Under certain experimental conditions peptide solutions can generate sufficient NMR data for structure determination Yet, unlike proteins, peptides typically prevail as ensembles of interconverting structures, and therefore, the experimental variables (which are intensities of cross peaks in nuclear Overhauser enhancement spectroscopy (NOESY) spectra, or NOEs) are average quantities. The process of structure elucidation is complex, and many related questions are still open to debate. We have developed a new theoretical methodology for treating ensembles of interconverting conformations which is based on purely thermodynamic grounds. The peptide is described by a potential energy function, and its conformational space is viewed as a collection of microstates, which are local conformational regions around energy minima. The overall methodology enables one to identify the thermodynamically most stable microstates, determine their populations, calculate the individual microstate NOEs, and obtain the overall NOEs as averages over the individual contributions, weighted by the microstate populations. In this paper we develop for the first time theoretical methods for calculating the relative contribution of microstates to the-partition function, as their minimum energy is increased above the global energy minimum(GEM). This is necessary for determining which microstates should be considered in detail in the next stage of the analysis.