## 3 Antioxidant Properties of C\({}_{30}\) Carotenoids
Carotenoids with higher numbers of double bonds tend to have higher scavenging activities, and acyclic compounds have greater scavenging activities than cyclic carotenoids [36]. Additionally, carotenoid functional groups influence antioxidant activity and solubility. Aldehyde groups increase carotenoid solubility, which in turn affects bioavailability and stability, two critical factors in carotenoid end-product manufacture. Functional groups have a clear effect on C\({}_{30}\) carotenoid reactivity. Scavenging activity is greater in carotenoids with aldehyde or carboxylic groups. Carboxyl groups have slightly higher antioxidant capacities than aldehyde groups, but aldehyde-protein crosslinking increases the timely effects of cosmetic applications, which improves the viability of commercial production [37]. Carbonyl and hydroxyl groups have significant roles in the potency of the quenching capacity of a carotenoid, which is attributed to the affinity of the carotenoid to the ROS and the hydrophobicity of the experimental solvent. As such, the antioxidant activities of carotenoids can be enhanced by increasing the number of conjugated double bonds (C=C and C=O) or by lowering the hydrophobicity of the compound through the addition of more hydroxyl groups [56]. According to [PERSON] et al. [36], the activity of 4,4'-diapolycopene-4,4'-dial, which has 13 double bonds and two aldehyde groups, is greater than that of 4,4'-diaponeurosporene which only has 11 double bonds. Compared to DL-\(\alpha\)-tocopherol, which had a DPPH half-maximum inhibitory concentration (IC\({}_{50}\)) value of 33.2 uM, the tested C\({}_{30}\) carotenoids and their corresponding DPPH IC\({}_{50}\) values were: diapolycyopen-dial, 7.5 uM; diapolycoppene, 8.7 uM; diaponeurosporene acid, 9.7 uM; diaponeurosporen-al, 10.2 uM; diaponeurosporene, 11.6 uM; diapotorulene, 70.3 uM; and diapo-\(\beta\)-carotene, 77.8 uM. Their double bonds were 13, 11, 10, 10, 9, 8, and 5, respectively. The cyclic compounds diapotorulene and diapo-\(\beta\)-carotene had one and two additional double bonds, respectively, within their \(\beta\)-end groups [36]. In another study, 4_4'_-diaponeurosporene was isolated from _Lactiplantiobacillus plantarumum_ subsp. _plantarum_ KCCP11226T, a strain that showed high tolerance to oxidative stress, a high survival rate after H\({}_{2}\)O\({}_{2}\) treatment, and a DPPH free-radical scavenging ability of 27.41%, which was greater than that of butylated hydroxytoluene (BHT, 50 \(\upmu\)/mL) [29]. Staphyloxanthin from _S. gallinarum_ KX912244 has a DPPH free-radical scavenging activity of IC\({}_{50}\) 54.22 \(\upmu\)g/mL, a vitamin C equivalent antioxidant capacity (VEAC) while ascorbic acid had an IC\({}_{50}\) value of 35.54 \(\upmu\)g/mL [34]. Strong \({}^{1}\)O\({}_{2}\)-quenching activity was observed in methyl 5-glucosyl-5,6-dihydro-4,4'-diapolycopenoate (5.1 \(\upmu\)M), while the following intermediate carotenoids exhibited moderate \({}^{1}\)O\({}_{2}\)-quenching activity compared to astaxanthin (3.7 \(\upmu\)M): 5-glucosyl-5,6-dihydro-4,4'-diapolycopene (30 \(\upmu\)M), 5-hydroxy-5,6-dihydro-4,4'-diapolycopene (30 \(\upmu\)M), 4,4'-diaponeurosporene (45 \(\upmu\)M), 5-hydroxy-5,6-dihydro-4,4'-diaponeurosporene (56 \(\upmu\)M), and 15-_cis-4_,4'-diapophytoene (>100 \(\upmu\)M) [33]. 4_4'_-Diapolycopene-4,4'-dioic acid and its methyl esters (monomethyl ester and dimethyl ester) were tested for their antioxidant activity according to their quenching capabilities of singlet oxygen (\({}^{1}\)O\({}_{2}\)). Their IC\({}_{50}\) values were 5.8 \(\upmu\)M, 6.0 \(\upmu\)M, and 6.2 \(\upmu\)M, respectively, compared to that of astaxanthin (9.3 \(\upmu\)M) [46]. In addition to natural C\({}_{30}\) carotenoids, several studies have reported the synthesis of novel C\({}_{30}\) carotenoids, such as glycol-C\({}_{30}\) carotenoids, which have different antioxidative properties. [PERSON] et al.