Extracting Infrared Spectra of Protein Secondary Structures Using a Library of Protein Spectra and the Ramachandran Plot

Infrared (IR) spectra from 1200 to 1800 cm–1 of the pure α-helix and β-sheet secondary structures have been extracted using a covariant least-squares procedure which relates a library of 40 infrared (IR) solution protein spectra from the work of Dong, Carpenter, and Caughey and amino acid fractions of the proteins based on assignments by STRIDE (secondary structure identification) of Eisenhaber and Argos. The excitonic splitting of the β-sheet structures is determined for this library of solution proteins. The method is extended to find a set of spectral basis functions that analyze IR spectra of protein samples for α-helix and β-sheet content. A rigorous error analysis including covariance, the correlations between the input library spectra, was used to justify the results and avoid less meaningful results. The utility of the results on α-helix and β-sheet regions is demonstrated by detecting protein changes due to cancer in imaging Fourier transform IR (FTIR) spectra of liver tissue slices. This work ends with a method to extract IR spectra of less prominent torsional angle distributions.本研究通过协方差最小二乘法，关联董、卡彭特与考伊的研究中收录的40条溶液态蛋白质红外（IR）光谱库，以及基于艾森哈伯与阿戈斯开发的STRIDE（二级结构识别）算法赋值得到的蛋白质氨基酸组分，提取出纯α螺旋（α-helix）与β折叠（β-sheet）二级结构在1200~1800 cm⁻¹波数范围内的红外光谱。针对该溶液态蛋白质光谱库，我们测定了β折叠结构的激子分裂参数。本方法进一步拓展得到一组光谱基函数，可用于分析蛋白质样品红外光谱中的α螺旋与β折叠含量。研究采用纳入协方差与输入光谱库间相关性的严格误差分析方法，以验证结果的合理性并避免无意义的分析结论。通过检测肝组织切片的傅里叶变换红外（FTIR）光谱中因癌症引发的蛋白质变化，验证了该方法在α螺旋与β折叠区域的应用效能。本研究最终提出了一种提取低占比扭转角分布对应红外光谱的方法。