Monopeptide versus monopeptoid: insights on structure and hydration of aqueous alanine and sarcosine via X-ray absorption spectroscopy.

Despite the obvious significance, the aqueous interactions of peptides remain incompletely understood. Their synthetic analogues called peptoids (poly-N-substituted glycines) have recently emerged as a promising biomimetic material, particularly due to their robust secondary structure and resistance to denaturation. We describe comparative near-edge X-ray absorption fine structure spectroscopy studies of aqueous sarcosine, the simplest peptoid, and alanine, its peptide isomer, interpreted by density functional theory calculations. The sarcosine nitrogen K-edge spectrum is blue shifted with respect to that of alanine, in agreement with our calculations; we conclude that this shift results primarily from the methyl group substitution on the nitrogen of sarcosine. Our calculations indicate that the nitrogen K-edge spectrum of alanine differs significantly between dehydrated and hydrated scenarios, while that of the sarcosine zwitterion is less affected by hydration. In contrast, the computed sarcosine spectrum is greatly impacted by conformational variations, while the alanine spectrum is not. This relates to a predicted solvent dependence for alanine, as compared to sarcosine. Additionally, we show the theoretical nitrogen K-edge spectra to be sensitive to the degree of hydration, indicating that experimental X-ray spectroscopy may be able to distinguish between bulk and partial hydration, such as found in confined environments near proteins and in reverse micelles.