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-Aminopurine (2AP) is a structural isomer of adenine (A), in which the amino group is at C2 instead of C6, and can form stable Watson-Crick (WC) type base pairs with thymine (T) (Figure 1). [1,2] While natural nucleobases do not emit at all, 2AP shows appreciable fluorescence. More importantly, its fluorescence quantum yield decreases 100-fold upon duplex formation. [3] Numerous studies exploit 2AP fluorescence to investigate problems in structural biology and biophysics: methyltransferase-induced base flipping, [4][5][6] conformational changes and enzymatic cleavage of the hammerhead ribozyme, [7,8] promoter binding and clearance of T7 RNA polymerase, [9,10] binding and strand separation of primertemplate DNA by T4 DNA polymerase, [11][12][13] and chargetransfer mechanisms in DNA coupled to polar solvation. [14][15][16] Alternatively, structural changes can be monitored by a lowenergy circular dichroism band observed with 2AP, as was demonstrated with RNA and DNA hairpin loops. [17,18] The high number of publications relating to 2AP reflects its importance in studies of biological macromolecules.When structural transitions in biological systems are examined with a molecular probe, the assumption is that the modified system behaves like the natural one. Consequently the introduction of the probe must leave the structure and dynamics unchanged. Fluorophore-induced perturbations have been analyzed by solution NMR spectroscopy in many cases. [19][20][21][22][23] With 2AP, however, the changes are so small that the results in the original studies were inconclusive. [24,25] With high-field spectrometers and an expanded set of NMR parameters at hand we can now investigate 2AP-induced changes in detail.We present herein the NMR solution structure and basepair dynamics of two 13-mer DNA duplexes (Figure 1) with X = A in the reference sequence and X = 2AP in the modified sample (in the following called 13merRef and 13mer2AP, respectively). The only change introduced into the helix is the position of the amino group in A and 2AP. To what extent are structure and dynamics affected by this change? To answer this question we employed 2D NMR spectroscopy and measurements of residual dipolar couplings (RDC) in conjunction with simulated annealing calculations to determine the solution structure, selective NMR T 1 experiments to evaluate base-pair dynamics, and temperature-dependent absorption and fluorescence spectroscopy to characterize local melting. By combining information from these different approaches the effect of a single substitution A!2AP can be evaluated.All NMR resonances except for the severely overlapped H5'/H5'' signals could be assigned by intra-and internucleotide NOEs.[26] WC base-pairing of 2AP is evidenced by the imino proton signal of T20 which is observed-though broadened-for 13mer2AP at 298 K. [26] In contrast to an earlier report, [24] all cross peaks expected for regular B-DNA are present in the NOESY spectra of both samples. However, for the diagonal imino proton signal for T20, fast decay with increa...

Four flavonol glycosides were isolated from an extract of sea buckthorn pomace (Hippophaë rhamnoides) by Sephadex LH-20 gel chromatography and semipreparative HPLC. Their structures were elucidated by hydrolysis studies, ESI-MS(n), UV, and (1)H and (13)C NMR spectroscopy. The occurrence of the major flavonol glycoside kaempferol 3-O-beta-sophoroside-7-O-alpha-rhamnoside in sea buckthorn is described here for the first time. A further 21 flavonol glycosides of Sephadex LH-20 fractions of sea buckthorn pomace were characterized by HPLC-DAD-ESI-MS. The characteristic MS-MS and MS(3) fragmentation pattern of flavonol glycosides previously identified in sea buckthorn juice and of flavonol glycosides identified by NMR spectroscopy gave valuable indications for their identification. The results demonstrate that loss of the sugar moiety from C-7 of the aglycon is more favored than fission of the glycosidic linkage at the C-3 position. Thus, most of the compounds identified were 7-rhamnosides of isorhamnetin, kaempferol, and quercetin, which exhibit different substitution patterns at the C-3 position, mainly glucosides, rutinosides, and sophorosides. In addition, numerous flavonol glycosides were detected lacking a sugar moiety at C-7. Finally, eight flavonol derivatives were identified that are acylated by hydroxybenzoic or hydoxycinnamic acids.

The strict exclusion of tertiary amines during the otherwise standard synthetic approach to the synthesis of the guanidinium‐type peptide coupling reagents HBTU (X = CH, see scheme) and HATU (X = N) allows the synthesis of the uronium‐type isomers of these guanidinium compounds, whose existence was until now pure speculation. These O isomers (O‐HBTU and O‐HATU) are more efficient coupling reagents than the N isomers.

Blending in: A triazole‐modified DNA duplex is perturbed in structure and dynamics, but this is delocalized over five base pairs. The conformation remains B‐DNA and hydrogen bonds between the DNA phosphate oxygen and polymerases can be mimicked by the triazole nitrogen (see figure). The results explain the surprising biocompatibility of triazole‐linked DNA.

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A new cycloartane glycoside (1) was obtained from a minor triterpene fraction of the rhizome extract of Actaea racemosa (synonym: Cimicifuga racemosa) along with a known compound, cimigenol 3-O-beta-D-xylopyranoside. The structure of 1 was elucidated as 20(S),22(R),23(R),24(S)-12beta-acetoxy-16beta:23,23alpha:24-diepoxy-3beta,22beta,25-trihydroxy-9,19-cyclolanost-7-ene 3-O-beta-D-xylopyranoside (actaeaepoxide 3-O-beta-D-xylopyranoside) on the basis of spectral and chemical evidence.

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