In this contribution we present a novel methodology that enables accurate calculations of ensemble observables, such as Circular Dichroism (CD), Infrared (IR), Raman, and Raman Optical Activity (ROA) spectra, for disordered peptides. The methodology relies upon (i) prediction of the peptides' conformational ensembles by means of enhanced-sampling molecular dynamics (MD) methods; (ii) unbiased identification and characterization of a small number (order of 10 to 100) of representative conformers by means of clustering and dimensionality reduction methods; (iii) calculation and averaging of the spectral responses of those conformers with first-principle methods beyond the current state of the art. For our study, we chose a representative dodecapeptide with sequence HSSHHQPKGTNP, which was identified as a strong binder to single-crystalline ZnO using phage-display techniques. Theoretically predicted and experimentally acquired spectroscopic spectra are compared, showing the advantages and limitations of the methodology developed here and of the methods employed to predict the spectral responses of individual conformers. By doing so, we provide what we believe to be the first complete all-atom spectroscopic fingerprint of a disordered peptide in solution.