A Raman spectroscopic study on 5′‐rGMP gel and self‐aggregate

In order to analyze the melting behavior of 5′‐rGMP gel at acidic pH and self‐aggregate near neutral pH we have obtained Raman spectra of aqueous solutions of 5′‐rGMP at various temperatures. At low temperature the intensities of Raman peaks at 502, 585, 1083, 1179, 1322, 1366, 1487, and 1578 cm −1 decrease due to the formation of ordered structure (Raman hypochromism). In contrast, the peaks at 671, 725, 813, and 1338 cm −1 become stronger at low temperature (Raman hyperchromism). The Raman hyperchromism of the 671‐ and 813‐cm −1 peaks have been explained in terms of detailed structural models. Recently, the 668‐ and 682‐cm −1 peaks in the Raman spectrum of aqueous 5′‐rGMP solution have been attributed to the guanine ring breathing vibrations in C3′‐ and C2′‐ endo conformers [Benevides, J. B., Lemur, D. & Thomas, G. J., Jr. (1984) Biopolymers 23 , 1011–1024]. On the basis of this information our Raman data can be interpreted to suggest that the continuous helix model of 5′‐rGMP gel is right‐handed. The 1487‐cm −1 peak intensity has been used to monitor the melting profies at several pHs. Near neutral pH the melting profile shows a single transition, whereas at acidic pH it shows two transitions. From these observations we propose possible pathways for the melting of 5′‐rGMP gel formed at acidic pH and self‐aggregate formed near neutral pH.