Intrinsic (gas phase) thermodynamic stability of 2-adamantyl cation. Its bearing on the solvolysis rates of 2-adamantyl derivatives

The standard enthalpy of formation of gaseous 2-adamantyl chloride(2-Ad-Cl) was determined by calorimetric techniques. The standard Gibbs energy change for the chloride anion exchange between 1-adamantyl (1-Ad+) and 2-adamantyl (2-Ad+) cations in the gas phase was obtained by Fourier transform ion cyclotron resonance spectroscopy (FT ICR). Theoretical calculations at the G2(MP2) level were performed on these and other relevant species. This and data from the literature provided three highly consistent independent estimates of the relative stabilities of 2-Ad+ and 1-Ad+. This difference in gas-phase stability was compared to the differential structural effects on the rates of solvolysis of the corresponding chlorides and tosylates, and it was shown that the thermodynamic stability of the secondary cation is the leading factor determining the solvolytic reactivity of the precursors in the absence of solvent effects. Thus, under these conditions, the previously established linear free energy correlation between carbenium ion stability and solvolytic reactivity of bridgehead derivatives applies also to secondary derivatives.     

A new method for direct calculation of total energy of protein

A new scheme is developed for efficient quantum mechanical calculation of total energy of protein based on a recently developed MFCC (molecular fractionation with conjugate caps) approach. In this scheme, the linear-scaling MFCC method is first applied to calculate total electron density of protein. The computed electron density is then employed for direct numerical integration in density functional theory (DFT) to yield total energy of protein, with the kinetic energy obtained by a proposed ansatz. Numerical studies are carried out to calculate torsional energies of two polypeptides using this approach and the energies are shown to be in good agreement with the corresponding full system DFT calculation.     

An efficient approach for ab initio energy calculation of biopolymers

We present a new method for efficient total-energy calculation of biopolymers using the density-matrix (DM) scheme based on the molecular fractionation with conjugate caps (MFCC) approach. In this MFCC-DM method, a biopolymer such as a protein is partitioned into properly capped fragments whose density matrices are calculated by conventional ab initio methods which are then assembled to construct the full system density matrix. The assembled full density matrix is then employed to calculate the total energy and dipole moment of the protein using Hartree-Fock or density-functional theory methods. Using this MFCC-DM method, the self-consistent-field procedure for solving the full Hamiltonian problem is avoided and an efficient approach for ab initio energy calculation of biopolymers is achieved. Two implementations of the approach are presented in this paper. Systematic numerical studies are carried out on a series of extended polyglycines CH3CO-(GLY)n-NHCH3(n = 3-25) and excellent results are obtained.     

A new neolignan glycoside from the leaves of Acer truncatum

A new neolignan glycoside, (7R,8R)-7,8-dihydro-9'-hydroxyl-3'-methoxyl- 8-hydroxymethyl-7-(4-hydroxy-3-methoxyphenyl)-1'-benzofuranpropanol 9'-O-beta-D- glucopyranoside (1) was isolated from the leaves of Acer truncatum along with (7R,8R)-7,8-dihydro-9'-hydroxyl-3'-methoxyl-8-hydroxymethyl-7-(4-O-alpha-L-rhamno- pyranosyloxy-3-methoxyphenyl)-1'-benzofuranpropanol (2), schizandriside (3), lyoniresinol (4), berchemol (5), (-)-pinoresinol-4-O-beta-D-glucopyranoside (6), hecogenin (7), chlorogenic acid (8) and neochlorogenic acid (9). Their structures were elucidated on the basis of extensive spectroscopic data. The absolute configuration of compounds 1 was established by its CD spectrum. The antibacterial activities of compounds 1-7 were evaluated.