First-principles study on core-level spectroscopy of arginine in gas and solid phases.

First-principles simulations have been performed for near-edge X-ray absorption fine-structure (NEXAFS) spectra of neutral arginine at different K-edges in the solid phase as well as X-ray photoelectron spectra (XPS) of neutral, deprotonated, and protonated arginines in the gas phase. Influences of the intra- and intermolecular hydrogen bonds (HBs) and different charge states have been carefully examined to obtain useful structure-property relationships. Our calculations show a noticeable difference in the NEXAFS/XPS spectra of the canonical and zwitterionic species that can be used for unambiguously identifying the dominant form in the gas phase. It is found that the deprotonation/protonation always results in red/blue shifts of several electronvolts for the core binding energies (BEs) at all edges. The normal hydrogen bond Y-H···X (X, Y = N, O) can cause a blue/red shift of ca. 1 eV to the core BEs of the proton acceptor X/donor Y, while the weak C-H···Y hydrogen bond may also lead to a weak red shift (less than 1 eV) of the C1s BEs. Moreover, the influence of intermolecular interactions in the solid state is reflected as a broadening in the σ* region of the NEXAFS spectra at each edge, while in the π* region, these interactions lead to a strengthening or weakening of individual transitions from different carbons, although no evident visual change is found in the resolved total spectra. Our results provide a better understanding of the influences of the intra- and intermolecular forces on the electronic structure of arginine.