Topochemistry, optical anisotropy and FT-IR microspectroscopy of the cocoon of Lithurgus chrysurus (Hymenoptera, Megachilidae)

A previous study has not revealed the participation of a mucous component in the cocoon wall of the solitary bee, Lithurgus chrysurus, differing from the cocoon structure reported for many other bee species. However, uncertainty remains, because only the median and rear zones of this cocoon type have thus far been analyzed. Here, we studied the front zone of this cocoon, searching its components and their organization, to fill this knowledge gap. Topochemical assays, polarization microscopy and Fourier transform-infrared (FT-IR) microspectroscopy were used to study cross sections from L. chrysurus cocoon. Three main layers differing in structural organization were found to compose the cocoon wall. Silk fibroins were assumed to constitute the filamentous threads of the inner and outer layers and the laminar structure of the intermediate layer. Deduced from its topochemical properties and FT-IR spectral signature, a foamy material containing mucin glycoproteins and carboxylated acid glycosaminoglycans was found in the intermediate layer. FT-IR analysis using a Savitzky-Golay 2nd-derivative and absence of linear dichroism and birefringence phenomena suggest that a random-coil secondary structure predominates in the foam component. Co-existence of α-helical and β-sheet conformations is also hypothesized for the fibroin component of this cocoon. It is thus concluded that in addition to fibroin elements, a mucous component, likely contributed by a Malpighian tubule secretion, integrates the composition of the front zone of the cocoon wall of L. chrysurus. In addition, the FT-IR analysis of the inner layer silk of this cocoon suggests significant differences in comparison to the silk fibroins of the silkworm, and some minor spectral differences in comparison to published data on the honeybee silk, with respect to protein secondary structure.