Kink sites on high Miller index surfaces are either left-or right-handed and can be thought of as chiral, when the step lengths on either side of the kink site are unequal. A silver single crystal was oriented and cut to expose the Ag(643) surface on one side and the Ag(6 h4 h3 h) surface on the other side. A system is proposed for naming these surfaces as Ag(643) S and Ag(643) R , respectively , in analogy with the CahnIngold-Prelog rules used in the nomenclature of organic stereoisomers. The left hand/right hand relationship of the two surfaces was manifested by the direction of the splitting of the low-energy electron diffraction (LEED) spots. The interaction of the enantiomers of a chiral alcohol ((R)-2-butanol and (S)-2-butanol) with each surface was studied using temperature-programmed desorption (TPD) measurements in order to ascertain the magnitude of the effect of surface chirality on the heats of adsorption. Desorption of the alcohols following exposure to the clean surfaces was molecular and exhibited first-order kinetics. No difference was observed between (R)-and (S)-2-butanol in either desorption temperature (225 K) or peak shape. Upon exposure to preoxidized surfaces, the alcohols deprotonated to form (R)-and (S)-2-butanoxide, both of which decomposed upon heating via -hydride elimination. The decomposition product, 2-butanone, desorbed at 282 K. Again, no difference in the reaction kinetics of the enantiomeric alkoxides was observed on the two surfaces. From these results it can be concluded that the difference in (a) the heat of adsorption of the enantiomeric alcohols and (b) the difference in the energy barrier to -hydride elimination for the enantiomeric alkoxides is less than 0.1 kcal/mol.

Angiotensin-converting enzyme 2 (ACE2) and accessory proteases (TMPRSS2 and CTSL) are needed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cellular entry, and their expression may shed light on viral tropism and impact across the body. We assessed the cell-type-specific expression of ACE2, TMPRSS2 and CTSL across 107 single-cell RNA-sequencing studies from different tissues. ACE2, TMPRSS2 and CTSL are coexpressed in specific subsets of respiratory epithelial cells in the nasal passages, airways and alveoli, and in cells from other organs associated with coronavirus disease 2019 (COVID-19) transmission or pathology. We performed a meta-analysis of 31 lung single-cell RNA-sequencing studies with 1,320,896 cells from 377 nasal, airway and lung parenchyma samples from 228 individuals. This revealed cell-type-specific associations of age, sex and smoking with expression levels of ACE2, TMPRSS2 and CTSL. Expression of entry factors increased with age and in males, including in airway secretory cells and alveolar type 2 cells. Expression programs shared by ACE2 + TMPRSS2 + cells in nasal, lung and gut tissues included genes that may mediate viral entry, key immune functions and epithelial-macrophage cross-talk, such as genes involved in the interleukin-6, interleukin-1, tumor necrosis factor and complement pathways. Cell-type-specific expression patterns may contribute to the pathogenesis of COVID-19, and our work highlights putative molecular pathways for therapeutic intervention.

Kinked-stepped, high Miller index surfaces of metal crystals are chiral and, therefore, exhibit enantiospecific properties. Previous temperature-programmed desorption (TPD) spectra have shown that the desorption energies of R-3-methylcyclohexanone (R-3-MCHO) on the chiral Cu(643)(R) and Cu(643)(S) surfaces are enantiospecific (J. Am. Chem. Soc. 2002, 124, 2384). Here, a comparison of the TPD spectra from Cu(111), Cu(221), Cu(533), Cu(653)(R&S), and Cu(643)(R&S) surfaces reveals that the enantiospecific desorption occurs from the chiral kink sites on the Cu(643) surfaces. Titration of the chiral kink sites with I atoms confirms this assignment of desorption features in the TPD spectra. Finally, the enantiospecific difference in the desorption energies of R- and S-3-MCHO has been used as the basis for demonstration of an enantioselective, kinetic separation of racemic 3-MCHO into its purified components during adsorption and desorption on the Cu(643)(R&S) surfaces.

Temperature-programmed desorption (TPD) experiments have been conducted to investigate enantiospecific desorption from chiral single-crystal surfaces. The (643) and (six four three) planes of face-centered cubic metals such as Cu have kinked and stepped structures which are nonsuperimposable mirror images of one another and therefore are chiral. These chiral surfaces are denoted Cu(643)(R) and Cu(643)(S). We have observed that the desorption energies of (R)-3-methylcyclohexanone and (R)- and (S)-propylene oxides from the Cu(643)(R) and Cu(643)(S) surfaces depend on the relative handedness of the adsorbate/substrate combination. Since the (643) surface is comprised of terraces with local (111) orientation which are separated by kinked monatomic steps, it is instructive to perform TPD experiments with these chiral compounds on the achiral Cu(111) surface. These experiments have given some insight into the adsorption sites for the chiral molecules on the Cu(643) surfaces. There are several high-temperature features in the TPD spectra of the chiral compounds that only appear in the spectra from the (643) surfaces and thus are attributed to molecules adsorbed at or near the kinked steps. In addition there are lower temperature desorption features observed on the Cu(643) surfaces which occur in the same temperature range as desorption features observed on the Cu(111) surface. These features observed on the (643) surfaces are attributed to desorption from the flat (111) terraces.

The dynamics of oligomer desorption from surfaces have been studied by measuring the desorption kinetics of a set of n-alkanes from the surface of single crystalline graphite. Desorption rates were measured using a set of 21 monodispersed n-alkanes (C N H 2Nϩ2 ,5рNр60) each adsorbed at coverages in the range Ͻ0.1 to Ͼ1 monolayers. Desorption is observed to be a first-order process with a desorption barrier (⌬E des ‡ ) that is independent of coverage. The pre-exponential of the desorption rate constant is independent of the oligomer chain length and has a value of ϭ10 19.6Ϯ0.5 s Ϫ1 . We also find that ⌬E des ‡ has a nonlinear dependence on chain length and takes the empirical form ⌬E des ‡ ϭaϩbN ␥ , with the exponent having a value of ␥ϭ0.50Ϯ0.01. More interestingly, we have proposed a mechanism for the desorption process and a model for the energetics and the entropy of the oligomers on the surface that provide an extremely good quantitative fit to the observed chain length dependence of ⌬E des ‡ . ⌬E des ‡ is given by the difference in energy between the gas phase n-alkane and the conformation of the adsorbed n-alkane with the minimum free energy at the desorption temperature. These results reveal that conformational isomerism plays a significant role in determining the desorption kinetics of oligomers from surfaces.

The surfaces of chemically synthesized Au nanoparticles have been modified with d- or l-cysteine to render them chiral and enantioselective for adsorption of chiral molecules. Their enantioselective interaction with chiral compounds has been probed by optical rotation measurements during exposure to enantiomerically pure and racemic propylene oxide. The ability of optical rotation to detect enantiospecific adsorption arises from the fact that the specific rotation of polarized light by (R)- and (S)-propylene oxide is enhanced by interaction with Au nanoparticles. This effect is related to previous observations of enhanced circular dichroism by Au nanoparticles modified by chiral adsorbates. More importantly, chiral Au nanoparticles modified with either d- or l-cysteine selectively adsorb one enantiomer of propylene oxide from a solution of racemic propylene oxide, thus leaving an enantiomeric excess in the solution phase. Au nanoparticles modified with l-cysteine (d-cysteine) selectively adsorb the (R)-propylene oxide ((S)-propylene oxide). A simple model has been developed that allows extraction of the enantiospecific equilibrium constants for (R)- and (S)-propylene oxide adsorption on the chiral Au nanoparticles.

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The dynamics of oligomer desorption from surfaces has been studied by measuring the desorption kinetics of a set of straight chain alkanes [H͑CH 2 ͒ n H, with n 5 to 60] from the surface of single crystalline graphite. Desorption is observed to be a first-order process and the preexponent of the desorption rate constant has a value n 10 19.660.5 sec 21 and is independent of the oligomer chain length. More interestingly, we find that the barrier to desorption has a nonlinear dependence on chain length and takes the form DE This Letter reports the barriers to desorption ͑DE z des ͒ of straight chain alkanes [H͑CH 2 ͒ n H, with n 5 to 60] from the surface of graphite into vacuum and the finding of a nonlinear dependence of DE z des on chain length, n. This nonlinear behavior reveals something of the dynamics of the complex process or mechanism by which oligomeric species desorb from surfaces. Independent of its fundamental interest, this result has implications for a number of technologically important surface phenomena such as the evaporation of thin lubricant films from the surfaces of magnetic data storage disks and the desorption of alkanes from the surfaces of Fischer-Tropsch catalysts.The vast majority of measurements of molecular desorption kinetics from surfaces have used relatively small species for which the desorption process is considered simply as a displacement along the surface normal. This desorption mechanism can be adequately modeled using a single well-defined reaction coordinate through a fairly simple potential energy surface describing the interaction of the molecule with the surface. The desorption rate constant is usually considered to have the form given by the transition state theory [1]where h is Planck's constant and k B is Boltzmann's constant. The DE z des is the difference between the zero-point energies of the adsorbed state and the transition state for desorption. At best the multidimensional nature of the adsorbate-substrate potential energy surface influences the desorption kinetics through the partition functions for the adsorbed state ͑q͒ and the transition state for desorption ͑q z ͒.Consider the desorption of an oligomeric species such as H͑CH 2 ͒ 60 H from a surface. Many scanning tunneling micrographs (STM) of alkanes adsorbed on graphite at room temperature reveal an all-trans conformation stretched out and interacting with the surface along its length [2][3][4][5]. Since the desorbed state has no segments interacting with the surface one might predict that the DE z des should be linear in the chain length. There have been several prior sets of measurements which observe the effects of alkyl chain length on the desorption kinetics of species such as alkyl alcohols and simple alkanes adsorbed on metal surfaces [6][7][8][9]. In these cases the range of alkyl chain lengths has been limited to n # 12. Needless to say, over this limited range the measured values of DE