Apiogalacturonans (AGA) are a type of pectin found in the plant cell wall of some small, fast growing aquatic monocots [1]. Among the subfamily of Lemnoideae, Lemna minor and Spirodela polyrhiza have primary cell walls that contain large amounts of AGA that distinguish them from most other flowering plants. Moreover, it appears that this type of pectin decreased substantially as the Lemnoideae diversified while xylogalacturonan is far more abundant [2]. Pagliuoso and colleagues suggested that the presence of AGA in aquatic plants could be an evolutionary adaptation of this family related to their rapid growth on the water surface [3].

Structural studies revealed that AGA are composed of (1 → 4)-linked α-d-Galacturonic acid backbone substituted with apiose side chains via β1-2 or β1-3 links [4,5]. The linkage groups were significantly similar between the species, but the degree of apiose extension varied considerably, with higher ratios of two-apiose substitutions in Spirodela polyrhiza and lower ratios in Wolffia australiana [6].

Apiose-containing cell wall fragments may be involved in signaling cascades which orchestrate defense against pathogens [1]. It has also been shown they have anti-viral and cryoprotective applications. However, while their structure suggested substantial hygroscopic properties, there is no study aiming to investigate the intrinsic property of AGA.

Hydration is a key aspect of the skin that influences its physical and mechanical properties [7], especially in its outer layer, the Stratum corneum (SC) [8]. Environmental factors such as dry climate, exposure to wind or cold weather, hormonal variations or even drugs induce skin dehydration, mainly affecting the SC. Water loss regulation by physiological processes such as production of lipids and natural moisturizing factors could be impacted. Thereby, the skin needs the use of moisturizing treatments. Hence, it could be possible to increase the water content with humectants or reducing water loss by using occlusive oils and lipid-modulating agents [9]. Humectants correspond to hygroscopic agents that attract and retain water into the SC and thus increase the cutaneous hydration level.

The aim of this study was thus to investigate the hygroscopicity of AGA to use them as active ingredient displaying humectant properties. For this purpose, we were interested in Spirodela polyrhiza, one of the species with the higher content of AGA and whose structure is detailed in the literature. First, molecular modeling study was conducted to generate the tridimensional structure of representative molecules of AGA. Then, molecular dynamics (MD) simulations were run to estimate contact frequencies between AGA and water molecules. After a dedicated and patented process aiming to isolate and purify AGA from Spirodela polyrhiza, their hygroscopic potential was evaluated in vivo by tracking deuterated water (D2O) using confocal Raman microspectroscopy. Altogether, these experimentations demonstrated that AGA are natural molecules displaying a high hygroscopic potential, able to capture and retain water in the SC.