Preface

**Chapter 7 121**

Aliasing Compromises Staggered-Rotamer Analysis of Polypeptide

Sidechain Torsions *by Jürgen M. Schmidt*

**II**

Nuclear magnetic resonance (NMR) has evolved as a versatile tool in chemistry and biology. The scientific technique is based on the detection of magnetic moments of atomic nuclei arising due to an intrinsic property called spin because of their precession in static magnetic fields. Nuclei are excited by radio frequency (RF) magnetic fields and subsequently their precession is observed by the voltage they induce on an induction coil as they precess. The signal gives valuable information on the precession frequency of nuclei, which depends on the applied magnetic field and the local magnetic fields. At a field of say 14 tesla, protons precess at 600 MHz, carbon at 150 MHz, and nitrogen at 60 MHz. The frequency information is obtained by Fourier transform, which gives a characteristic spectrum. This local field is characteristic of the chemical environment of the nuclei and is termed chemical shift. Chemical shift gives each molecule a fingerprint spectrum with peaks dispersed in the kHz range and helps to identify the molecule from its spectrum. NMR spectroscopy is therefore an important tool in organic chemistry that aids synthetic chemistry. The spectrum of compounds displays characteristic chemical shifts and magnetic couplings between the atomic nuclei. In addition to giving frequency information the NMR signal displays a characteristic decay rate. This decay rate is important in MRI as it helps to provide contrast between the biological tissue being imaged. The decay rate broadens lines in a spectrum of large molecules and makes it difficult to resolve the frequency content of the spectrum.

Modern high-field NMR is able to record the spectrum of large molecules and resolve this spectrum by use of ingenious methods called 2D NMR. These methods rely on carefully tailored RF pulses that correlate frequency of the coupled nuclei by transferring magnetization between coupled spins.

These 2D NMR experiments coupled with relaxation measurements form the basis of the structural analysis of biological molecules, which give information on the dynamics and structure of biological macromolecules. NMR spectroscopy, which started as a tool for the analysis of compounds in organic chemistry, has now matured into a major discipline for the structural and dynamic study of large molecules.

NMR studies are not limited to molecules in solution but are also performed on samples in solid (powder or crystalline) form. These studies on solid-state powders involve spinning the sample to average an anisotropic interaction and obtain a resolved spectrum. These methods have evolved from the study of polymers to the study of biological molecules like membrane proteins.

In this book, we present some of the most exciting developments in the field of NMR: for example, new developments in NMR instrumentation, new magnet technology, RF coil design, the design of novel NMR sensors, and new developments of methods in solution and solid-state NMR. These range from new methods for fast the acquisition of 2D spectrum to NMR studies of molecular interactions in ionic solutions. Solid-state methods for the analysis of polyvinyl chloride and NMR studies of torsion angles in polypeptides are also included.

The book will be a useful reference for practitioners in the field and at the same time will appeal to a broad audience interested in the general area of NMR.

> **Navin Khaneja** Indian Institute of Technology Bombay, Mumbai, India

> > **1**

Section 1

NMR Instrumentation

Section 1
