Sodium-ion binding to DNA: detection by ultrafast time-resolved stokes-shift spectroscopy

The nature of the counterion is shown to have a pronounced effect on the apparent dynamic polarity in the interior of DNA. Time-resolved Stokes-shift measurements in the 80 ps to 40 ns time range were made on a polarity-sensitive fluorophore (coumarin 102) that replaces a base pair in an oligonucleotide. With sodium counterions, the emission spectrum narrows with time, whereas with tetrabutylammonium counterions, it does not. Our interpretation is that a subpopulation of helices have sodium cations bound in a fashion that slows the normal dynamics.     

Limit lemmas and jump inversion in the enumeration degrees

We show that there is a limit lemma for enumeration reducibility to 0_e', analogous to the Shoenfield Limit Lemma in the Turing degrees, which relativises for total enumeration degrees. Using this and `good approximations' we prove a jump inversion result: for any set W with a good approximation and any set X<_eW such that W_eX' there is a set A such that X_eA<_eW and A'=W'. (All jumps are enumeration degree jumps.) The degrees of sets with good approximations include the ⁰₂ degrees and the n-CEA degrees. The results in this paper form part of the author's doctoral dissertation written under the supervision of Prof. Steffen Lempp at the University of Wisconsin Madison. The author is grateful to an anonymous referee for helpful comments and suggestions.     

Phosphoryl-substituted 1-azadienes for use in intra-molecular diels-alder cyclizations

The preparation and isolation of 2-(diethoxyphosphoryl)-1-azadienes and their use in the formation of functionalized quinolizidine and indolizidine ring systems via an intramolecular Diels-Alder reaction is described.     

Propargyl radical: ab initio anharmonic modes and the polarized infrared absorption spectra of matrix-isolated HCCCH2

The propargyl radical has twelve fundamental vibrational modes, gamma(vib)(HCCCH2) = 5a1 [symbol: see text] 3b1 [symbol: see text] 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HC=:C-:CH2 matrix frequencies (cm(-1)) and polarizations are a1 modes--3308.5 +/- 0.5, 3028.3 +/- 0.6, 1935.4 +/- 0.4, 1440.4 +/- 0.5, 1061.6 +/- 0.8; b1 modes--686.6 +/- 0.4, 483.6 +/- 0.5; b2 modes--1016.7 +/- 0.4, 620 +/- 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HC=:C-:CH2 2 X (2)B1. From an analysis of the vibrational spectra, the small electric dipole moment, mu(D)(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol(-1), we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HC=:C-:CH2.     

Scandium(III) catalysis of transimination reactions. Independent and constitutionally coupled reversible processes

Sc(OTf)(3) efficiently catalyzes the self-sufficient transimination reaction between various types of C=N bonds in organic solvents, with turnover frequencies up to 3600 h(-)(1) and rate accelerations up to 6 x 10(5). The mechanism of the crossover reaction in mixtures of amines and imines is studied, comparing parallel individual reactions with coupled equilibria. The intrinsic kinetic parameters for isolated reactions cannot simply be added up when several components are mixed, and the behavior of the system agrees with the presence of a unique mediator that constitutes the core of a network of competing reactions. In mixed systems, every single amine or imine competes for the same central hub, in accordance with their binding affinity for the catalyst metal ion center. More generally, the study extends the basic principles of constitutional dynamic chemistry to interconnected chemical transformations and provides a step toward dynamic systems of increasing complexity.     

The design of spin-lattice relaxation experiments

The method of determining spin-lattice relaxation time (T ₁) of liquids by NMR is considered from two aspects: the nature of the pulse sequence and the choice of the experimental independent variable. It is shown that prediction analysis may be used to select the range, distribution, and number of measurements which minimize the amount of time required for the determination ofT ₁ to a given precision.A preliminary report of this work was presented at the 3rd National NMR Conference, Brisbane, Queensland, May 1981.     

Tuning the photophysical behavior of luminescent cyclam derivatives by cation binding and excited state redox potential

The emission from two photoactive 14-membered macrocyclic ligands, 6-((naphthalen-1-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L1) and 6-((anthracen-9-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L2) is strongly quenched by a photoinduced electron transfer (PET) mechanism involving amine lone pairs as electron donors. Time-correlated single photon counting (TCSPC), multiplex transient grating (TG), and fluorescence upconversion (FU) measurements were performed to characterize this quenching mechanism. Upon complexation with the redox inactive metal ion, Zn(II), the emission of the ligands is dramatically altered, with a significant increase in the fluorescence quantum yields due to coordination-induced deactivation of the macrocyclic amine lone pair electron donors. For [ZnL2]2+, the substituted exocyclic amine nitrogen, which is not coordinated to the metal ion, does not quench the fluorescence due to an inductive effect of the proximal divalent metal ion that raises the ionization potential. However, for [ZnL1]2+, the naphthalene chromophore is a sufficiently strong excited-state oxidant for PET quenching to occur.     

The binding of free oligopeptides to cyclodextrins: The role of the tyrosine group

The formation of-cyclodextrin (-CD) and,-cyclodextrin (-CD) inclusion complexes with free tyrosine and the tyrosine residues within two free oligopeptides were investigated using steady-state fluorescence spectroscopy. The oligopeptides consist of five amino acids (pentapeptide) and the tyrosine residues are located at then-termini. The two peptides used in this study have well-known biological functions and are known to bind selectively to specific cell receptors. Cyclodextrins were used to model this receptor-peptide (protein-ligand) interaction. Equilibrium binding constants and the enthalpy and entropy of binding were recovered. Molecular size of the tyrosine-containing species and pH (7.0 vs. 10.0) were found to have little affect on-CD binding. However, tyrosine binding to-CD was dependent on the size (free tyrosine vs. peptide), structure, and pentapeptide conformation.