Cisplatin forms the cis-Pt(NH3)2(d(GpG)) intrastrand cross-link with DNA. Recently our experiments showed that the phosphodiester backbone can have a normal (1) or opposite (2) backbone propagation direction, leading to four conformer classes, HH1, HH2, ΔHT1, and ΛHT2, with the bases in either a head-to-head or head-to-tail orientation. In addition, since each G residue can be syn or anti and the base canting can be left (L) or right (R) handed, 32 variants of this cross-link are conceivable. Reported evidence supported the existence of only the two anti,anti HH1 variants, L in single strands and in cis-Pt(NH3)2(d(GpG)) and R in duplexes and in the ribo analogue, cis-Pt(NH3)2(GpG); in this regard, the latter is an excellent simple model of the DNA lesion. To test such interpretations, we used retro-model adducts (complexes with carrier ligands designed to slow dynamic motion in the d(GpG) cross-link). In retro-model d(GpG) adducts, anti,syn ΔHT1 L (5‘-G anti and 3‘-G syn) and anti,anti HH2 R variants have energy comparable to the previously known anti,anti HH1 variants; our work has led to the hypothesis that cis-Pt(NH3)2 adducts may actually be mixtures of conformers exchanging rapidly on the NMR time scale (Marzilli et al. J. Am. Chem. Soc. 1999, 121, 9133−9142). To test this hypothesis, we have now conducted NMR and CD spectroscopic studies of GpG adducts. Retro models containing the Bip (2, 2‘-bipiperidine) carrier ligand in two enantiomeric forms, (R,S,S,R)-Bip and (S,R,R,S)-Bip (N, C, C, and N chelate ring atoms having the respective R or S configurations), control, respectively, the R and L base canting direction. For low pH (both G N1H's still protonated), (R,S,S,R)-BipPt(GpG) is almost entirely anti,anti HH1 R, but (S,R,R,S)-BipPt(GpG) is a mixture of anti,anti HH1 L and anti,syn ΔHT1 L forms, both new low pH forms for a GpG adduct. This HT variant grew to dominance after ∼3 d at pH ∼10 (both G N1's deprotonated). By pH jump experiments, we obtained NMR and deconvoluted CD spectra of both L variants of (S,R,R,S)-BipPt(GpG) at low and high pH. Spectral features of these L variants are present in cis-Pt(NH3)2(GpG) spectra, suggesting that the anti,anti HH1 R variant is not exclusively present but that ∼30% of other variants are present; N1H deprotonation alters the distribution of forms as found also for retro models. The results suggest that the spectroscopic and structural properties for retro models are directly relevant to cisplatin adducts.

Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.

NMR and CD spectroscopy and molecular mechanics and dynamics (MMD) calculations were used to characterize (Me2DAP)Pt(G)2 complexes (G = N9-substituted guanine derivative; Me2DAP = 2,4-bis(methylamino)pentane with N, C, C, and N stereochemistries of S,R,S,R, S,R,R,R, and R,R,R,R). NMR and MMD results indicated that the favored Me2DAP chelate ring conformations were chair. There are two possible head-to-tail rotamers (ΔHT and ΛHT) and, depending on the Me2DAP stereochemistry, one or two head-to-head (HH) rotamers. Rotation of the G bases around the Pt−N7 bond was found to be rapid on the NMR time scale for all compounds in D2O at room temperature; in contrast, slow rotation was reported for (Me2DAB)Pt(G)2 (Me2DAB = 2,3-bis(methylamino)butane by Xu et al.) Because of the additional flexibility of the six-membered chelate ring in the Me2DAP systems versus the five-membered ring in Me2DAB, the N-methyl groups of Me2DAP can occupy more pronounced axial positions, allowing a low-energy path to rotation as suggested by MMD calculations. The fast rotation necessitated that the rotamer preference be assessed by CD spectroscopy. For the S,R,S,R complexes, the G N9 substituent strongly influenced which HT rotamer was preferred. The ΛHT rotamer was favored for the R,R,R,R complexes at pH 3 regardless of the G used. MMD calculations on the [(R,R,R,R)-(Me2DAP)Pt(9-EtG)2]2+ complex suggested amine-O6 hydrogen bonding in the ΛHT rotamer and indicated an unfavorable nonbonded interaction between the G O6 and an axial cis C-methyl group in the ΔHT rotamer. This nonbonded interaction was also observed in calculated structures of the [(S,R,R,R)-(Me2DAP)Pt(9-EtG)2]2+ complex, for which experimental data showed a preference for the ΛHT rotamer as well. Thus, the orientations of the N- and C-methyl groups appear to be important in determining both the rate of rotation and the rotamer preference of the (Me2DAP)Pt(G)2 systems.

Calcium (Ca) is an essential element for almost all living organisms. Here, we examined global variation and controls of freshwater Ca concentrations, using 440 599 water samples from 43 184 inland water sites in 57 countries. We found that the global median Ca concentration was 4.0 mg L −1 with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L −1 , a threshold considered critical for the survival of many Ca-demanding organisms. Spatially, freshwater Ca concentrations were strongly and proportionally linked to carbonate alkalinity, with the highest Ca and carbonate alkalinity in waters with a pH around 8.0 and decreasing in concentrations towards lower pH. However, on a temporal scale, by analyzing decadal trends in >200 water bodies since the 1980s, we observed a frequent decoupling between carbonate alkalinity and Ca concentrations, which we attributed mainly to the influence of anthropogenic acid deposition. As acid deposition has been ameliorated, in many freshwaters carbonate alkalinity concentrations have increased or remained constant, while Ca concentrations have rapidly declined towards or even below pre-industrial conditions as a consequence of recovery from anthropogenic acidification. Thus, a paradoxical outcome of the successful remediation of acid deposition is a globally widespread freshwater Ca concentration decline towards critically low levels for many aquatic organisms.

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A tetracationic supramolecular helicate, [Fe2L3]4+ (L = C25H20N4), with a triple-helical architecture is found to induce the formation of a three-way junction (3WJ) of deoxyribonucleotides with the helicate located in the center of the junction. NMR spectroscopic studies of the interaction between the M enantiomer of the helicate and two different oligonucleotides, [5'-d(TATGGTACCATA)]2 and [5'-d(CGTACG)]2, show that, in each case, the 2-fold symmetry of the helicate is lifted, while the 3-fold symmetry around the helicate axis is retained. The 1:3 helicate/DNA stoichiometry estimated from 1D NMR spectra supports a molecular model of a three-way junction composed of three strands. Three separate double-helical arms of the three-way junction are chemically identical giving rise to one set of proton resonances. The NOE contacts between the helicate and DNA unambiguously show that the helicate is fitted into the center of the three-way junction experiencing a hydrophobic 3-fold symmetric environment. Close stacking interactions between the ligand phenyl groups and the nucleotide bases are demonstrated through unusually large downfield shifts (1-2 ppm) of the phenyl protons. The unprecedented 3WJ arrangement observed in solution has also been found to exist in the crystal structure of the helicate adduct of [d(CGTACG)2] (Angew. Chem., Int. Ed. 2006, 45, 1227).

was mainly inhabited by ultramicrobacteria related to the LD12-lineage of Alphaproteobacteria and to Actinobacteria; the latter group preferred the shallow regions. Cytophaga-Flavobacteria, in particular a population related to Fluviicola sp., were more frequent in and below the layer of maximal P. rubescens abundances. Betaproteobacteria on the other hand were highly abundant in the epi-and hypolimnion, but not in the P. rubescens layer. Four betaproteobacterial subpopulations with contrasting longitudinal and/or vertical habitat preferences were distinguished: Putatively methylotrophic bacteria of the LD28 lineage (beta IV) preferentially inhabited the hypolimnion, Polynucleobacter acidiphobus was found throughout the epilimnion, Limnohabitans (R-BT065) more in the shallow regions of the lake, and Polynucleobacter necessarius ssp. asymbioticus only in hypoxic waters. Our results stress the potential importance of spatial niche differentiation in freshwater bacterioplankton. This variability should be taken into account, e.g., in studies of seasonal community changes derived from single sampling locations and depths.