Hydroxymethylated Dioxobilins in Senescent Arabidopsis thaliana Leaves: Sign of a Puzzling Biosynthetic Intermezzo of Chlorophyll Breakdown

1‐Formyl‐19‐oxobilin‐type tetrapyrroles are characteristic, abundant products of chlorophyll breakdown in senescent leaves. However, in some leaves, 1,19‐dioxobilin‐type chlorophyll catabolites (DCCs) lacking the formyl group accumulate instead. A P450 enzyme was identified in in vitro studies that removed the formyl group of a primary fluorescent chlorophyll catabolite (pFCC) and generated fluorescent DCCs. These DCCs are precursors of isomeric nonfluorescent DCCs (NDCCs). Here, we report a structural investigation of the NDCCs in senescent leaves of wild‐type Arabidopsis thaliana. Four new NDCCs were characterized, two of which carried a stereoselectively added hydroxymethyl group. Such formal DCC hydroxymethylations were previously found in DCCs in leaves of a mutant of A. thaliana. They are now indicated to be a feature of chlorophyll breakdown in A. thaliana, associated with the specific in vivo deformylation of pFCC en route to NDCCs.     

Stereo‐ and Regioselective Phyllobilane Oxidation in Leaf Homogenates of the Peace Lily (Spathiphyllum wallisii): Hypothetical Endogenous Path to Yellow Chlorophyll Catabolites

In senescent leaves, chlorophyll typically is broken down to colorless and essentially photo‐inactive phyllobilanes, which are linear tetrapyrroles classified as “nonfluorescent” chlorophyll catabolites (NCCs) and dioxobilane‐type NCCs (DNCCs). In homogenates of senescent leaves of the tropical evergreen Spathiphyllum wallisii, when left at room temperature and extracted with methanol, the major endogenous, naturally formed NCC was regio‐ and stereoselectively oxidized (in part) to a mixture of its 15‐hydroxy and 15‐methoxy derivative. In the absence of methanol in the extract, only the 15‐OH‐NCC was observed. The endogenous oxidation process depended upon molecular oxygen. It was inhibited by carbon monoxide, as well as by keeping the leaf homogenate and extract at low temperatures. The remarkable “oxidative activity” was inactivated by heating the homogenate for 10 min at 70 °C. Upon addition of a natural epimeric NCC (epiNCC) to the homogenate of senescent or green Sp. wallisii leaves at room temperature, the exogenous epiNCC was oxidized regio‐ and stereoselectively to 15‐OH‐epiNCC and 15‐OMe‐epiNCC. The identical two oxidized epiNCCs were also obtained as products of the oxidation of epiNCC with dicyanodichlorobenzoquinone (DDQ). Water elimination from 15‐OH‐epiNCC occurred readily and gave a known “yellow” chlorophyll catabolite (YCC). The endogenous oxidation process, described here, may represent the elusive natural path from the colorless NCCs to yellow and pink coloured phyllobilins, which were found in (extracts of) some senescent leaves.     

Chlorophyll Breakdown in Maize: On the Structure of Two Nonfluorescent Chlorophyll Catabolites

Summary. In extracts of senescent leaves of the maize plant Zea mays, two colorless compounds with UV/Vis-characteristics of nonfluorescent chlorophyll catabolites (NCCs) were detected and tentatively named Zm-NCCs. The constitution of the two polar Zm-NCCs was determined by spectroscopic means, which confirmed both of these tetrapyrroles to have the basic ligand structure typical of the NCCs from (other) senescent higher plants. In the less polar catabolite, named Zm-NCC-2, the core structure was conjugated at the 8²-position with a glucopyranose unit. Zm-NCC-2 had the same constitution as Nr-NCC-2, an NCC from tobacco (Nicotiana rustica). Indeed, the two NCCs were identified (further) based on their HPL-chromatographic and NMR-spectroscopic properties. The more polar NCC from maize, Zm-NCC-1, differed from Zm-NCC-2 by carrying a dihydroxyethyl side chain instead of a vinyl group at the 3-position. In earlier work on polar NCCs, only separate glucopyranosyl- and dihydroxyethyl-functionalities were detected. Zm-NCC-1 thus is a new constitutional variant of the structures of NCCs from senescent higher plants.     

蔬菜叶片中叶绿素a和叶绿素b的一阶导数分光光度法测定

本文提出用一阶异数分光光度法同时测定蔬菜叶片中叶绿素ａ和叶绿素ｂ的含量。讨论了同时存在于蔬菜叶片中的类胡萝卜素对两者测定的影响，并提出用适当浓度的类胡萝卜素丙酮溶液作为参比溶液，用以消除类胡萝卜素对叶绿素测定的影响。     

分光光度法快速测定玉米叶片中的叶绿素

以玉米叶片为材料,利用双波长双光束紫外可见分光光度计,用最大决定系数增量回归算法,研究了叶片叶绿素a、b及SPAD值同时、快速测量方法.结果表明,用3个波长点建立的模型可以达到良好的预测效果,预测叶绿素a、b及SPAD值的相关系数分别为0.9919、0.9816和0.9757;标准差分别为1.52、0.43和1.96;预测相对标准差分别为4.64%、5.50%和4.88%.同时研究了仪器波长误差和带宽变化对测定结果的影响, 发现波长偏移超过0.2 nm,误差快速增大,且波长向长波方向偏移时对测量的影响要大于向短波方向偏移的影响;仪器带宽变大,预测误差也就越大.     

高光谱图像技术检测黄瓜叶片的叶绿素叶面分布

以黄瓜叶片为材料,利用高光谱图像技术结合独立分量法(ICA),研究了叶绿素浓度叶面分布的快速、无损检测方法.用高光谱相机采集了80片黄瓜叶子的高光谱图像(408～1117 nm),利用ICA方法提取了高光谱图像的8个独立分量信号,通过逐步线性回归(SMLR)优选出第1、第2和第5个ICA信号,并在此基础上建立了叶绿素浓...     

Chlorophyll Breakdown in Senescent Banana Leaves: Catabolism Reprogrammed for Biosynthesis of Persistent Blue Fluorescent Tetrapyrroles

Chlorophyll breakdown is a visual phenomenon of leaf senescence and fruit ripening. It leads to the formation of colorless chlorophyll catabolites, a group of (chlorophyll‐derived bilin‐type) linear tetrapyrroles. Here, analysis and structure elucidation of the chlorophyll breakdown products in leaves of banana (Musa acuminata) is reported. In senescent leaves of this monocot all chlorophyll catabolites identified were hypermodified fluorescent chlorophyll catabolites (hmFCCs). Surprisingly, nonfluorescent chlorophyll catabolites (NCCs) were not found, the often abundant and apparently typical final chlorophyll breakdown products in senescent leaves. As a rule, FCCs exist only fleetingly, and they isomerize rapidly to NCCs in the senescent plant cell. Amazingly, in the leaves of banana plants, persistent hmFCCs were identified that accounted for about 80 % of the chlorophyll broken down, and yellow leaves of M. acuminata display a strong blue luminescence. The structures of eight hmFCCs from banana leaves were analyzed by spectroscopic means. The massive accumulation of the hmFCCs in banana leaves, and their functional group characteristics, indicate a chlorophyll breakdown path, the downstream transformations of which are entirely reprogrammed towards the generation of persistent and blue fluorescent FCCs. As expressed earlier in related studies, the present findings call for attention, as to still elusive biological roles of these linear tetrapyrroles.     

A Robust Near-Infrared Calibration Model for the Determination of Chlorophyll Concentration in Tree Leaves with a Calibration Transfer Method

A method based on piecewise direct standardization was developed to directly predict leaf chlorophyll concentrations by correction of near-infrared spectra to construct a robust calibration model. Chinar, camphor, and gingko leaves collected from two growth intervals were evaluated. Spectral pretreatment methods and wavelength selection were investigated. The first derivative combined with stability competitive adaptive reweighted sampling before piecewise direct standardization provided the best performance. Under the optimized parameters, the root mean square error of prediction was significantly reduced by using piecewise direct standardization. This study demonstrates that the calibration model may be used to rapidly characterize chlorophyll concentrations across species and growth intervals.     

用化学计量法—分光光度法测定活体叶片中叶绿素a,b的含量

本文研究了用逐步回归分析法、主成分回归法、偏最小二乘法与人工神经网络法测定活体叶片中叶绿素a、b的含量。活体叶片的光谱由PC微机采集,测定叶绿素a与b的相关系数分别达到0.927～0.958与0.873～0.908;相对标准偏差约5％～9％(叶绿素a 9.4μg/2ml,n=20)。本方法可用于农业研究。     

Formation of the Main Cores Present in Natural Products by Tandem Additions

A rapid route to 5,5‐ and 5,6‐ bicyclic systems is provided by an 1,3‐alkyl‐shift process mediated by a hypervalent iodine reagent on aromatics. The structures obtained contain several unsaturations with different behaviors and reactivities. Such diversity allows further elaborations for the rapid formation of compact systems present in a variety of natural products. The potential for further transformations has been demonstrated by performing a double Michael addition. This cyclization process is regio‐ and stereoselective due to the presence of a former benzylic substituent. Furthermore, an extension of this approach has been accomplished on indole derivatives.     

Guapsidial A and Guadials B and C: Three New Meroterpenoids with Unusual Skeletons from the Leaves of Psidium guajava

A novel sesquiterpene‐based Psidium meroterpenoid, possessing an unusual coupling pattern, and two new monoterpene‐based meroterpenoids with unprecedented skeletons were isolated from the leaves of Psidium guajava. Their structures and absolute configurations were elucidated by spectroscopic, X‐ray diffraction, and computational methods. The plausible biosynthetic pathway of these meroterpenoids as well as their cytotoxicities toward HepG2 and HepG2/ADM cells were also discussed.     

Biology‐Oriented Synthesis of a Withanolide‐Inspired Compound Collection Reveals Novel Modulators of Hedgehog Signaling

Biology‐oriented synthesis employs the structural information encoded in complex natural products to guide the synthesis of compound collections enriched in bioactivity. The trans‐hydrindane dehydro‐δ‐lactone motif defines the characteristic scaffold of the steroid‐like withanolides, a plant‐derived natural product class with a diverse pattern of bioactivity. A withanolide‐inspired compound collection was synthesized by making use of three key intermediates that contain this characteristic framework derivatized with different reactive functional groups. Biological evaluation of the compound collection in cell‐based assays that monitored biological signal‐transduction processes revealed a novel class of Hedgehog signaling inhibitors that target the protein Smoothened.