Conformational studies on angiotensin-II. Least-squares fit of carbon-13 nuclear magnetic resonance relaxation times to extended and folded conformations

Abstract

We have fitted the carbon-13 nuclear magnetic spin-lattice relaxation time ( T 1) data of the proton-bearing carbons in angiotensin-II to T 1 values obtained from a plausible model for reorientational relaxation. In the absence of X-ray data, we used conformations that were deduced from energy calculations. In particular, we assessed the conformational sensitivity of the calculated (best fit) relaxation parameters by comparing results for a low energy folded structure with the fully extended one. Furthermore, we estimated the relative importance and physical relevance of interpreting the observed relaxation behaviour of angiotensin-II in aqueous solution either in terms of rigid overall isotropic molecular tumbling only or with the additional assumption of internal motion in the side chains. A non-linear least-squares procedure was used to fit the theoretical T 1 values to the measured ones. For a given conformation the T 1 values were calculated from stochastic rotational diffusion models for both overall (isotropic and anisotropic) and internal molecular motion. The computational results are consistent with nearly isotropic overall tumbling of angiotensin-II. Furthermore, it seems essential to invoke internal motion for some of the linear side chains and all methyl groups in order to arrive at a physically meaningful description of the molecular dynamics. Without the premise of internal motion, an anisotropic rigid body model of the overall tumbling cannot explain the experimental relaxation results. The distinction between isotropic and anisotropic overall motion is not crucial, provided internal motion is also considered. The calculated effective correlation times for internal motion about side-chain CC bonds are rather insensitive to overall conformation, particularly beyond the C gaC β bond. This observation leads to the important conclusion that side chain-side chain interactions and their influence on the flexibility of adjacent residues in peptides and peptide hormones can be profitably studied via 13C relaxation without precise knowledge of the overall conformation.