JP6704800B2 - Indicator substance for sweat odor determination - Google Patents

Description

TECHNICAL FIELD The present invention relates to an index substance used for determining a sweat odor generated from a textile product, a method for determining a sweat odor using the same, and a pseudo sweat odor composition.

2. Description of the Related Art In recent years, an increasing number of people are worried about unpleasant odors around them, for example, body odors generated by people and odors generated from textile products such as clothes when working or commuting, due to increasing interest of consumers in their living environments. In particular, an odor generated from a textile product such as clothes when worn is recognized as an unpleasant odor as a so-called “sweat odor when worn”. Hitherto, it has been considered that a plurality of short-chain and medium-chain fatty acid odors make a large contribution to human sweat odor. For example, Non-Patent Document 1 describes that the main odor component of sweat odor is isovaleric acid, and in addition, fatty acids having 2 to 6 carbon atoms are used as synthetic odor.

In addition, as an odor component related to a plurality of short-chain and medium-chain fatty acid odors, there is an unpleasant odor that tends to be generated when leaving a textile product such as clothes, towels and bedding in an insufficiently dried state for a long time. A certain "dry odor" is known. This raw dry odor (room-dried odor) is a "moldy odor" such as medium-chain aldehydes, medium-chain alcohols and ketones, a "sour odor" such as short-chain fatty acids and medium-chain fatty acids, and a "fresh odor" such as nitrogen compounds. It is a complex odor composed of a sulfur compound and a sulfur compound, and it is reported that the contribution of medium-chain fatty acids is particularly large (Non-Patent Document 2). Here, it is presumed that the main component of the raw dry odor is a “mixture of fatty acids containing 7 to 9 carbon atoms with branching/unsaturation”, and these are also included in odors such as sweat. That is, neither Non-Patent Document 1 nor Non-Patent Document 2 suggests the presence of a component peculiar to sweat odor generated from a textile product when worn.

On the other hand, in Patent Documents 1 and 2, three types of methylhexene, such as 4-methyl-3-hexenoic acid, are used as a causative substance of an unpleasant odor (dry dry odor) generated when the textile product after washing is insufficiently dried. The acid is disclosed as a substance with a high contribution.

Kiyohito Sawano, "What is body odor?", Fragrance, June 1994, No.182, pp.123-128 Hanbara, Sonoda, “Detergent to control room odor”, Fragrance, September 2004, No.223, p.109-116

JP 2009-244094 JP Japanese Patent Laid-Open No. 2011-75385

However, none of the documents mentions a component specific to sweat odor generated from a textile product when worn. Conventionally, as a method for determining a sweat odor generated from a textile product and the effectiveness evaluation of a textile product treatment agent product for preventing the occurrence of a sweat odor, it is assumed that the odor during wearing and the odor after wearing are the same, It relied on a sensory evaluation test to judge the smell of clothing after it was worn. Further, in the sensory evaluation test, there is a large room for the subjective judgment of the evaluator, so that there is a problem that the quantitative judgment is difficult and the objectivity is lacking.

The present invention is an indicator substance that makes it possible to objectively and quantitatively determine a sweat odor generated from a textile product when worn, and a method for determining a sweat odor of a textile product using the indicator substance, and a sweat odor masking effect, etc. The present invention relates to a simulated raw dry odor composition suitable for evaluation.

The present inventors have conducted a study on causative components of clothes having sweat odor generated from clothes when they are worn. As a result, methylpentenoic acid and methylpentanoic acid having a specific structure have a large contribution to sweat odor and a quantitative determination. It has been found that it is present in clothing at a possible concentration, and that a group of compounds having structures close to these compounds can be used as an objective index for quantitatively determining the degree of sweat odor.

The present invention contains a carboxylic acid represented by the general formula (1) and at least one selected from the group consisting of carboxylic acid derivatives obtained by introducing an atom or an atomic group into the carboxy group of the carboxylic acid, a fiber product The present invention provides an indicator substance for determining the sweat odor.

[In the formula, the broken line indicates that it may be a double bond. ]

The present invention also provides a method for determining a sweat odor of a textile product using the carboxylic acid or its derivative as an index.

Further, the present invention provides a pseudo sweat odor composition containing the carboxylic acid.

The carboxylic acid represented by the general formula (1) has an odor that is extremely close to the odor of sweat of a textile when worn, and is useful as an indicator substance for determining a sweat odor of the textile. Therefore, the strength of sweat odor of the textile product can be objectively and quantitatively evaluated based on the amount of the compound present.

The indicator substance for sweat odor determination of the present invention comprises a carboxylic acid represented by the general formula (1) (hereinafter referred to as carboxylic acid (1)) and an atom or an atomic group introduced into the carboxy group of the carboxylic acid. It contains at least one selected from the group consisting of carboxylic acid derivatives.

The causative substance of sweat odor in a textile product when the present inventors have found that, among the compounds included in the carboxylic acid (1), 4-methyl-3-pentenoic acid represented by the following formula (1a): (Hereinafter, referred to as 4M3P) and 4-methylpentanoic acid represented by the following formula (1b) (hereinafter, referred to as 4MP), each of which has an odor that is very close to the sweat odor generated from the textile when worn.

4M3P (1a) is a compound known as an intermediate of industrial raw materials [US3082251A], 4MP (1b) is a compound used as a flavor component, but both are components of sweat odor. It has never been reported. In addition, since it has the following characteristics, it is considered that the amount and state of existence of clothes having sweat odor indicate the degree of sweat odor when worn.

(A) 4M3P and 4MP are not detected in the clothing when worn, and sweat odor is clearly generated in the clothes in which 4M3P and 4MP are detected. That is, the above two compounds are specifically present in the sweat odor of clothing when worn.
(B) The higher the amount of 4M3P and 4MP contained in clothes, the stronger the sweat odor.

Carboxylic acid (1) includes the above 4M3P and 4MP as well as compounds having a very similar chemical structure. Specific examples of the carboxylic acid (1) other than 4M3P and 4MP include 4-methyl-2-pentenoic acid and 4-methyl-4-pentenoic acid.

Carboxylic acid (1) can be synthesized by a known method and can be purchased as a reagent, so by stably supplying a synthetic product of constant quality, the sweat odor can be objectively evaluated at any time and place, It is also suitable as an indicator substance because it can be judged.

The carboxylic acid (1) may be a simple substance or a mixture of a plurality of carboxylic acids (1). The higher the purity, the more preferable, but it may contain impurities as long as it does not affect the odor.

In addition, the carboxylic acid (1) may be used after being chemically modified, that is, by introducing an atom or an atomic group into the carboxy group, as long as the detection function as the indicator compound is not lost. For example, in order to improve the analytical sensitivity in instrumental analysis, the carboxy group is colored by acylation, esterification, trimethylsilylation, amidation, carboxylic acid conversion, or by making the indicator substance visible. You can also introduce groups.

As a derivatization reagent, UV reagents such as O-(p-nitrobenzyl)-N,N-diisopropylisourea (PNBDI) and p-bromophenacyl bromide (PBPB), 4-bromomethyl-7-methoxycoumarin ( Br-MmC) and other fluorescent reagents, silylating agents such as N-trimethylsilylimidazole (TMSI) and N,O-bis(trimethylsilyl)acetamide (BSA), acylating agents such as trifluoroacetic anhydride and trifluoroacetylimidazole Can be used.

When a chromophore in the visible region is used as the carboxylic acid (1) labeling compound, the concentration of the labeling compound-a color standard sample was prepared, and the odorant extract collected from the sample was colored with the same reagent. It is also possible to visually judge the degree of sweat odor as compared with.

As a method for determining the presence or absence and the amount of carboxylic acid (1) using the color reaction,
i) Method of directly introducing a chromophore into the carboxy group of carboxylic acid (1)
ii) A method of introducing a chromophore into the derivative after converting the carboxylic acid (1) into the derivative
iii) A method of introducing a chromophore into the decomposed product after decomposing the carboxylic acid (1), and the like.

The color reagent used in the method for directly introducing a chromophore into the carboxy group of the carboxylic acid (1) of i) is a reagent for introducing a carboxylic acid into a color-forming acid hydrazide in the presence of a condensing agent to give a color, There are a reagent for leading a carboxylic acid to a color-forming ester to give a color and a reagent for leading a carboxylic acid to a color-forming amide to give a color.

As a reagent for introducing a color of a carboxylic acid into a chromic acid hydrazide to give a color, 2-nitrophenylhydrazine, 6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone-3-propionylcarboxylic acid hydrazide (DMEQ- H), p-(4,5-diphenyl-1H-imidazol-2-yl)-benzohydrazide, p-(1-methyl-1H-phenanthro-[9,10-imidazol-2-yl)-benzohydrazide, Examples thereof include p-(5,6-dimethoxy-2-benzothiazoyl)-benzohydrazide.

As a reagent for coloring a carboxylic acid into a color-forming ester, 9-anthryldiazomethane, 1-naphthyldiazomethane, 1-(2-naphthyl)diazoethane, 1-pyrenyldiazomethane, 4-diazomethyl-7-methoxy Coumarin, 4-bromomethyl-7-methoxycoumarin, 3-bromomethyl-6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone, 9-bromomethylacridine, 4-bromomethyl-6,7-methylenedioxycoumarin , N-(9-acridinyl)-bromoacetamide, 2-(2,3-naphthylimino)ethyl trifluoromethanesulfonate, 2-(phthalimino)ethyltrifluoromethanesulfonate, N-chloromethylphthalimide, N-chloromethyl-4-nitro Examples thereof include phthalimide, N-chloromethylisatin, o-(p-nitrobenzyl)-N,N'-diisopropylisourea and the like.

Examples of the reagent that introduces a carboxylic acid into a chromophoric amide to cause coloration include monodansylcadaverine, 2-(p-aminomethylphenyl)-N,N'-dimethyl-2H-benzotriazol-5-amine, and the like. ..

In the method of converting a carboxylic acid into a derivative of ii) and then introducing a chromophore into the derivative, examples of the carboxylic acid derivative that can be used in the color reaction include an inorganic salt and an acid chloride.

An inorganic salt of a carboxylic acid can be reacted with an aromatic halogen to form a color-forming ester, and an acid chloride can be formed into a color-forming amide. Examples of the method of converting the carboxylic acid into an inorganic salt include a method of neutralizing the carboxylic acid by mixing it with an alkaline substance such as a sodium hydrogen carbonate solution, a sodium carbonate solution, a sodium hydroxide solution, a potassium hydroxide solution. Aromatic halogens that can react with inorganic salts of hydroxycarboxylic acids to form color-forming esters include p-nitrobenzyl bromide, phenacyl bromide, p-chlorophenacyl bromide, p-bromophenacyl bromide, p-iodo. Phenacyl bromide, p-nitrophenacyl bromide, p-phenylphenacyl bromide, p-phenylazophenacyl bromide, N,N′-dimethyl-p-aminobenzeneazophenacyl chloride, and the like can be mentioned.

Examples of the method of converting carboxylic acid into acid chloride include a method of reacting carboxylic acid with oxalyl chloride. Examples of the method of converting the acid chloride into a chromophoric amide include a method of reacting with 9-aminophenanthrene in the presence of triethylamine.

After decomposing the carboxylic acid of iii), a method of introducing a chromophore into the decomposition product is as follows. Acyl-CoA synthetase in the presence of adenosine triphosphate (ATP) and coenzyme CoA in the carboxylic acid. To produce acyl-CoA, which is then treated with acyl-CoA oxidase to produce enoyl-CoA and hydrogen peroxide, which is then treated with catalase to formaldehyde, which Examples include a method of reacting 4-amino-3-hydrazino-5-mercapto-1,2,3-triazole (AHMT), which is a color reagent, and comparing the resulting purple color.

As described above, in the present invention, the reagent used for the color reaction of the carboxylic acid is not particularly limited as long as it reacts with any one of the carboxylic acid, the carboxylic acid derivative and the carboxylic acid decomposed product. That is, the sweat odor determination reagent that changes the color of the carboxylic acid (1) can surely and quickly and easily determine the degree of sweat odor without using an expensive analyzer such as gas chromatography or liquid chromatography. Since it is possible, it can be used when making a determination in an environment where there is no special analytical instrument.

In the present invention, it is possible to quantitatively determine the degree of sweat odor in a textile product by using these index substances.

As a method of using carboxylic acid (1) or a derivative thereof as an indicator substance for sweat odor determination, for example, when measuring by GC-MS, carboxylic acid (1) or a derivative thereof is calibrated as a standard substance (standard). A line may be created, the calibration curve may be used to identify the peak of the carboxylic acid (1) contained in the collected clothing, and the amount may be measured.

In addition, when performing sensory evaluation, carboxylic acid (1) was diluted in several steps to prepare odor standard samples of various concentrations, and the odor of the evaluation sample prepared from clothes having sweat odor was compared with the standard sample. The amount of carboxylic acid (1) contained in clothing may be determined by sensory evaluation. In this way, the degree of sweat odor of the clothing is determined by quantitatively determining the smell of the sweat using the odor or the parameter other than the odor.

In addition, even if a large amount of carboxylic acid (1) is produced in clothing, but it is converted to a salt-free odor-free or weak derivative, despite the presence of a latent sweat odor, In some cases, the sensory evaluation may not accurately evaluate the latent state of sweat odor. On the other hand, in the present invention, by measuring the carboxylic acid (1) or a derivative thereof that can be analyzed by the necessary chemical treatment, whether the evaluation sample is a sample that may generate sweat odor, that is, Potential evaluation can be performed.

As described above, the indicator substance of the present invention can be used for any of chemical analysis, instrumental analysis, sensory evaluation, etc., and enables highly objective quantitative determination, but in particular, it is measured by chemical analysis or instrumental analysis. By expressing the value as the amount of carboxylic acid (1) present, subjectivity can be excluded from the determination result.

Furthermore, in the present invention, the effectiveness of the textile product treating agent product targeting sweat odor can be objectively and quantitatively determined using an indicator substance containing a carboxylic acid (1) or a derivative thereof. ..

In the method for determining the effectiveness of a treatment agent product for a textile product, the indicator substance is used alone, or other components, for example, a solvent for dissolution or dilution, a stabilizer, an antibacterial agent, an antibacterial agent, An additive such as a surfactant, an antioxidant, a fragrance, or a plant extract may be added to prepare a composition suitable for practical use such as storage or use in a judgment test.

As a treatment agent for textile products targeting sweat odor, for example, a treatment agent that suppresses decomposition of sweat, sebum, etc. remaining in clothing by sterilizing bacteria adhered to the clothing, decomposed into an odorless derivative Alternatively, a treating agent that changes or a treating agent that has an effect such as a treating agent that masks odor can be used. To use the carboxylic acid (1) or a derivative thereof as an index substance for determining the effectiveness of a treatment agent product for a textile product, it may be used in conformity with the action mechanism and evaluation method of the treatment agent product for a textile product.

Specifically, a carboxylic acid (1) or its derivative as an active ingredient, preferably 4M3P, 4MP or a derivative thereof at a predetermined concentration is attached to a fiber such as clothing with an indicator substance, and a predetermined amount of a textile product is treated. By adding the agent sample and quantifying the change state of the indicator substance by an appropriate method, the effectiveness of the textile agent treatment agent sample can be objectively and quantitatively determined.

As a specific example of the method for quantifying the change state of the indicator substance, in the case of a textile product treatment agent sample that decomposes carboxylic acid (1) or induces another compound to reduce odor, a calibration curve of the indicator substance is prepared in advance. It is prepared in advance and instrumental analysis is performed using this calibration curve, or the changed or unchanged form of the indicator substance is quantified by chemical analysis such as titration or extraction. In the case of a textile product treatment agent sample that masks sweat odor, the indicator substance is diluted in several steps to prepare odor standard samples, and the indicator substance smell added with the textile product treatment agent sample is added. The masking effect is judged by sensory evaluation by comparing with a standard sample.

Further, the present invention provides a pseudo sweat odor composition containing carboxylic acid (1), which can be used for deodorizing sweat odor and for evaluating the masking effect. By using this pseudo sweat odor composition, the deodorant base can be screened and the sweat odor deodorizing effect of the deodorant composition can be evaluated accurately and with good reproducibility.

When this carboxylic acid (1) is used for the sweat odor determination in sensory evaluation or the like, the sweat odor closer to the actual situation can be reproduced by mixing known sweat odor constituents at a predetermined ratio. That is, the simulated raw dry odor composition of the present invention contains the carboxylic acid (1) as the component (A), and can appropriately contain the compounds of the components (B) and (C) shown below. Component (A) preferably contains at least either 4M3P or 4MP. The content of the component (A) in the pseudo sweat odor composition is preferably 0.5% by mass or more, more preferably 1% by mass or more, and preferably 60% by mass or less, more preferably 56% by mass or less. is there.

Component (B) is a lower fatty acid having 2 to 5 carbon atoms, and by adding these compounds to the composition, a sweat-like sour odor is imparted, and the sweat odor generated from a textile product when worn is brought closer to it. be able to. Examples of the component (B) include acetic acid, propionic acid, butyric acid, 2-methylbutyric acid, valeric acid and isovaleric acid, and any one of them can be used alone or in combination of two or more. The content of the component (B) in the pseudo sweat odor composition is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less. is there.

In the present invention, the mass ratio of the component (B) to the component (A) (B)/(A) is preferably 0.005 or more, more preferably 0.01 or more, and preferably 50 or less, more preferably 25 or less. Is.

Component (C) is a C7 saturated or unsaturated methyl-branched fatty acid, and the addition of these compounds to the composition imparts a peculiar sweat-like odor generated from the textile when worn, and It can be brought closer to the sweat odor generated when wearing. Examples of the component (C) include 4-methyl-3-hexenoic acid (4M3H), 4-methylhexanoic acid (4MH), 5-methyl-2-hexenoic acid (5M2H), 5-methyl-4-hexenoic acid ( 5M4H) and the like, and any of them can be used alone or in combination of two or more. The content of the component (C) in the pseudo sweat odor composition is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and preferably 15% by mass or less, more preferably 10% by mass or less. is there.

In the present invention, the mass ratio (C)/(A) of the component (C) to the component (A) is preferably 0.001 or more, more preferably 0.01 or more, and preferably 10 or less.

The pseudo sweat odor composition of the present invention further contains, as the component (D), a straight chain saturated fatty acid having a carbon chain of 6 to 12 which is detected from human sebum or sweat, and has a sweetness peculiar to human origin. It is preferable because it can give a muddy acid odor and can bring it closer to the sweat odor generated from the textile product when worn. The component (D) is selected from hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid and dodecanoic acid, and any one of them can be used alone or in combination of two or more. The content of the component (D) in the pseudo sweat odor composition is preferably 0.5% by mass or more, more preferably 5% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less. is there.

The mass ratio (D)/(A) of the component (D) to the component (A) is preferably 0.01 or more, more preferably 0.1 or more, and preferably 100 or less, more preferably 50 or less.

The simulated sweat odor composition of the present invention is a compound other than the components (A) to (D), for example, compounds derived from musty odors such as pyrazines, pyridines, dimethyldisulfide, dimethyltrisulfide, myristic acid, palmitic acid, and palmitoyl. By including higher fatty acids such as acids, stearic acid, oleic acid, and linoleic acid, the sweat odor can be brought closer to the actual sweat odor.

Further, the pseudo sweat odor composition of the present invention, if necessary, contains a diluent such as water, diethylphthalate, dipropylene glycol, propylene glycol, triethyl citrate, butyl diglycol or a solvent such as ethanol. Can be made The amount can be appropriately determined according to the intended use, intended use, etc. of the pseudo sweat odor composition of the present invention.

Further, the pseudo sweat odor composition of the present invention can be used for evaluating the sweat odor intensity of a textile product as in the case of using the carboxylic acid (1) as the indicator substance. For example, the pseudo sweat odor composition is diluted in several stages to prepare odorant standard samples of various concentrations, and the odor of the evaluation sample prepared from clothing having sweat odor is compared with the standard sample to determine the strength of the odor attached to the clothing. Can be quantitatively measured by sensory evaluation. Furthermore, these compositions are used for deodorant agents for deodorizing mixed odors, for deodorizing care odors, indoor deodorants, detergents for clothing, deodorants for clothing, detergents for homes, etc. It may be applied to the development of agents and the like.

Test example 1
10 g of clothes having a sweat odor was finely cut and immersed in 30 mL of diethyl ether, and ultrasonic waves were applied for about 15 minutes to obtain an odor component extract. The odorant extract was filtered through a filter (0.4 micron pores), 10 mL of 1N NaOH aqueous solution was added, and the mixture was stirred well. After the stationary layer separation, the upper diethyl ether layer was removed, 10 mL of fresh diethyl ether was added to the remaining lower layer (aqueous layer), and the mixture was well stirred. After the stationary layer separation, the upper diethyl ether layer was removed. After adding 3 mL of diethyl ether to the lower layer (aqueous layer), hydrochloric acid was added to neutralize, and the upper layer (containing diethyl ether/free fatty acid) was filtered (0.2 micron pores) to obtain a sample for analysis.
The sample for analysis was subjected to sniffing GC, and the GC retention time (RT) of the component having a sweat-like odor was specified. Subsequently, the sample for analysis was analyzed by an apparatus in which a large-scale injection unit (CIS4) manufactured by GERSTEL was connected to a GC-MS manufactured by Agilent, and a component having an odor like sweat odor was analyzed. The respective analysis conditions are shown.

Smell GC conditions
GC: Agilent 6890N
Column DB-1 (trade name, manufactured by Agilent), length 30 m, inner diameter 530 μm, film thickness 1.0 μm
40℃-2℃/min-280℃ (30min.)
Bulk injection unit (CIS4) conditions:
After injecting 50 μL of sample, vent the solvent at 0℃ and inject at 180℃.
GC-MS conditions:
GC: Agilent 6890N (trade name, manufactured by Agilent)
MS: Agilent 5975 (trade name, manufactured by Agilent)
TDS desorption conditions: 250°C, purge flow rate 50 mL/min, purge time 3 min.
Column: DB-1 (trade name, manufactured by Agilent), length 60 m, inner diameter 250 μm, film thickness 0.25 μm
30℃(6min.)-10℃/min-60℃-5℃/min-95℃-3℃/min-300℃

As a result of the analysis, two kinds of carboxylic acids, 4M3P and 4MP, were identified as components having a characteristic sweat-odor. Each compound was identified by comparing the GC retention time and the mass fragment ion (m/z(+)) of the detected component with a standard product purchased separately as a reagent. The m/z(+) of the two carboxylic acids are as follows.
4-Methyl-3-pentenoic acid (4M3H) m/z(+): 41(82), 42(8), 43(10), 45(7), 53(14), 55(13), 67 (11), 68(16), 69(100), 81(3), 96(7), 114(48)
4-Methylpentanoic acid (4MP) m/z(+): 41(45), 42(11), 43(45), 45(14), 55(53), 56(16), 57(100) , 59(15), 60(30), 73(63), 74(64), 83(20), 87(7), 101(5), 115(0.32), 116(0.03)

The odor of each compound was sensory evaluated by a professional evaluator. The results are shown in Table 1.

Examples 1 to 7, Comparative Example 1
The pseudo sweat odor composition shown in Table 2 was prepared and sensory evaluation was performed based on the following evaluation methods. The content of each component in Table 2 is% by mass.
(Evaluation methods)
Each simulated sweat odor composition was diluted 1000 times with ion-exchanged water to an unused towel (made of cotton) of 5×5 cm in a room kept at room temperature (25° C.) and humidity of 65%. 0.1 mL of the product was applied. A sensory evaluation of 10 levels (10: very sweaty odor to 1: not sweaty odor) was performed by four expert evaluators to find out how close the odor of this towel was to sweat odor. The average value is shown in Table 2.

Claims (2)

Contains the following components (A) , (B) and (C) , the content of the component (A) is 0.5 mass% or more and 60 mass% or less, the content of the component (B) is 0.5 mass% or more and 60 mass% or less. Hereinafter, a composition having a pseudo sweat odor, in which the content of the component (C) is 0.01% by mass or more and 15% by mass or less .
(A) Carboxylic acid represented by the general formula (1)

[In the formula, the broken line indicates that it may be a double bond. ]
(B) Lower fatty acid having 2 to 5 carbon atoms
(C) Saturated or unsaturated methyl branched fatty acid having 7 carbon atoms Further, the composition having a pseudo sweat odor according to claim 1 , further comprising the following component (D).
(D) Straight chain saturated fatty acid having 6 to 12 carbon atoms