After synthesis of a deuterated 4-hydroxy-2-nonenal (4-HNE) standard, the formation of 4-HNE during heating of peanut oil and whole peanuts, respectively, was measured by GC-MS. Whereas a significant increase in 4-HNE levels was observed for peanut oil, the amount of 4-HNE decreased when whole peanuts were roasted due to lipation reactions with amino acid side chains of the proteins. The epsilon-amino group of lysine was identified as the favored reaction partner of 4-HNE. After heating N-alpha-acetyl-l-lysine and 4-HNE, a Schiff base, a novel pyridinium derivative, a 2-pentylpyrrol derivative and, following reduction and hydrolysis, a reduced, cyclized Michael adduct were identified. 2-Amino-6-(2-pentyl-1H-pyrrol-1-yl)hexanoic acid (2-PPL) was synthesized and quantitated in peanut proteins, which had been incubated with various amounts of 4-HNE by HPLC-ESI-MS/MS after enzymatic hydrolysis. At low 4-HNE concentrations the modification of lysine could be entirely explained by the formation of 2-PPL. Additionally, 2-PPL was quantified for the first time in peanut samples, and an increase depending on the roasting time was observed. 2-PPL represents a suitable marker to evaluate the extent of food protein lipation by 4-HNE.