PH-induced changes in β-casomorphin 7 structure studied by 1H nuclear magnetic resonance and Fourier-transform infrared spectroscopy

Abstract

β-Casomorphins are opioid peptides that can be liberated from β-casein during digestion, with some suggested effects on human physiology linked to their properties. In this study, the structural behaviour of β-casomorphin 7 (BCM7; Tyr–Pro–Phe–Pro–Gly–Pro–Ile) was studied at pH 2.3 and 6.7 using 1H nuclear magnetic resonance (1H NMR) and Fourier-transform infrared (FTIR) spectroscopy. Cis–trans isomerism involving the Tyr1–Pro2, Phe3–Pro4 and Gly5–Pro6 bonds of BCM7 was observed. Cis and trans isomers of Tyr1–Pro2 and Phe3–Pro4 and trans isomers for Gly5–Pro6 were observed at both pH 6.7 and 2.3 but cis isomers of Gly5–Pro6 were only observed at pH 2.3 and not at pH 6.7. These pH-induced conformational changes at the Gly5–Pro6 bond of BCM7 may play important role in enzymatic release of this peptide from the parent protein β-casein under gastro-intestinal conditions, as well as further cleavage of BCM7 into smaller peptides.