Intramolecular Vibrations in Low-Frequency Normal Modes of Amino Acids: l‑Alanine in the Neat Solid State

Abstract

This paper presents a theoretical analysis of the low-frequency phonons of l-alanine by using the solid-state density functional theory at the Γ point. We are particularly interested in the intramolecular vibrations accessing low-frequency phonons via harmonic coupling with intermolecular vibrations. A new mode-analysis method is introduced to quantify the vibrational characteristics of such intramolecular vibrations. We find that the torsional motions of COO^– are involved in low-frequency phonons, although COO^– is conventionally assumed to undergo localized torsion. We also find the broad distributions of intramolecular vibrations relevant to important functional groups of amino acids, e.g., the COO^– and NH₃⁺ torsions, in the low-frequency phonons. The latter finding is illustrated by the concept of frequency distribution of vibrations. These findings may lead to immediate implications in other amino acid systems