Chlorophyll (Chl)-c₁ and Chl-c₂ were extracted from a commercially available diatom Chaetoseros calcitrans, and the former (8-ethyl) and the latter (8-vinyl) were efficiently separated by reverse-phase HPLC using a polymeric octadecylsilyl column to afford analytically pure compounds in an amount adequate for further chemical modification. The conformation of the unique acrylate moiety at the 17-position of isolated Chls-c in THF was unambiguously determined to be “cisoid” around the C17–C17¹ bond using ¹H-¹H NOE correlations: C17¹C17² was on the same side as C17C18. Interestingly, correlations originating from the “transoid” conformer could not be observed under the present NMR conditions, indicating that the rotation of the acrylate was considerably restricted. To elucidate the function of the rigid acrylate in Chls-c, we examined their electronic absorption properties using two synthetic types of esters possessing a porphyrin π-system: acrylate-type (17-CHCH–COOR) prepared by esterification of natural Chl-c₁ and Chl-c₂, and propionate-type (17-CH₂–CH₂–COOR) by 17,18-oxidation of natural Chl-a and its 8-vinyl analog. The Soret absorption bands at around 450 nm of the acrylate-type were red-shifted and broadened more than those of the propionate-type. Consequently, the unique acrylate in Chls-c serves as an aid for expanding the absorption region around 400–500 nm in order to capture intense irradiation from the sun for photosynthesis.