Michal Tencer

A Possible Path to the RNA World: Enantioselective and Diastereoselective Purification of Ribose

A theoretical mechanism resulting in the prebiotic appearance of enantiopure ribose, which would be needed for the origin of RNA and the “RNA world” is proposed. The mechanism simultaneously explains the emergence of biological homochirality and could answer the question of why nucleic acids are based on ribose rather than another sugar. Cleavage of certain non-chiral mineral crystals is known to lead to formation of chiral surfaces. In a chromatography-like process a mixture of racemic carbohydrates originating from the formose reaction is proposed to have been separated on such a chiral surface. Monosaccharides interact with surfaces through their hydroxyl groups, either by hydrogen bond formation or complex formation with metal ions. α-Ribopyranose, which has all hydroxyl groups on one side of the ring, is known to interact more strongly than other sugars with surfaces, as corroborated by certain chromatographic and electrophoresis data. A similar scenario leading to enantiopure ribose is separation on a flat, but not necessarily chiral surface in the presence of a strong electric field capable of orienting highly polar derivatives of sugars.

Toposelective Electrochemical Desorption of Thiol SAMs from Neighboring Polycrystalline Gold Surfaces

We describe a method for the selective desorption of thiol self-assembled monolayers from gold surfaces having micrometer-scale separations on a substrate. In an electrolyte solution, the electrical resistance between the adjacent areas can be much lower than the resistance between a surface and the counter electrode. Also, both reductive and oxidative thiol desorption may occur. Therefore, the potentials of the surfaces must be independently controlled with a multichannel potentiostat and operating windows for a given thiol/electrolyte system must be established. In this study operating windows were established for 1-dodecanethiol-based SAMs in phosphate buffer, phosphate-buffered saline, and sodium hydroxide solution, and selective SAM removal was successfully performed in a four-electrode configuration.

Electrochemical Differentiation and TOF-SIMS Characterization of Thiol-Coated Gold Features for (Bio)chemical Sensor Applications

A method to chemically differentiate small closely spaced lithographically defined Au features with different thiol-based self-assembled monolayers (SAMs) was developed. The key step in the method is the reductive electrochemical desorption of a SAM from a specific Au feature without affecting SAMs on neighboring Au features by simultaneously controlling their potentials with a multichannel potentiostat. The method is demonstrated by chemically differentiating the arms of a Au plasmonic Mach-Zehnder interferometer such that the arms have different affinities toward an analyte, thus rendering the interferometer useful for (bio)chemical sensing. The simultaneous presence of a poly(ethylene glycol)-terminated SAM on one arm, and of a biotin-terminated SAM on the other, was verified by imaging with time-of-flight secondary ion mass spectrometry (TOF-SIMS) and phase-shift atomic force microscopy. The method can be applied generally to chemically differentiate a large number of electrically isolated Au features simultaneously, leading to low cost wafer-scale functionalization of (bio)chemical sensors and other devices.

Confinement and deposition of solution droplets on solvophilic surfaces using a flat high surface energy guide

A method of depositing small amounts of solution on flat micron scale surface areas on a hydrophilic substrate or die was developed. This method utilizes the capillarity of a flat, high surface free energy guide. Interfacial forces confine the solution between the guide and the substrate surface. The liquid follows the movement of the guide along the surface and can be moved to the desired area. The thermodynamic background of the method is given and its application to coat one arm of a gold plasmonic Mach-Zehnder interferometer with bovine serum albumin is described. This method, which is related to but different from microcontact printing and dip-pen microlithography, can be utilized in the manufacturing of biosensors and other lab-on-a-chip structures, and is particularly suitable to development stage devices.

Mechanical resolution of chiral objects in achiral media: where is the size limit?

Macroscopic chiral objects (boats and planes with turned rudders, shoes, etc.) get separated from their mirror-image counterparts by motion in achiral media. However, chiral molecules are not enantio-differentiated without the presence of a chiral environment, which may be due to other chiral molecules in the medium. This article explores the reasons of this micro/macro difference as well as the size borderline between the two regimes. There are two major demarcation lines, both related to the objects chaotic thermal motion. The first one is due to destruction of the necessary spatial orientation by the fast rotational diffusion. Only particles larger than 1 μm can maintain their original orientation for 1 sec or longer. For smaller particles, an additional external orienting factor, e.g., a strong electric field has to be applied. The second limitation is defined by the ratio of the hydrodynamic separation of the enantiomers (which is directly proportional to time) to their displacement due to the translational Brownian motion (which is proportional to square root of time). On the laboratory time scales (up to a year), the chiral objects have to be larger than 0.25 μm to be resolved. On evolutionary time scales, much smaller object could be resolved. For enantiomers approaching the molecular size, periods comparable to the age of the universe would be required.

A Comment on “Separation of Chiral Molecules: A Way to Homochirality”

Keywords Chiral.Homochirality.Enantioselectiveseparation.Brownianmotion.Macroscopicandmicroscopicobjects.Resolution.Pre-biotic.HydrodynamicIn a recent paper Atencio (2012) proposes a mechanism which in the author’s mind could beresponsible for a pre-biotic separation of chiral molecules in the absence of a chiralenvironment. The author provides a thorough hydrodynamic analysis of particle separationin an aquifer based on their chirality. Without getting into the details of this analysis, weassume that it is correct. However, we think that the author fell into a trap by not consideringfundamental behavioural differences between different types of objects related to their sizescale.Mechanical and hydrodynamic separation of macroscopic chiral object in achiral media isan everyday occurrence, as everyone who watches sleigh rides on a hill or a sailing boatturning knows. The phenomenon, observed also in nature by (Nagle 1967, Welch 1998) wassubjected to theoretical analysis (de Gennes 1999, Howard et al. 1976) and even patented(Hirschfelder et al. 1977).The critical aspect here, not taken into account by Atencio (2012), is the effects of therandom thermal motions differentiating between the macro- and micro- scale chiral objects.This effect is negligible with the former but not with latter. De Gennes (1999) estimated theregime transition size to be ca. 1 μm. More detailed considerations of the effect of two kindsof random motion (Brownian motion and rotational diffusion) on the efficiency of hydro-dynamic resolution as a function of the object’s size (Tencer and Bielski 2011) showed thatrotational diffusion destroys the object’s spatial orientation necessary for the hydrodynamicdifferentiation and only objects larger than 1 μm can maintain such orientation longer than1 s, in agreement with de Gennes (1999) estimate. Even if the orientation can be maintained