecular interaction between monolayer components and aqueous template molecules.…”

Chiral recognition at the air–water interface

“…Most importantly they found that molecular recognition at the air-water interface generally occurs with enhanced binding constants relative to those in bulk media [7]. This was also theoretically interpreted using quantum chemical calculations [8,9]. Unfortunately, chiral discrimination was not seriously discussed in Kunitake's reports although they demonstrated recognition of guest substances with chiral structures, such as sugars [10], nucleotides [11], amino acids [12], and peptides [13][14][15][16].…”