Recognition and Dissociation Kinetics in the Interfacial Molecular Recognition of Barbituric Acid by Amphiphilic Melamine-Type Monolayers

Quantum chemical assessment of the molecular area corresponding to the onset of the LE-LC phase transition for amphiphilic 2D monolayers at the air/water interface

An approach for the assessment of the area per surfactant molecule in a monolayer at the onset of the LE-LC phase transition (A_c) is proposed based on the quantum chemical approach and a thermodynamic model for amphiphilic monolayers, which takes into account the nonideality of the mixing entropy. The values of the Gibbs' clusterization energy for small surfactant associates, as well as the geometric parameters of the monolayer unit cells, were used, previously calculated using the semiempirical PM3 method for eight classes of amphiphilic compounds: saturated and ethoxylated alcohols, saturated and unsaturated cis-carboxylic acids, α-hydroxylic and α-aminoacids, N-acyl-substituted alanine and dialkyl-substituted melamine. The obtained A_c values are in satisfactory agreement with the available experimental data. This allows using the proposed approach for prognostic purposes in the cases where there are no corresponding π-A isotherms for necessary surfactants, but there are calculated thermodynamic and structural parameters of its clusterization.

Thermodynamic, kinetic and conformational analysis of proteins diffusion-sorption on a solid surface

In this paper we examine particularly some of the more fundamental properties of protein conformational changes at a solid surface coupled with diffusion from the bulk of an aqueous solution and with the adsorption-desorption processes. We focus our attention on adsorbed protein monolayers upon a solid surface using a thermodynamic and kinetic analytical development. Account is also taken of the effects on the overall rate of the conformational change on a solid surface of deviation from ideality, of protein flexibility, of surface free energy and of interaction with reactive solid sites. Our theory applied to steady states is illustrated by examples such as folding-misfolding-unfolding of RNase and SNase on a solid surface after diffusion and adsorption from an aqueous solution. For this purpose, we put forward the determining steps which shall lead to the steady state. The existence of three situations is highlighted according to the values of the typical constants relevant for the protein considered: reaction rate determining step, diffusion and sorption determining steps, mixed adsorption diffusion and reaction rate. Finally, we have tried to link the developments of our theories to a large literature based on experimental results encountered during proteins diffusion-sorption-reaction processes, fundamental topics that has been since long investigated by Miller's team in MPKG.

Self-assembly and molecular recognition of adenine- and thymine-functionalized nucleolipids in the mixed monolayers and thymine-functionalized nucleolipids on aqueous melamine at the air-water interface

Self-assembly and molecular recognition of the monolayers composed of an equimolar mixture of adenine- and thymine-functionalized nucleolipids at the air-water interface have been investigated in detail using surface pressure-molecular area isotherms and in situ infrared reflection absorption spectroscopy (IRRAS). Prior to molecular recognition, the adenine moieties in the monolayer were almost oriented on an end-on mode through π-stacking and hydrogen bonding interactions, and the C-C-C planes of the alkyl chains were preferentially oriented perpendicular to the water surface, while the thymine moieties in the monolayer were involved in hydrogen bonding almost with a flat-on orientation. On aqueous subphases containing complementary bases, no significant molecular recognition was observed for the monolayers of individual nucleolipids. In the monolayer of equimolar mixture, molecular recognition occurred between the adenine and thymine moieties through hydrogen bonding probably with the development of cyclic structures of adenine-thymine-adenine-thymine quartets. Although molecular recognition between the monolayer of thymine-functionalized nucleolipids and aqueous melamine took place through triple hydrogen bonds, no melamine binding to the monolayer of equimolar mixture was observed, which reflects the formation of the quartets in the mixed monolayers at the air-water interface. FTIR and small-angle X-ray diffraction (XRD) results of the corresponding Langmuir-Blodgett films support the hydrogen bonding recognition and molecular orientation.

Quantum-chemical description of the thermodynamic characteristics of clusterization of melamine-type amphiphiles at the air/water interface

The semiempiric PM3 method is used to calculate the thermodynamic parameters of the formation of monomers, dimers, trimers, and tetramers of the amphiphilic melamine-type series of 2,4-di(n-alkylamino)-6-amino-1,3,5-triazine (2C(n)H(2n+1)-melamine) with n = 9-16. The most stable conformations are determined, which then are used to construct the clusters. The peculiar feature of these structures is the existence of a bend at one of the alkyl chains. Thus, the formation of infinite films becomes possible because of their spatial arrangement. From the calculation of the relative amount of various conformers in the mixture, it follows that, if the alkyl chain length is lower than 11-12 carbon atoms, the mixture is composed mainly of the monomers that do not contain any intramolecular interactions, whereas for higher alkyl chain lengths the monomers that involve such interactions prevail in the mixture. For all clusters thus considered, the thermodynamic parameters (enthalpies, entropies, and Gibbs' energies) of clusterization are calculated. It is shown that the dependencies of these parameters on the alkyl chain length either exhibit stepwise shape or are represented by the combination of a linear and stepwise function. This depends on the different number of hydrogen-hydrogen interactions in the structures considered. Five types of clusters that are capable of the formation of infinite 2D films are considered in detail. For each of these types, the dependencies of the clusterization enthalpy, entropy, and Gibbs' energy on the alkyl chain length in the constituting monomers are derived. Using these dependencies, it becomes possible to calculate these thermodynamic characteristics for clusters of any size, and also for infinite 2D films. It is shown that the spontaneous clusterization of 2C(n)H(2n+1)-melamine becomes possible if the alkyl chain length exceeds 9 carbon atoms.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.