While the fluorescence decay kinetics of tyrosine model compounds [Laws, W. R., Ross, J. B. A., Wyssbrod, H. R., Beechem, J. M., Brand, L., & Sutherland, J. C. (1986) Biochemistry 25, 599-607] and the tyrosine residue in oxytocin [Ross, J. B. A., Laws, W. R., Buku, A., Sutherland, J. C., & Wyssbrod, H. R. (1986) Biochemistry 25, 607-612] can be explained in terms of heterogeneity derived from the three ground-state chi 1 rotamers, a similar correlation has yet to be directly observed for a tryptophan residue. In addition, the asymmetric indole ring might also lead to heterogeneity from chi 2 rotations. In this paper, the time-resolved and steady-state fluorescence properties of [tryptophan2]oxytocin at pH 3 are presented and compared with 1H NMR results. According to the unrestricted analyses of individual fluorescence decay curves taken as a function of emission wavelength and a global analysis of these decay curves for common emission wavelength-independent decay constants, only three exponential terms are required. In addition, the preexponential weighting factors (amplitudes) have the same relative relationship (weights) as the 1H NMR-determined chi 1 rotamer populations of the indole side chain. 15N was used in heteronuclear coupling experiments to confirm the rotamer assignments. Inclusion of a linked function restricting the decay amplitudes to the chi 1 rotamer populations in the individual decay curve analyses and in the global analysis confirms this correlation. According to qualitative nuclear Overhauser data, there are two chi 2 populations. Depending upon the degree of correlation between chi 2 and chi 1, there may be from three to six side-chain conformations for the tryptophan residue. The combined fluorescence and NMR results are consistent with a rotamer model in which either (i) the chi 2 rotations are fast compared to the fluorescence intensity decay of the tryptophan residue, (ii) environmental factors affecting fluorescence intensity decay properties are dominated by chi 1 interactions, or (iii) the chi 2 and chi 1 rotations are highly correlated.

The interaction of iron(Ill) and uranyl ions with dissolved silica is studied by a spectrophotometric technique. The interaction with iron is studied as a function of the degree of polymerization of the silica. For uranyl ion, a complex with monomeric silica is inferred. An equilibrium constant PQ1-2.0 for the equilibrium UO22+ + Si(OH)4 = UO2SiO(OH)31+ + H ÷ is measured.

The dinuclear copper(II) complexes [Cu(2)(tmihpn)(prz)](ClO(4))(2).2CH(3)CN (6) and [Cu(2)(tmihpn)(O(2)CCH(3))](ClO(4))(2).CH(3)CN (7) were prepared, where tmihpn is the deprotonated form of N,N,N',N'-tetrakis[(1-methylimidazol-2-yl)methyl]-1,3-diaminopropan-2-ol and prz is the pyrazolate anion. The crystal structures of 6 and 7 were determined and revealed that both complexes contain bridging alkoxide ligands as well as bridging pyrazolate and acetate ions, respectively. Crystal data: compound 6, triclinic, P&onemacr;, a = 18.089(2) Å, b = 22.948(3) Å, c = 9.597(2) Å, alpha = 93.37(2) degrees, beta = 94.49(2) degrees, gamma = 81.69(2) degrees, V = 3925.1 Å(3), Z = 4; compound 7, triclinic, P&onemacr;, a = 12.417(2) Å, b = 15.012(3) Å, c = 10.699(2) Å, alpha = 104.76(2) degrees, beta = 102.63(2) degrees, gamma = 99.44(2) degrees, V = 1830.1 Å(3), Z = 2. In compound 6, the coordination geometry around both copper centers resembles a distorted square pyramid, while the stereochemistry around the copper centers in 7 is best described as trigonal bipyramidal. Both complexes display well-resolved isotropically shifted (1)H NMR spectra. Selective substitution studies and integration data have been used to definitively assign several signals to specific ligand protons. Results from the solution (1)H NMR studies suggest that the basal and apical imidazole groups do not exchange rapidly on the NMR time scale and the solid state structures of the complexes are retained in solution. In addition, the magnetochemical characteristics of 6 and 7 were determined and provide evidence for "magnetic orbital switching". Antiferromagnetic coupling in 6 (J = -130 cm(-)(1)) is strong, while the copper centers in compound 7 are ferromagnetically coupled (J = +16.4 cm(-1)). Differences in the magnetic behavior of the two copper centers have been rationalized using the "ligand orbital complementary" concept. The ground state magnetic orbitals involved in spin coupling in 6 (d(x)()()2(-)(y)()()2) are different from those in 7 (d(z)()()2).

The spatial distribution and temporal disappearance of Rohon-Beard cells in the spinal cords of larval and newly metamorphosed Rana pipiens were studied histologically to provide a basis for further research dealing with this particular cell type. It was found that a maximum number of approximately 250 Rohon-Beard neurons have differentiated within the spinal cord of Rana pipiens by larval stage I. The majority of these cells are located in the cephalic end of the spinal cord, with a large number found near the boundary of the anterior quarter and the second quarter of the spinal cord. Fewer cells are found in the third and fourth quarters. There is a gradual degeneration of these cells during larval life so that by stage XXV, which is the completion of the metamorphic climax, there are no normally appearing Rohon-Beard cells present. Degeneration of the Rohon-Beard cells does not progress cephalocaudally with development, but rather there appears to be a nearly equal proportional loss of Rohon-Beard cells in each quarter of the spinal cord at each of the stages studied. All Rohon-Beard cells present during larval development of Rana pipiens are located in a dorsal position close to the midline. At all ages and in each quarter of the spinal cord there are no significant bilateral differences in number of these cells.

Ligand reactivity was demonstrated for sulfide alkylation, thiolate dealkylation, and dithiophosphate de-esterification within molybdenum(V) dimers. The cationic complex [Mo(2)(NC(6)H(4)Me)(2)(S(2)P(OEt)(2))(2)(&mgr;-O(2)CMe)(&mgr;-SR)(2)](+) was inductively activated toward thiolate C-S and dithiophosphate C-O heterolyses. The dealkylations were studied using anionic nucleophiles, and various reactivity patterns were characterized. The de-esterification of the diethyl dithiophosphate ligands produced complexes containing the rare monoester EtO(O)PS(2)(2)(-) ligand. This ligand's phosphoryl group was poorly nucleophilic but weakly basic. Crystallographic comparisons between the activated cation and the neutral complex [Mo(2)(NC(6)H(4)Me)(2)(S(2)P(OEt)(2))(2)(&mgr;-O(2)CMe)(&mgr;-S)(&mgr;-SMe)] were conducted to delineate structural differences related to the activation. A crystallographic study was also done of the complex [Mo(2)(NC(6)H(4)CH(3))(2)(S(2)P(OEt)(2))(S(2)P(O)OEt)(&mgr;-O(2)CMe)(&mgr;-SEt)(2)], which provided internal comparison of monoester EtO(O)PS(2)(2)(-) and diester (EtO)(2)PS(2)(-) ligand types.

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The extraction of water by several crown ethers into chloroform + carbon tetrachloride mixtures has been investigated using a proton NMR technique. The equilibrium is well described by formation of a 1:1 water−crown complex in rapid exchange with uncomplexed ligand and water. The fraction (k) of crown ether complexed with water increases with crown cavity size, varying from (15 ±1)% for 12-crown-4 to (97 ± 5)% for 18-crown-6. Addition of carbon tetrachloride to chloroform lowers the k value for all crown ethers in equilibrium with water, and the value is close to zero in pure CCl4. The partition coefficient follows the opposite trend:  the amount of crown ether in the organic phase increases with the percentage of CCl4 in this phase. The chemical shifts of free and complexed water also vary with solvent composition. Interaction of water with crown ether depends on solvation environment and may play a significant role in liquid−liquid extraction of metal ions using macrocyclic polyethers as extractants.

Specific synthesis of some oligoadenylates including A2'p5'A2'p5'Ap(2'), the 2'-phosphorylated oligoribonucleotide core of the recently discovered protein synthesis inhibitor pppA2'p5'A2'p5'A is described using a novel solid-phase method. The CD spectra of A2'p5'Ap(2'), A2'p5'A2'p5'Ap(2') and A2'p5'A2'p5'A (derived by treatment of the phosphorylated synthetic trimer with E. coli alkaline phosphatase) are presented. Comparison of the latter spectrum with that of A2'p5'A2'p5'A obtained similarly from a biologically derived sample of pppA2'p5'A2'p5'A provides further evidence that this molecule is in fact the first naturally-occurring 2'-5'-linked oligoribonucleotide.