Polymer amide as a source of the cosmic 6.2 μm emission and absorption

Abstract

ABSTRACT Cosmic infrared emission and absorption spectra often carry a well-defined and invariant 6.2 $\mu \rm m$ band that has been proposed to emanate from very small dust grains that may carry polyaromatic hydrocarbons. Hemoglycin, a well-defined polymer of glycine that also contains iron, has been found in meteorites of the primordial CV3 class and therefore originated in the solar protoplanetary disc. Here, we suggest that the polymer hemoglycin should also be considered as a source of the cosmic 6.2 $\mu{\rm m}$ emission and absorption. In quantum calculations, the principal amide I infrared absorption band of hemoglycin is centred, before splitting, at 6.0 $\mu\rm m$. Multiple hemoglycin polymers interact to split amide I into the strong (a-) band in the region of 6.2 $\mu\rm m$ and the much weaker (a+) band in the region of 5.8 $\mu\rm m$. Experimentally, these two components are seen in extracts of the Sutter’s Mill meteorite and in stromatolite ooid. The two 11-mer glycine antiparallel chains of hemoglycin have an exact structural analogue in antiparallel poly-l-lysine beta sheet crystals which in the laboratory have an (a-) absorption peak at 6.21 $\mu\rm m$. This wavelength coincidence, the demonstrated propensity of hemoglycin 4.9 nm rods to form accreting lattice structures, and its proven existence in the solar protoplanetary disc suggest that the cosmic 6.2 $\mu\rm m$ emission and absorption could be from small grains that are hemoglycin lattices or shell-like vesicles carrying internal organic molecules of various types. Calculated hemoglycin ultraviolet absorptions associated with iron in the molecule match the observed ultraviolet extinction feature at nominal 2175 Å.