Microlensing surveys have proven to be tremendously fruitful in providing valuable data products for many fields of astrophysics, from eclipse lightcurves for substellar candidates to limb darkening in stellar atmospheres. We report on a program of modeling observables from microlensing of circumstellar envelopes, particularly those of red giant stars that are the most likely to show finite source effects. We will summarize work for how polarization light curves can be used to infer envelope properties and will describe recent modeling of the time dependent spectral energy distributions (SEDs) for microlensing of dusty winds. One of the most exciting developments is the possibility of measuring variable polarization from microlensing in a suitable source using the RINGO polarimeter at La Palma. Also quite interesting is the possibility of probing a dusty wind using IRAC data for a suitable source in the event that Spitzer has a “warm” cycle.

In the present work the vertical and adiabatic ionization energy thresholds (IET) of adenine, and its amino and imino tautomers complexed with 1-3 water molecules are presented. The vertical and adiabatic IETs have been calculated at the B3LYP and P3 levels of theory, using the standard 6-31++G(d,p) basis set. The results show that there is hardly any effect of microhydration on the vertical ΔIET of adenine, which is at odds with the experimental values determined by Kim et al, (J. Phys. Chem. 1996, 100, 7933). In an attempt to assign the experimental ΔIET values, calculations have been performed on the microhydrated amino and imino tautomers of adenine. Vertical ΔIET calculations and adiabatic ΔIET calculations on adenine N7H tautomers complexed with water are in better agreement with the experimental results than are calculations involving the canonical (N9H) form of adenine.

The vertical ionization energies of the four nucleotides have been computed. Geometries have been chosen to mimic orientations as they appear in B-DNA. The negative charge on the phosphate was neutralized by protonation, and also by the inclusion of counterions. Calculations have been performed with electron propagator methods (P3), Møller-Plesset second-order perturbation theory, and density functional theory to determine the nature of the orbitais associated with the highest lying ionization energies. Calculations at the MP2/6-311G(d,p)//P3/6-311G(d,p) level of theory yield vertical ionization energies for 5′-dTMP 9.05 eV, for 5′-dCMP 8.40 eV, for 5′-dAMP 8.16 eV and for 5′-dGMP 7.96 eV. In all cases the highest occupied molecular orbital resides on the base moieties.

A recent paper by Hou et al. (Hou, R.; Gu, J.; Xie, Y.; Yi, X.; Schaefer, H. F. J. Phys. Chem. B 2005, 109, 22053) on 2′-deoxyadenosine-5′- phosphate (5′-dAMP) reports calculations on one-electron oxidation of the 5′-dAMP anion. The paper presents a very interesting observation that, for the radical produced by electron removal, the unpaired spin density resides on both the phosphate and the adenine base moieties. There are also indications that this radical has a weakened C5′-O5′ bond, and it is said that this may be the origin of a single-strand break in DNA. New calculations have been performed to show that the spin density on the phosphate is dependent on the charge on the phosphate. The use of the B3LYP method with the 6-31G(d) basis set yields results very similar to those obtained with the much larger B3LYP/DZP++ basis set in computing the structures of one electron oxidized 5′-dAMP. New calculations on the isotropic hyperfine couplings in 5′-dAMP are presented to show under just what conditions one might expect to see small amounts of unpaired spin density on the phosphates. Results show that this may occur in gas-phase studies of nucleotides but, most likely, not in DNA.

In a recent article we reported calculations of the ionization energy thresholds (IET) of microhydrated thymine (Close; et al. J. Phys. Chem. A, 2006, 110, 7485). Calculations showed a distinct effect of microhydration on the IET's of thymine. The first water molecule was seen to decrease the IET by about 0.1 eV, and the second and third water molecules caused a further decrease of less than 0.1 eV each. These changes in IET calculated for the canonical form of thymine with 1-3 waters of hydration are smaller than the experimental values determined by Kim et al. (J. Phys. Chem. C 1996, 100, 7933). In the present study it has been shown that there is considerable reorientation of the water molecules in microhydrated thymine upon ionization. This leads to the expectation that the experimental ionization energies may therefore represent an adiabatic process. The results presented here show that the changes in experimental ionization energies determined by Kim et al. for microhydrated thymine are in good agreement with the calculated adiabatic ionization energies.

The equilibrium geometries of the exohedral and endohedral complexes of the polyhedral oligomeric silsesquioxane (POSS) cage (HSiO3/2) 8 containing the transition metal atoms or ions Sc0,+, Cr0,+, Fe0,+, Co0,+, Ni0,+, Cu 0,+, Zn0,+, Mo0,+, W0,+, Ru 0,+, Os0,+ have been investigated at the B3LYP/LanL2DZ levels. All these species form endohedral complexes with the T8-POSS cage except Sc0,+, Mo0, and W0,+. The Mo 0 and W0,+ species as well as Cr0,+, Fe 0,+, Co0,+, Ni0,+, Cu+, Zn +, Ru0,+, Os0° form stable exohedral complexes. Geometries, electronic properties and ionization potentials were computed. The Si-O and Si-H bond lengths in the cationic endohedral complexes are shorter than in the corresponding complexes of neutral transition metal atoms. The zero-point corrected inclusion energies of the endohedral species X(SiHO3/2)8 (X = Fe+, Co+, Ni +, Cu+, Os+) are all negative, suggesting that these complexes are more stable than their isolated components. All exohedral complexes have energies that are lower than their corresponding endohedral analogs. Transition metal atom encapsulation raised the HOMO and lowered the LUMO energies, reducing the HOMO-LUMO gaps of every complex compared to that of the pure cage. The HOMO-LUMO gap of the empty cage is 8.1 eV while the endohedral complexes exhibit gaps between 1.2 and 4.96 eV. Insertion of Cr, Fe, Co, Ni, Cu, or Zn into the POSS cage is more favorable in water than in the gas phase. However, insertion of Co+, Ni+, Cu+, or Zn+ into the POSS cage is less favorable in water than in the gas phase. Overall, both the neutral and ionic endohedral transition metal complexes, X(SiHO3/2)io, (X = Cr0,+, Fe0,+, Co0,+, Ni0,+, Cu0,+, Zn0,+, Ru 0,+, Os0,+) appear to be viable synthetic targets.

Metallofullerenes of composition GdxM3-xN@C 80 with 0 ≤ x ≤ 3 and M = Sc, Sm, Lu, were investigated with respect to their geometric, electronic, energetic, and magnetic properties by Density Functional Theory, using the generalized gradient approximation in combination with an on-site correlation approach. Equilibrium structures were identified for both the free GdxM3-xN (0 ≤ x ≤ 3, M = Sc, Sm, Lu) units and the composite of the trimetallic nitride core and the fullerene cage. Although the core clusters generally tend toward planarity upon encapsulation into C80, some species containing Sm atoms show a reversal of this trend. The electronic interaction between the cage and the core is characterized by the formation of a covalent region between both components. This effect proves to be strongest in those cases where no core rotation has been detected by experiment. Ferromagnetic ordering is favored by the free trimetallic nitride clusters involving Gd and Sm constituents. For Gd xM3-xN@C80 (0 ≤ x ≤ 3 and M = Sc, Sm, Lu), the ferromagnetically and antiferromagnetically ordered isomers are seen to be near-degenerate. A slight preference for the former alternative is found for Sm3N @C80 while complexes containing Gd tend toward the latter.

Redox potentials for the DNA nucleobases and nucleosides, various relevant nucleoside analogues, Watson-Crick base pairs, and seven organic dyes are presented based on DFT/B3LYP/6-31++G(d,p) and B3YLP/ 6-311+G(2df,p)//B3LYP/6- 31+G* levels of calculations. The values are determined from an experimentally calibrated set of equations that correlate the vertical ionization (electron affinity) energy of 20 organic molecules with their experimental reversible oxidation (reduction) potential. Our results are in good agreement with those estimated experimentally for the DNA nucleosides in acetonitrile solutions (Seidel et al. J. Phys. Chem. 1996, 100, 5541). We have found that nucleosides with anti conformation exhibit lower oxidation potentials than the corresponding syn conformers. The lowering in the oxidation potential is due to the formation of an intramolecular hydrogen bonding interaction between the 5′-OH group of the sugar and the N(3) of the purine bases or C(2)=O of the pyrimidine bases in the syn conformation. Pairing of adenine or guanine with its complementary pyrimidine base decreases its oxidation potential by 0.15 or 0.28 V, respectively. The calculated energy difference between the oxidation potential for the G-C base pair and that of the guanine base is in good agreement with the experimental value estimated recently (0.34 V: Caruso, T.; et al. J. Am. Chem. Soc. 2005, 127, 15040). The complete and consistent set of reversible redox values determined in this work for the DNA constituents is expected to be of considerable value to those studying charge and electronic energy transfer in DNA.

Radioprotection of DNA from direct-type radiation damage by histones has been studied in model systems using complexes of positively charged polypeptides (PCPs) with DNA. PCPs bind to DNA via ionic interactions mimicking the mode of DNA-histone binding. Direct radiation damage to DNA in films of DNA-PCP complexes was quantified as unaltered base release, which correlates closely with DNA strand breaks. All types of PCPs tested protected DNA from radiation, with the maximum radioprotection being approximately 2.5-fold compared with non-complexed DNA. Conformational changes of the DNA induced by PCPs or repair of free radical damage on the DNA sugar moiety by PCPs are considered the most feasible mechanisms of radioprotection of DNA. The degree of radioprotection of DNA by polylysine (PL) increased dramatically on going from pure DNA to a molar ratio of PL monomer:DNA nucleotide ∼1:2, while a further increase in the PL:DNA ratio did not offer more radioprotection. This concentration dependence is in agreement with the model of PCP binding to DNA that assumes preferential binding of positively charged side groups to DNA phosphates in the minor groove, so that the maximum occupancy of all minor-groove PCP binding sites is at a molar ratio of PCP:DNA = 1:2.

Single crystals of the 1:1 complex of the nucleic acid base cytosine and the dipeptide N-formylglycine (C· NFG) have been irradiated at 10 and 273 K to doses of about 70 kGy and studied at temperatures between 10 and 293 K using 24 GHz (K-band) and 9.5 GHz (X-band) electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) spectroscopy. In this complex, the cytosine base is hydrogen bonded at positions N3 and N4 to the carboxylic group of the dipeptide, and the N3 position of cytosine has become protonated by the carboxylic group. At 10 K, two major radicals were characterized and identified. One of these (R1) is ascribed to the decarboxylated N-formylglycine one-electron oxidized species. The other (R2) is the N3-protonated cytosine one-electron reduced species. A third minority species (R3) appears to be a different conformation or protonation state of the one-electron reduced cytosine radical. Upon warming, the R2 and R3 radicals decay at about 100 K, and at 295 K, the only cytosine-centered radicals present are the C5 and C6 H-addition radicals (R5, R6). The R1 radical decays at about 150 K, and a glycine backbone radical (R4) grows in slowly. Thus, in the complex, a complete separation of initial oxidation and reduction events occurs, with oxidation localized at the dipeptide moiety, whereas reduction occurs at the nucleic acid base moiety. DFT calculations indicate that this separation is driven by large differences in electron affinities and ionization potentials between the two constituents of the complex. Once the initial oxidation and reduction products are trapped, no further electron transfer between the two constituents of the complex takes place.