In some embodiments, the invention provides modified pyrazine derivatives containing a central pyrazine ring with two secondary amine groups attached directly to the central pyrazine ring and two amino carbonyl groups attached directly to the central pyrazine ring. The secondary amine groups are terminated by alkyl groups containing from 1 to 6 carbons. The amino carbonyl groups can be terminated by a wide range of substituents including, but not limited to, alkyl and alkylene groups, polyether groups, including poly(ethylene glycol) groups, secondary and tertiary amine groups, polyhydroxylated alkyl groups, amino carbonyl groups, amino thioketone groups, and combinations thereof. In some embodiments, the invention provides modified pyrazine derivatives useful as optical agents in a wide variety of biomedical imaging procedures, including diagnostic and imaging procedures. In an embodiment, for example, modified pyrazine derivatives are provided which are useful in monitoring organ and system functioning, for example in monitoring renal system functioning.