We report the first cocrystal as an intermediate in a solidstate organic reaction wherein molecules of barbituric acid and vanillin assume a favorable orientation for the subsequent Knoevenagel condensation.The Knoevenagel condensation is an important carbon-carbon bond forming reaction. More than a hundred years after the original report by Knoevenagel, 1 Suzuki 2 and Kaupp 3 demonstrated an efficient and quantitative Knoevenagel condensation in the solid state achieved by milling. Other studies of solvent-free Knoevenagel condensation reactions soon followed. 4-10 The reaction of barbituric acid (barb) and vanillin (van) was even used as a model mechanochemical organic reaction for assessing energetics of milling, 11,12 to test twin-screw extrusion for solid-state organic synthesis, 13 and latest, to reveal a peculiar deviation of solid-state reaction kinetics from the one observed in solution, stemming from changes in the rheology of the milled sample. 14 However, studies of barb-van Knoevenagel condensation were thus far limited to ex situ reaction monitoring by, e.g., solution UV-Vis 11 or NMR spectroscopies. 14 In this work, we employ real-time in situ Raman spectroscopy monitoring 15,16 to reveal that the solid-state Knoevenagel condensation (Scheme 1) of barb and van proceeds through a cocrystal intermediate. In the cocrystal, packing of barb and van is such that molecules of barb are suitably positioned for the nucleophilic

International audienceThe effect of metal ions on the excited states of guanine nanostructures, short d(TG4T)4 quadruplexes and long G4-wires, are studied by fluorescence spectroscopy. The steadystate emission spectra show that both systems exhibit a strong cation effect. Fluorescence decays and fluorescence anisotropy decays, recorded from the femtosecond to the nanosecond timescale, reveal the following picture. In the presence of K+, emission arises mainly from delocalized ππ* states (excitons), whose decay spans several decades of times. In contrast, the fluorescence in the presence of Na+ is dominated by emission from charge transfer excited states decaying essentially on the subnanosecond time-scale. Such a difference is not due to the initially populated (Franck−Condon) states. The interproton distances derived from two-dimensional NMR measurements on the ground state of d(TG4T)4 show that the geometrical arrangement of guanines, governing the electronic coupling, is the same for both cations, in line with the UV absorption spectra. The observed cation effect is correlated with the excited state relaxation: the increased mobility of Na+ ions within the quadruplex favors trapping of ππ* excitons by charge transfer excited states, whereas such a process is hindered for the larger K+ ions. This is rationalized by quantum calculations on two stacked guanine tetrads

NMR study has shown that DNA oligonucleotide d(G(3)T(4)G(4)) adopts an asymmetric bimolecular G-quadruplex structure in solution. The structure of d(G(3)T(4)G(4))(2) is composed of three G-quartets, overhanging G11 residue and G3, which is part of the loop. Unique structural feature of d(G(3)T(4)G(4))(2) fold is the orientation of the two loops. Thymidine residues T4-T7 form a diagonal loop, whereas T15-T18 form an edge type loop. The G-quadruplex core of d(G(3)T(4)G(4))(2) consists of two stacked G-quartets with syn-anti-anti-anti alternation of dG residues and one G-quartet with syn-syn-anti-anti alternation. Another unusual structural feature of d(G(3)T(4)G(4))(2) is a leap between G19 and G20 over the middle G-quartet and chain reversal between G19 and G20 residues. The presence of one antiparallel and three parallel strands reveals the hitherto unknown G-quadruplex folding motif consisting of antiparallel/parallel strands and diagonal as well as edge type loops. Further examination of the influence of different monovalent cations on the folding of d(G(3)T(4)G(4)) showed that it forms a bimolecular G-quadruplex in the presence of K+, Na+, and NH4+ ions with the same general fold.

Infection with human papillomaviruses (HPVs) is one of the most common sexually transmitted infections and can lead to development of head and neck, skin, and anogenital cancer, including cervical cancer, which represents one of the world's most significant health problems. In this study, we analyze G-rich regions in all known HPV genomes in order to evaluate their potential to fold into G-quadruplex structure. Interestingly, G-rich loci fulfilling the criteria for G-quadruplex formation were found in only 8 types of HPV. Nevertheless, viral G-quadruplexes in 7 sequences derived directly from HPVs are confirmed here for the first time. G-rich regions with the capacity to form G-quadruplexes are located in the LCR, L2, E1, and E4 regions of the HPV genome; therefore we assume that regulation processes in viruses could be affected by G-quadruplex formation. Our results represent a starting point for the design of specific ligands with viral G-quadruplex motifs and suggest novel methods for the control of viral replication and transcription.