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C99 (also known as β-CTF) is the 99 residue transmembrane C-terminal domain (residues 672-770) of the amyloid precursor protein and is the immediate precursor of the amyloid-β (Aβ) polypeptides. To test the dependence of the C99 structure on the composition of the host model membranes, NMR studies of C99 were conducted both in anionic lyso-myristoylphosphatidylglycerol (LMPG) micelles and in a series of five zwitterionic bicelle compositions involving phosphatidylcholine and sphingomyelin in which the acyl chain lengths of these lipid components varied from 14 to 24 carbons. Some of these mixtures are reported for the first time in this work and should be of broad utility in membrane protein research. The site-specific backbone (15)N and (1)H chemical shifts for C99 in LMPG and in all five bicelle mixtures were seen to be remarkably similar, indicating little dependence of the backbone structure of C99 on the composition of the host model membrane. However, the length of the transmembrane span was seen to vary in a manner that alters the positioning of the γ-secretase cleavage sites with respect to the center of the bilayer. This observation may contribute to the known dependency of the Aβ42-to-Aβ40 production ratio on both membrane thickness and the length of the C99 transmembrane domain.

Hydrogen bonds play critical roles in protein structure, stability, and function. Conventionally, hydrogen bonds are mainly determined by X-ray crystallography and NOE-based NMR spectroscopy in indirect manners. In recent years, it was demonstrated that hydrogen bonds can be directly detected through NMR measurements of trans-hydrogen-bond scalar coupling constants. Here we report across hydrogen-bond protium/deuterium isotope effects in a 35 kDA protein observed with the isotopomer-selective TROSY NMR technique (Liu et al. J. Biomol. NMR 2006, 36, 205−214; Liu et al. J. Magn. Reson. 2007, 186, 319−326) and show that such isotope effects can be used to detect a most common type of bifurcated hydrogen bonds, in which a heavy atom, usually oxygen, is involved in two hydrogen bonds, including a pair of bifurcated hydrogen bonds involving a bound water molecule.