The EtOAc extract of a Taiwanese soft coral, Caldiella australis, yielded four new eunicellin-based diterpenoids, australins A-D (1-4). The chemical structures of these metabolites were determined on the basis of extensive spectroscopic (including 1D and 2D NMR) analyses and by comparison of their NMR spectral data with those of related metabolites. Metabolite 2 was found to possess a moderate cytotoxic activity against selected breast and liver cancer cell lines.
We present an alternative scheme for determining the frequencies of cesium (Cs) atom 6S-8S Doppler-free transitions. With the use of a single electro-optical crystal, we simultaneously narrow the laser linewidth, lock the laser frequency, and resolve a narrow spectrum point by point. The error budget for this scheme is presented, and we prove that the transition frequency obtained from the Cs cell at room temperature and with one-layer μ-metal shielding is already very near that for the condition of zero collision and zero magnetic field. We point out that a sophisticated linewidth measurement could be a good guidance for choosing a . Two research groups have tried to obtain the transition frequencies by very different approaches [1,7], namely, "direct comb laser spectroscopy" and "frequency chain." We here demonstrate a third approach for obtaining new frequencies of the two Dopplerfree 6S-8S transitions with 1 order of magnitude higher precision over the latest published measurement [1]. The accuracy of the transition frequency is discussed after we have inspected ten Cs cells. We suggest the frequency to be determined from the cells that yield the Lorentian-part linewidth near the theoretical natural linewidth. Figure 1 shows a simplified block diagram of our experimental design, which allowed us to simultaneously achieved the following three primary tasks of narrowing the laser linewidth, stabilizing the laser frequency, and scanning the laser carrier frequency for high-precision spectroscopy. The extended cavity diode laser (ECDL) system (Fig. 1) served as the master laser [5] in this experiment. The output light after a tapered amplifier (TA) was sent to a spatial filter, and then a 200 mW spatially regulated laser beam was split equally into the electro-optical modulator (EOM) area and acoustic-optical modulator (AOM) area in which the time base of the two modulator's drivers were all phase locked against the time base of a Cs clock. The EOM area is set up for both laser stabilization and laser carrier frequency (ω c ) scanning. The driving frequency of the EOM is provided by a function generator (Agilent 81150A) that allows us to dither the EOM modulation frequency (Δ 0 ) with an external dither source (ω d , Agilent 33250A). Consequently, the optical phase (ϕ) was modulated as ϕ M cos Δt with a dither frequency ω d embedded in the modulation frequency Δ, that is, Δ Δ 0 A cos ω d t.Here, a modulation index (MI) of 0.4 rad is used; Δ 0 is 106 MHz; A is 1 MHz; ω d is 28 kHz. Dithering the EOM frequency was for obtaining a first-derivative spectrum from Cs cell #1 (81°C wall temperature). The smooth curve on the center of the Pound-Drever-Hall (PDH) signal (green dashed inset, Fig. 1), for reducing laser jitter to 20 kHz bandwidth, proves an interesting fact that the aforementioned dither on sidebands was just canceled out in the heterodyned PDH signal. That is, dither Fig. 1. Simplified block diagram for simultaneously narrowing laser linewidth, stabilizing laser carrier frequency by cell #1, and resolvin...
The metabolites, (24S)-3b-acetoxyergost-5-en-21-oic acid (2), 5¢-O-acetylthymidine (3), 3¢-O-acetylthymidine (4), and p-vinylbenzyl alcohol (5), along with a known steroid (1) were isolated from the EtOAc extract of the Formosan soft coral Caldiella australis. The structures of new metabolites were determined on the basis of spectroscopic (including 1D and 2D NMR) analyses and by comparison of their NMR spectral data with those of related compounds. Except for 3, all compounds exhibited cytotoxic activity of various degrees of potency against a limited panel of human liver and breast cancer cell lines.
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