title: UVA illumination-induced optical coupling between tryptophan and natural dissolved organic matter.
abstract: Exposure of tryptophan (Trp) in aqueous solutions to UVA radiation resulted in decrease of Trp (C1) but generated an unknown fluorescent component (C2) with fluorescence emission maxima extending into wavelength range characteristic of humic substance (HS)-like material. The intensity of the two components (C1 and C2) could be operationally fit to linear functions of the illumination time t (0~40 h). However, C1 and C2 decreased and increased nonlinearly respectively in a mixture (Trp mixed with a reference sample of natural organic matter, i.e., NOM) which was exposed to the same UVA illumination, and the change of both C1 and C2 was faster than that in the absence of NOM. Moreover, the UV-Vis absorption maximum (ex = 278 nm) of Trp was faster removed for the mixture (after 5 h) than for Trp solutions without NOM (after 20 h). These observations suggested NOM-facilitated photobleaching of Trp and photoproduction of a new FDOM component under UVA illumination. Meanwhile, the fluorescence of the NOM in the absence of Trp was well represented by two HS-like components which decayed monotonically upon exposure to UVA light, while the photoinduced decay became nonmonotonic in the presence of Trp, and one component even increased with illumination during certain time window, indicating Trp-facilitated production of HS-like fluorescence signatures from NOM. The findings show that UVA-induced optical signature changes of tryptophan and HS-like materials are coupled and highlight the potential impact of absorption of solar UVA light by natural dissolved organic matter (DOM) on using the optical signatures to trace sources and sinks of DOM.
authors.affiliation: College of Chemistry and Chemical Engineering, Inner Mongolia University, 235 West University Road, Hohhot, 010021, China.
authors.forename: Xuechun
authors.initials: X
authors.lastname: Wang
authors.affiliation: State Environmental Protection Key Laboratory in Coastal and Estuarine Research, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China. chenhao@craes.org.cn.
authors.forename: Hao
authors.initials: H
authors.lastname: Chen
authors.affiliation: State Environmental Protection Key Laboratory in Coastal and Estuarine Research, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China.
authors.forename: Kun
authors.initials: K
authors.lastname: Lei
authors.affiliation: College of Chemistry and Chemical Engineering, Inner Mongolia University, 235 West University Road, Hohhot, 010021, China.
authors.forename: Zhong
authors.initials: Z
authors.lastname: Sun
mesh_terms: D006812:Humic Substances / Q000528:radiation effects*; D038761:Photobleaching; D013050:Spectrometry, Fluorescence / Q000379:methods; D013472:Sunlight; D014364:Tryptophan / Q000737:chemistry* / Q000528:radiation effects*; D014466:Ultraviolet Rays
chemical_list: D006812:Humic Substances; D014364:Tryptophan
keywords: Fluorescence; Natural organic matter (NOM); Parallel factor analysis; Photochemistry; Tryptophan; UVA irradiation