Structure elucidation and proposed de novo synthesis of an unusual mono-rhamnolipid by Pseudomonas guguanensis from Chennai Port area

Abstract

In this paper, we describe the isolation of an unusual type of high molecular weight monorhamnolipid attached to esters of palmitic, stearic, hexa and octadecanoic acids as against the routinely reported di-rhamnolipids linked to hydroxydecanoic acids. The bioemulsifier was column-purified and the chemical nature of the compound was elucidated using FT-IR, GC-MS and 1D [¹H and¹³C] and 2D NMR. This monorhamnolipid is extracted from a bacterium, Pseudomonas guganensis and is not reported to have biological activities, let alone emulsification abilities. The bacterium continually produced rhamnolipids when nourished with n-hexadecane as its lone carbon source. The extracellularly secreted monorhamnolipids are capable of degrading hydrocarbons, with most preference to n-hexadecane [EI₂₄ of 56 ± 1.42% by 2 mL of the spent medium]. Whilst the crude ethyl acetate partitioned extract had an EI₂₄ of 65 ± 1.43%; the purified rhamnolipid product showed 78 ± 1.75% both at 12.5 mg/mL concentration. The used-up n-hexadecane is biotransformed to prepare its own rhamnolipids which in return is utilized to degrade n-alkanes thus creating a circular pathway which is proposed herein. This bacterium can be seen as a new source of bioemulsifier to reduce hydrocarbon in polluted waters.

Figure 1

Laboratory-scale production of bioemulsifer by P. guguanensis [left] and dehydrated crude extract [right] in the top. Emulsification activity [left] and TLC pattern [run in EA:MeOH 5:5] of active fraction [right] at the bottom.

Figure 2

NMR spectroscopies [¹H and ¹³C] and GC-MS fragmentation patterns of compound 1.

Figure 3

NMR spectroscopies [¹H and ¹³C] and GC-MS fragmentation patterns of compound 2.

Figure 4

NMR spectroscopies [¹H and ¹³C] and GC-MS fragmentation patterns of compound 3.

Figure 5

Proposed de novo synthetic pathway of monorhamnolipids in Pseudomonas guguanensis.