每周分子 | Molecule of the Week

Original ACS 美国化学会 ACS美国化学会 2022-07-01

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每周分子：柠檬烯

Molecule of the Week: Limonene

十一月是每周分子风味和香味月！

柠檬烯是一种单萜类化合物，在自然界中以两种对映异构体的形式存在：(S)-柠檬烯[又称(-)-柠檬烯，D -柠檬烯]和(R)-柠檬烯[又称(+)-柠檬烯，L-柠檬烯]。此处展示的是(S)-柠檬烯。

柠檬烯的两种对映体都具有大家熟悉的风味和香味。比较常见的(S)-柠檬烯被发现存在于柑橘油中，具有橙子的风味和香味。量较少的(R)-柠檬烯则存在于香菜、莳萝和佛手柑植物中；(R)-柠檬烯类似松树的气味为这些植物的可食用部位增添了风味和香味。

除了作为商业食品香料外，柠檬烯还被用作工业清洁溶剂、润湿剂、空气清新剂和个人护理产品中的香精。柠檬烯的全球产量约为5万吨/年，2020年的市场价值为3.23亿美元。

像柠檬烯这类化合物最近被报道具有不好的一面。早在2018年进行的一项研究中，美国国家海洋和大气管理局(博尔德，科罗拉多) 的Matthew Coggon及其同事们从纽约市的不同地点收集了空气样本，并分析了样本中的挥发性有机化合物(VOCs)，这些挥发性有机化合物是生成臭氧的资源。

2021年8月公布的研究结果显示：一半的挥发性有机化合物是由化石燃料燃烧产生的，而化石燃料燃烧被认为是产生臭氧的罪魁祸首；另一半来自化学产品，包括涂料和粘合剂的溶剂，以及典型的臭氧前体。但在被确定的化学物质中，有一半的化学物质主要为单萜类化合物(如柠檬烯)以及其他用于个人护理产品的物质。这项研究为控制人口密集地区臭氧形成提供了新的思路。

从积极一点的方面来看，在上个月，柠檬烯作为一种利用工程改造菌的生物合成过程的中间体出现在新闻中，该中间体可以用来制备非天然来源的萜烯。美国加州大学伯克利分校和劳伦斯伯克利国家实验室的John Hartwig和他的同事们设计并构建了一株大肠杆菌(Escherichia coli)，可以在其细胞内生成(S)-柠檬烯、Sulfolobus acidocaldarius酶和卟啉特异性转运蛋白。

在含有铱中卟啉和重氮乙酸乙酯的介质中，大肠杆菌工程菌吸收卟啉，在大肠杆菌工程菌中，卟啉与酶结合，促进柠檬烯和重氮化合物之间的插入反应，形成环丙烯酯。该工艺模拟天然萜烯改性反应，具有较高的立体选择性。

柠檬烯不应与柠檬苦素(一种苦味、高度复杂的柑橘类果实的产物)或lemonene(芳香化合物联苯的模糊名称)混淆，芳香化合物联苯天然存在于煤焦油和原油中。

(S)-柠檬烯信息速览

CAS 登记号	5989-54-8
SciFinder命名	Cyclohexene, 1-methyl-4-(1-methylethenyl)-, (4S)-
分子式	C₁₀H₁₆
摩尔质量	136.23 g/mol
外观	无色至淡黄色液体
沸点	177–178°C
水溶性	14 mg/L

(S)-柠檬烯危害信息

GHS*分类：易燃液体，分类3
H226-易燃液体和蒸气
GHS分类：吸入有害，分类1
H304-吞咽并进入呼吸道可能致命
GHS分类：皮肤腐蚀/刺激，分类2A
H315-引起皮肤刺激
GHS分类：皮肤过敏，分类1
H317-可能引起皮肤过敏反应
GHS分类：严重眼损伤/眼刺激，分类2
H319-造成严重的眼刺激
GHS分类：短期(急性)水生危害，分类1
H400-对水生生物有害

【关于每周分子】

自2001年以来，“每周分子”已成为广受欢迎的栏目。多数分子由读者推荐。每个结构都经由科学家审查，并以3-D和平面图像的形式显示，同时附有该分子简介。可以通过美国化学文摘社 (CAS) 提供的CAS SciFinderⁿ中的CAS REGISTRY获取“每周分子”的更多信息。CAS REGISTRY中的每条物质信息都展示了物质的CAS登记号、索引名、通用名、商品名、书目信息等其他更多信息。发送邮件至motw@acs.org推荐您感兴趣的分子吧。

November is flavor and aroma month at MOTW!

Limonene is a monoterpene that exists in nature in two enantiomers: (S)-limonene [aka (–)-limonene, D-limonene] and (R)-limonene [aka (+)-limonene, L-limonene]. The (S)-isomer is shown here.

Both enantiomers have well-recognized flavors and aromas. The more common (S)-limonene is found in citrus oils and has the flavor and fragrance of oranges. Less abundant (R)-limonene is produced by caraway, dill, and bergamot plants; its piny odor contributes to the flavors and aromas of the plants’ edible portions.

In addition to their commercial use as food flavorings, the limonenes are used in industrial cleaning solvents, wetting agents, air fresheners, and fragrances in personal care products. Worldwide production is ≈50,000 t/year with a 2020 market value of US$323 million.

Compounds like the limonenes were recently reported to have a downside. In a study conducted in 2018, Matthew Coggon and co-workers at the National Oceanic and Atmospheric Administration (Boulder, CO) collected air samples from various locations in New York City and analyzed the samples for volatile organic compounds (VOCs), which are notorious ozone generators.

The results, published in August 2021, showed that half of the VOCs were produced by fossil fuel combustion, an expected culprit for creating ozone. The other half came from chemical products, including solvents for coatings and adhesives, also typical ozone precursors. But fully half of the identified chemicals consisted largely of monoterpenes, such as limonene, and other substances used in personal care products. The study provides new clues for potential control of ozone formation in densely populated areas.

On the more positive side, (S)-limonene was in the news last month as an intermediate in an engineered biosynthetic process to make an unnatural terpene. John Hartwig and colleagues at the University of California, Berkeley, and Lawrence Berkeley National Laboratory created a strain of Escherichia coli to produce (S)-limonene, a Sulfolobus acidocaldarius enzyme, and a porphyrin-specific transporter protein inside its cells.

In a medium that contains an iridium mesoporphyrin and ethyl diazoacetate, the engineered E. coli imbibes the porphyrin, where it combines with the enzyme to promote an insertion reaction between the limonene and the diazo compound to form a cyclopropylterpene ester. The process mimics natural terpene-modification reactions with high stereoselectivity.

The limonenes should not be confused with limonin, a bitter, highly complex citrus product, or lemonene, an obscure name for the aromatic compound biphenyl, which occurs naturally in coal tar and crude oil.

(S)-Limonenefast facts

CAS Reg. No.	5989-54-8
SciFinder nomenclature	Cyclohexene, 1-methyl-4-(1-methylethenyl)-, (4S)-
Empirical formula	C₁₀H₁₆
Molar mass	136.23 g/mol
Appearance	Colorless to pale yellow liquid
Boiling point	177–178°C
Water solubility	14 mg/L

(S)-Limonene hazard information

Flammable liquids, category 3
H226-Flammable liquid and vapor
Aspiration hazard, category 1
H304-May be fatal if swallowed and enters airways
Skin corrosion/irritation, category 2A
H315-Causes skin irritation
Skin sensitization, category 1
H317-May cause an allergic skin reaction
Serious eye damage/eye irritation, category 2
H319-Causes serious eye irritation
Short-term (acute) aquatic hazard, category 1
H400-Harmful to aquatic life

About Molecule of the Week

MOTW has been a popular feature on ACS website since 2001. Many molecules are suggested by our website visitors. Every structure is reviewed by a scientist and displayed in 3-D and flat images with a brief description. Each week’s molecule also links to a sample record from the CAS REGISTRY, which is searched using CAS SciFinderⁿ. Each record displays the registry number, index name and synonyms, bibliographic information, and more. Send us a molecule suggestion at motw@acs.org.

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