Biophysical techniques for protein secondary structure analysis: a comprehensive review

The determination of protein secondary structure constitutes a fundamental step in understanding protein function, folding mechanisms, and disease pathology. This comprehensive review presents a systematic methodological comparison of the principal biophysical techniques employed for secondary structure elucidation, spanning seven decades of technological development from Pauling and Corey's foundational discoveries to contemporary high-resolution methods. We examine spectroscopic approaches, including circular dichroism (CD) and Fourier-transform infrared (FTIR) spectroscopy, which provide rapid global quantification of secondary structure content with minimal sample requirements, alongside high-resolution techniques, including nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography, and cryo-electron microscopy (cryo-EM), which offer atomic-level structural details. For each technique, we analyze theoretical foundations, experimental protocols, deconvolution algorithms, accuracy benchmarks, and optimal application domains. Comparative analysis reveals that no single technique achieves optimal performance across all secondary structure classes: FTIR generally provides improved accuracy for β-sheet-rich systems, particularly when sheet topology discrimination is required. NMR uniquely provides residue-level resolution with dynamics information across multiple timescales, whereas crystallography and cryo-EM define atomic-resolution standards for static structures. Complementary techniques, including small-angle X-ray scattering (SAXS), hydrogen-deuterium exchange mass spectrometry (HDX-MS), and Förster resonance energy transfer (FRET), provide essential orthogonal information on global shape, protection patterns, and distance constraints. We present detailed decision frameworks for technique selection based on sample characteristics and research objectives, propose integrative multi-technique workflows, and identify critical gaps in current methodology, including the need for comprehensive multi-method benchmarking and standardized protocols for challenging protein systems such as membrane proteins and intrinsically disordered proteins. This review provides practical guidance for researchers designing experimental strategies for protein secondary structure characterization in the modern era of integrative structural biology.