Oligomers of Diphenylalanine Interrogated by Cold Ion Spectroscopy and Neural Network-based Conformational Search

Abstract

Diphenylalanine (Phe2) is the primary building block of many self-assembling nanostructures that are important in biology and material science. Understanding the detailed mechanism of their formation requires knowledge of the structural motives the smallest oligomers attain at the very first steps of the process. Here we, first, employ high-resolution mass spectrometry to assign protonated Phe2 and its 2- to 13-unit oligomers formed in the gas phase from solution by electrospray ionization and then use cold ion spectroscopy to record their UV and, for monomer, dimer and hexamer, IR spectra. UV spectroscopy suggests the likely lack of specific strong proton- interactions in the oligomers larger than octamer, implying their certain structural stabilization. The IR spectroscopy and quantum chemical calculations, enhanced by neural network-based conformational search, jointly have determined the lowest-energy structures of the Phe2 monomer and dimer.