C‐Nucleosides are characterized by a C−C rather than a C−N linkage between the heterocyclic base and the ribofuranose ring. While the biosynthesis of pseudouridine‐C‐nucleosides has been studied, less is known about the pyrazole‐C‐nucleosides such as the formycins and pyrazofurin. Herein, genome screening of Streptomyces candidus NRRL 3601 led to the discovery of the pyrazofurin biosynthetic gene cluster pyf. In vitro characterization of gene product PyfQ demonstrated that it is able to catalyze formation of the C‐glycoside carboxyhydroxypyrazole ribonucleotide (CHPR) from 4‐hydroxy‐1H‐pyrazole‐3,5‐dicarboxylic acid and phosphoribosyl pyrophosphate (PRPP). Similarly, ForT, the PyfQ homologue in the formycin pathway, can catalyze the coupling of 4‐amino‐1H‐pyrazole‐3,5‐dicarboxylic acid and PRPP to form carboxyaminopyrazole ribonucleotide. Finally, PyfP and PyfT are shown to catalyze amidation of CHPR to pyrazofurin 5′‐phosphate thereby establishing the latter stages of both pyrazofurin and formycin biosynthesis.

Showdomycin is aC -nucleoside bearing an electrophilic maleimide base.H erein, the biosynthetic pathwayo f showdomycin is presented. The initial stages of the pathway involve non-ribosomal peptide synthetase (NRPS) mediated assembly of a2 -amino-1H-pyrrole-5-carboxylic acid intermediate.This intermediate is prone to air oxidation whereupon it undergoes oxidative decarboxylation to yield an imine of maleimide,which in turn yields the maleimide upon acidification. It is also shown that this pyrrole intermediate serves as the substrate for the C-glycosidase SdmA in the pathway.A fter coupling with ribose 5-phosphate,t he resulting C-nucleoside undergoes as imilar sequence of oxidation, decarboxylation and deamination to affordshowdomcyin after exposure to air. These results suggest that showdomycin could be an artifact due to aerobic isolation;h owever,t he autoxidation maya lso serve to convert an otherwise inert product of the biosynthetic pathway to an electrophilic C-nucleotide thereby endowing showdomycin with its observed bioactivities. Results and DiscussionTo address these unresolved questions,t he sdm gene cluster was analyzed, and acassette consisting of sdmC, D, E, Figure 1. C-Ribosylnucleoside antibiotics.

Excited‐state relaxation of linear merocyanine dyes in solution is investigated using time‐resolved spectroscopy techniques and quantum chemical calculations. The merocyanine L‐Mero4 and phenyl‐substituted P‐L‐Mero4 have a S‐trans and S‐cis structure, respectively, consisting of indole moiety as the donor, indandione as the acceptor, and the tetramethine as the bridge. The time‐correlated single‐photon counting (TCSPC) picosecond measurements after excitation at wavelength 515 nm to the ππ* state yield emission curves with a short component τ1 in the range of 27–160 ps and a second component τ2 of 200–780 ps for L‐Mero4. In P‐L‐Mero4, τ1 lies in the range of 18–150 ps and τ2 220–520 ps. The subfemtosecond transient absorption measurements yield a short component around 0.4–1.4 ps, and the second/third components are similar to those in the TCPSC measurements. The analysis of the experimental data demonstrates that the ground state recovery exhibits a biexponential rise and rapidly indicates that the conversion back to the electronic ground state provides a fast, nonradiative pathway. Quantum chemical calculations on the electronic structures and their dependence on the molecular confirmation are performed. We identify the excited states and the relaxation path along the twist of the center double bonds in tetramethine that might be the nonradiative pathway. The C=C double bond is weakened in the ππ* state. The phenyl substitution in the conjugated double bond weakens this C=C bond, lowers the isomerization barrier, increases the nonradiative rate, and reduces the emission quantum yield. In polar solvents, the energy of the perpendicular conformer along the trans–cis isomerization path is increased to achieve less coupling to the ground state surface. Because of the small barrier to the trans form, these two conformers establish an equilibrium condition. The trans form, which lies at a lower energy, gains more population and thus has a higher emission yield.