ORIGIN OF ANOMALOUS 13-C IR ENHANCEMENT IN ANTI-PARALLEL BETA-SHEET AGGREGATES

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Experimentally we have observed that the anti-parallel b-sheet aggregates e.g., (Trp-Leu-Leu-Leu-*Leu-Leu)n, n=10-40, show very strong enhancement in 13-C-peak. The objective is to understand this anomalous enhancement from the point of view of that the amide units are coupled to each other to certain extend. It is on which our model is based. The knowledge of amide unit interactions is essential in order to obtain structure-spectrum relationship in linear IR and 2D-IR as well. We are working on a transition charge-charge interaction model, in which a transition charge entity is designated for each amide unit. This model allows us to calculate the IR spectrum of almost all the mid-sized secondary motifs based on atomic coordinates that are available at Protein Data Bank. Preliminary results showed that the dominant factors responsible for the observed 13-C-peak enhancement are the inter-chain and intra-chain nearest neighbor amide unit interactions, which are covalently bonded or hydrogen bonded. The enhancement is dependent on the length of aggregation and on the 13-C-labeling positions, which can be predicted. We are also refining the coupling strengths of the nearest neighbor units, by using the ab initio normal model calculations that take into account of both the through-space and through-bond contributions.