The tertiary interactions between amide-I vibrators on the separate helices of transmembrane (TM) dimers were probed by ultrafast two-dimensional vibrational photon echo spectroscopy (2D IR)¹. The 2D IR proves to be useful for the study of membrane bound structures. The isotopomers and their 50:50 mixtures of KKITLIIFG₇₉VMAGVIGTILLISWG₉₄IKK labeled at G₇₉ and G₉₄ with ¹³C=¹⁶O or ¹³C=¹⁸O formed helical dimers in sodium dodecyl sulfate (SDS) micelles. The 2D IR spectra showed homodimers, where both helices had either ¹³C=¹⁶O or ¹³C=¹⁸O substitution, and the heterodimer where one had ¹³C=¹⁶O and the other ¹³C=¹⁸O. The cross peaks in the heterodimer 2D IR and the splitting of the homodimer peaks show that the amide-I mode is delocalized across a pair of helices. The excitation exchange coupling (4.3-6.3 cm^-1) arises from through-space interactions between amide units on different helices. A structural picture of the hydrophobic interface that is consistent with results from NMR on helix dimers was obtained from 2D IR, which also exposed aspects of the ultrafast dynamics through the decay of the frequency distributions of the isotopomer transitions. Energy transport between neighboring amide-I modes in the helix through the cross-peak dynamics is observed. This research was supported by NIH RR01348 and NIH GM12592 to RMH. (1) Fang, C., Senes, A., Cristian, L., DeGrado, W. F. & Hochstrasser, R. M. (2006) Proc. Natl. Acad. Sci. USA 103 (45), 16740-16745.