Tire tread particles turn streams toxic: For coho salmon in the U.S. Pacific Northwest, returning to spawn in urban and suburban streams can be deadly. Regular acute mortality events are tied, in particular, to stormwater runoff, but the identity of the causative toxicant(s) has not been known. Starting from leachate from new and aged tire tread wear particles, Tian et al. followed toxic fractions through chromatography steps, eventually isolating a single molecule that could induce acute toxicity at threshold concentrations of ∼1 microgram per liter. The compound, called 6PPD-quinone, is an oxidation product of an additive intended to prevent damage to tire rubber from ozone. Measurements from road runoff and immediate receiving waters show concentrations of 6PPD-quinone high enough to account for the acute toxicity events. Science , this issue p. 185

Molecular imaging requires the specific accumulation of contrast agents at the target. To exploit the superb resolution of MRI for applications in molecular imaging, gadolinium chelates, as the MRI contrast agents (CA), have to be conjugated to a specific vector able to recognize the epitope of interest. Several Gd(III)-chelates can be chemically linked to the same binding vector in order to deliver multiple copies of the CA (multimers) in a single targeting event thus increasing the sensitivity of the molecular probe. Herein three novel bifunctional agents, carrying one functional group for the bioconjugation to targeting vectors and four Gd(III)-AAZTA chelates functions for MRI contrast enhancement (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) are reported. The relaxivity in the tetrameric derivatives is 16.4±0.2 mMGd-1 s-1 at 21.5 MHz and 25°C, being 2.4–fold higher than that of parent, monomeric Gd(III)-AAZTA. These compounds can be used as versatile building blocks to insert pre-formed, high relaxivity and high density Gd-centres to biological targeting vectors. As an example, we describe the use of these bifunctional Gd(III)-chelates to label a fibrin-targeting peptide.